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Space weather and the MOST microsatellite Skaret, Kristina A. 2001

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SPACE W E A T H E R A N D T H E M O S T  by K R I S H N A A .  M I C R O S A T E L L I T E  S K A R E T  B . S c . , M c G i l l University,  A THESIS  1998  S U B M I T T E D IN P A R T I A L F U L F I L M E N T  T H E R E Q U I R E M E N T S  F O R T H E D E G R E E  M A S T E R OF  OF  SCIENCE  in  T H E F A C U L T Y OF G R A D U A T E  Department of Physics and  We  a c c e p t t h i s t h e s i s as  STUDIES  Astronomy  conforming  to the r e q u i r e d s t a n d a r d  T H E UNIVERSITY  OF BRITISH  April,  ©  C O L U M B I A  2001  K r i s h n a Skaret,  2001  OF  In p r e s e n t i n g this thesis i n partial f u l f i l m e n t o f the r e q u i r e m e n t s f o r a n a d v a n c e d  degree  at t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e for reference a n d study. I further agree that p e r m i s s i o n for extensive c o p y i n g o f this thesis for scholarly purposes m a y be granted b y the h e a d o f m y department or b y his or her representatives.  It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r  financial gain shall not be a l l o w e d without m y written permission.  Department o f Physics and Astronomy The University of British Columbia Vancouver, Canada  II  Abstract The MOST (Mcrovariability and Oscillations of Stars) microsatellite is designed to obtain the most precise stellar photometry (AL/L -10" ) ever achieved. In preparation 6  for the launch of the first satellite devoted to asteroseismology, a complete evaluation of space weather in the baseline orbit including radiation analysis is performed, providing a 'weather forecast' for the mission in order to assist the M O S T instrument team with crucial planning decisions. Results of assessing the effects of space weather include recommendations for the choice of orbit, design structure, operating guidelines, and data reduction guidelines. This analysis has aided the MOST team to convince the Canadian Space Agency (CSA) to identify a launch vehicle capable of taking MOST to a polar sun-synchronous orbit. Preliminary shielding recommendations have been incorporated into the mechanical design of the telescope. Estimates of the amount of degradation the C C D will experience, including the number of Single Event Effects (SEEs, effects caused by interaction with a single cosmic ray), have influenced current operating procedure guidelines. It is shown that radiation doses to the C C D are not expected to cause critical failure of the detector. A minimum mission lifetime is established for a worse-case radiation environment and found to be of sufficiently long duration to meet all primary scientific objectives. As the impact of the radiation environment (and other orbital environment related factors) is less than critical thresholds, the forecast for the MOST microsatellite looks 'sunny and warm'.  •*•  Mi  Table of Contents Abstract  ii  Table o f Contents  $ h  List of Tables  VI  List o f Figures  ' VII  List of Acronyms  \X  Acknowledgments and Dedication  X (  C h a p t e r 1: I n t r o d u c t i o n  1  1.1  Space Weather and Astronomy  1  1.2  Space Weather and M O S T  3  1.3 T h e M O S T m i c r o s a t e l l i t e : A S p a c e S e i s m o l o g y P i o n e e r 1.4 C h a r g e - C o u p l e d D e v i c e s ( C C D s ) 1.4.1  The M O S T  C C D  4 8 11  C h a p t e r 2: S p a c e W e a t h e r  14  2.1  History  14  2.2  C h a r g e d Particle M o t i o n i n a M a g n e t i c F i e l d  15  2.3  2.4  2.5  2. 2.1  A d i a b a t i c Invariants  17  2.2.2  B and L Co-ordinates  20  The Geomagnetosphere  21  2.3.1  T h e South Atlantic A n o m a l y  23  2.3.2  Geomagnetospheric  24  2.3.3  Geomagnetic Storms  Shielding  25  C h a r g e d Particle Populations  26  2.4.1  Magnetospheric Particles  27  2.4.2  Solar Energetic Particles  30  2.4.3  Galactic Cosmic Radiation  31  2.4.4  T h e Anomalous Component of Galactic C o s m i c Radiation  32  Solar C y c l e M o d u l a t i o n  33  C h a p t e r 3: M o d e l i n g t h e R a d i a t i o n E n v i r o n m e n t  37  3.1 A p p r o a c h  37  3.2 A P 8 / A E 8 T r a p p e d P a r t i c l e M o d e l s  39  3:3  Geomagnetic Shielding models  40  3.4  C R E M E  40  3.5  Solar Energetic Particles  41  3.6 U n c e r t a i n t i e s  43  IV  C h a p t e r 4: C h o o s i n g a B a s e l i n e O r b i t  45  4.1  45  T h e M O S T Baseline Orbit  4.2 C o s m i c R a y H i t s  46  4.3  The Continuous Viewing Zone  47  4.4  The M O S T Duty Cycle  50  4.5 E c l i p s e S e a s o n  53  4.6  55  Stray L i g h t Effects  C h a p t e r 5: T h e W e a t h e r F o r e c a s t f o r t h e M O S T M i c r o s a t e l l i t e  59  5.1  The Geomagnetic Field  59  5.2 G e o m a g n e t i c S h i e l d i n g  .62  5.3 T r a p p e d P r o t o n s a n d E l e c t r o n s  63  5.4 G a l a c t i c C o s m i c R a d i a t i o n  64  5.5 A n o m a l o u s C o s m i c R a d i a t i o n  67  5.6  Solar Energetic Particles  68  5.7  Satellite S h i e l d i n g  70  5.8 C u m u l a t i v e D o s e s  76  5.9 A l t i t u d e v e r s u s D o s e  77  C h a p t e r 6: R a i n o r S h i n e ? I m p l i c a t i o n s o f S p a c e W e a t h e r o n M O S T  79  6.1  C C D Damage  80  6.1.1  82  6.2  D a r k Current  6.1.2  D a m a g e d Pixels  6.1.3  R T S  6.1.4  Flat B a n d V o l t a g e Shifts  86  6.1.5  C T E Degradation  87  6.1.6  I m p l i c a t i o n s f o r the P h o t o m e t r i c E r r o r B u d g e t  89  Single Event Effects (SEEs)  C h a p t e r 7: M i t i g a t i o n o f E n v i r o n m e n t a l D a m a g e 7.1  R e c o m m e n d a t i o n s f o r the M O S T M i c r o s a t e l l i t e  84 •  85  89 96 .96  7.2  Other Asteroseismology Missions  97  7.3  Future W o r k  98  References:..  100  A p p e n d i x A : M O S T T a r g e t Stars  104  A p p e n d i x B : Orbital Parameters  105  A p p e n d i x C : T a r g e t Star D w e l l T i m e i n the C V Z  109  Appendix D : M a p s of Trapped Proton and Electron Flux  113  Appendix E : Cumulative Doses  A p p e n d i x F : Specifications Sheet for M a r c o n i  CCD46-20  List of Tables: T a b l e 1.1 N o i s e l e v e l a n d d e t e c t i o n l i m i t s o f t h e M O S T m i c r o s a t e l l i t e  11  T a b l e 1.2 C C D 4 7 - 2 0 D e s c r i p t i o n  11  T a b l e 2.1 L o c a t i o n o f c e n t r o i d o f m i n i m u m o f S A A b e t w e e n 1 9 7 0  T a b l e 2.2  C o m p a r i s o n o f trapped particle populations  T a b l e 5.1 U n c e r t a i n t i e s i n r a d i a t i o n e n v i r o n m e n t m o d e l s  a n d 1993  23  27  76  V  VII  List of Figures: F i g u r e 1.1 P h o t o o f a m o c k - u p o f t h e M O S T m i c r o s a t e l l i t e  4  F i g u r e 1.2 F a b r y m i c r o l e n s p u p i l i m a g e f r o m t e s t i n g o f t h e M O S T m i c r o l e n s a r r a y  .6  F i g u r e 1.3 S c h e m a t i c o f t h e M O S T f o c a l p l a n e  .7  F i g u r e 1.4 S c h e m a t i c o f C C D f u n c t i o n  9  F i g u r e 1.5 T o p v i e w o f C C D , s h o w i n g t h r e e g a t e s  10  F i g u r e 1.6 S c h e m a t i c o f t y p i c a l b u r i e d c h a n n e l C C D , i n i n v e r t e d m o d e . . .  12  F i g u r e 2.1  M e r i d i a n projection o f a trapped charged particle  14  F i g u r e 2.2  A r t i s t s c o n c e p t i o n o f the V a n A l l e n radiation belts  15  F i g u r e 2.3  S c h e m a t i c o f c h a r g e d particle m o t i o n i n the g e o m a g n e t i c f i e l d  17  F i g u r e 2.4  S c h e m a t i c o f the Earth's geomagnetosphere  21  F i g u r e 2.5  C r o s s section o f the radiation belts s h o w i n g S A A  23  F i g u r e 2.6 R a d i a l d i f f u s i o n i n r e s p o n s e to a c o m p r e s s e d m a g n e t i c f i e l d  29  F i g u r e 2.7  34  S u n s p o t n u m b e r as a f u n c t i o n o f date  F i g u r e 2.8 R e c e n t s u n s p o t d a t a s h o w i n g s o l a r activity c y c l e f o r the n e x t d e c a d e  F i g u r e 3.1  A p p r o a c h to m o d e l i n g r a d i a t i o n e n v i r o n m e n t f o r the M O S T  .36  microsatellite...38  F i g u r e 4.1 D a t a f r o m t h e F U S E g u i d e c a m e r a c o m p a r i n g r e g i o n s i n a n d o u t o f S A A  46  F i g u r e 4.2  S c h e m a t i c o f the M O S T baseline orbit  47  F i g u r e 4.3  M O S T targets a n d l i m i t s o f C V A p r o j e c t e d o n the s k y  49  F i g u r e 4.4 T h e spectral w i n d o w f o r the M O S T microsatellite  52  F i g u r e 4.5 E c l i p s e d u r a t i o n f o r the M O S T b a s e l i n e orbit  54  F i g u r e 4.6 L i g h t c u r v e t a k e n b y the star-sensor o n b o a r d the W I R E satellite  55  F i g u r e 4.7  56  S c h e m a t i c o f the M O S T o r b i t i n g the E a r t h l o o k i n g d o w n o n N o r t h P o l e  viii  F i g u r e 4.8  T h e t a as a f u n c t i o n o f L T A N o v e r the c o u r s e o f a y e a r  57  F i g u r e 4.9 M a x i m u m a l l o w a b l e values o f L T A N for h e l i o s y n c h r o n o u s orbits  58  F i g u r e 5.1 M a g n e t i c f i e l d s t r e n g t h f o r M O S T b a s e l i n e o r b i t  59  F i g u r e 5.2 T r a p p e d p r o t o n s p e c t r a w i t h t h r e e d i f f e r e n t m a g n e t i c f i e l d m o d e l s  61  F i g u r e 5.3  62  G e o m a g n e t i c t r a n s m i s s i o n f u n c t i o n f o r the M O S T baseline orbit  F i g u r e 5.4 P r o t o n a n d e l e c t r o n f l u e n c e o v e r M O S T m i n i m u m m i s s i o n l i f e t i m e  63  F i g u r e 5.5 Integral f l u x e n e r g y s p e c t r u m o f G C R  65  F i g u r e 5.6 D i f f e r e n t i a l f l u x e n e r g y s p e c t r u m o f G C R  66  F i g u r e 5.7 Integral a n d d i f f e r e n t i a l f l u x e n e r g y s p e c t r a o f A C R  68  F i g u r e 5.8 D i f f e r e n t i a l e n e r g y s p e c t r a o f  69  S E P events  F i g u r e 5.9 D o s e v s . D e p t h c u r v e f o r s i m p l e s h i e l d i n g g e o m e t r y  71  F i g u r e 5.10  E n g i n e e r i n g s c h e m a t i c o f the p r e l i m i n a r y M O S T  72  F i g u r e 5.11  C o m p a r i s o n o f the p r o t o n f l u e n c e w i t h a n d w i t h o u t s h i e l d i n g  73  F i g u r e 5.12  L E T spectra o f trapped protons with different shielding  74  F i g u r e 5.13  L E T s p e c t r a o f solar e n e r g e t i c p r o t o n s as m o d e l e d b y J P L 9 1  75  F i g u r e 5.14  L E T s p e c t r a d u e to G C R  75  F i g u r e 5.15  D o s e at t h e c e n t e r o f a n A l s p h e r e w i t h v a r y i n g a l t i t u d e o r b i t s  77  F i g u r e 5.16  A p p r o x i m a t e b o u n d a r y o f t h e S A A at 6 0 0 ,  78  800,  satellite d e s i g n  a n d 1000  k m altitude  F i g u r e 6.1  C h a r g e generation or ionisation damage occur in a C C D  80  F i g u r e 6.2  Displacement damage  81  F i g u r e 6.3 N o n - i o n i s i n g e n e r g y l o s s a s a f u n c t i o n o f A l s h i e l d i n g t h i c k n e s s  88  F i g u r e 6.4 P r o t o n i n d u c e d s i n g l e e v e n t e f f e c t rate  92  F i g u r e 6.5  94  H e a v y i o n i n d u c e d upsets vs. sensitive v o l u m e a n d critical charge  ix F i g u r e 7.1  Ionising doses for M O S T , M O N S and C O R O T  F i g u r e A . 1 P o s i t i o n a l p l o t o f t r a p p e d p r o t o n f l u x >1.0  F i g u r e A . 2 P o s i t i o n a l plot o f t r a p p e d p r o t o n f l u x >10.0  M e V  M e V  97  113  114  Figure A . 3 Positional plot o f trapped proton flux >300.0 M e V  114  F i g u r e A . 4 P o s i t i o n a l p l o t o f t r a p p e d e l e c t r o n f l u x >1.0  115  Figure A . 5 Positional plot o f trapped electron flux  >5.0  M e V  M e V  115  LIST O F A C R O N Y M S A C R : Anomalous Cosmic Rays A C S : Attitude Control System C C D : Charge-Coupled Device C M E : Coronal Mass Ejection C R A N D :  Cosmic Ray Albedo Neutron Decay  C R E M E : C o s m i c R a y Effects on Micro-Electronics C R R E S : C o m b i n e d Release and Radiation Effects  Satellite  C V Z : Continuous Viewing Zone D A O : D o m i n i o n Astrophysical Observatory D S N U : D a r k SignalNon-Uniformity E S R T C : European Space Research and Technology Center F U S E : Far-infrared Ultraviolet Explorer F E S : Fine Error  Sensor  G C R : Galactic Cosmic Radiation H S T : Hubble Space  Telescope  I G R F : International G e o m a g n e t i c Reference  Field  EVIO: Inverted M o d e M I S : M e t a l Insulating  Semiconductor  M O S : Metal Oxide Semiconductor M O S F E T : Metal Oxide Semiconductor Field Effect M O S T : M i c r o v a r i a b i l i t y a n d O s c i l l a t i o n s o f Stars L E O : L o w Earth Orbit O N V : Orbit N o r m a l Vector Q E : Quantum Efficiency S A A : South Atlantic A n o m a l y S E B : Single Event Burnout S E E : Single Event  Effects  S E F : Single Event Failure S E F I : S i n g l e E v e n t F u n c t i o n a l Interrupt S E G R : Single Event Gate Rupture S E L : Single Event Latchup S E P : Solar Energetic S E U : Single Event  Particle  Upsets  S H E : Single Hard Error SOHO:  Solar and Heliospheric Observatory  S P E N V I S : Space Environment Information W E T : Whole Earth  Telescope  W I R E : Wide-field Infra-Red Explorer  System  Transistor  XI  Acknowledgements and Dedication M a n y p e o p l e h a v e contributed to this thesis i n m a n y different w a y s . O nthe  a c a d e m i c side, m y sincere thanks g o out to m y supervisor, J a y m i e M a t t h e w s , f o r always  s t a y i n g interested a n d patient, w i t h h i s sense o f h u m o u r constantly p r e v a i l i n g . I a m also  grateful to the M O S T  Johnston,  s c i e n c e t e a m at U B C , R a i n e r K u s c h n i g , G o r d o n W a l k e r , R o n  J o h n Pazder, a n d E v g e n y a S h k o l n i k f o r their comments a n d discussions, a n d  f o r t h e i r s u p p o r t o f m y w o r k , a n d t o o u r partners at C S A ( G l e n C a m p b e l l ) a n d D y n a c o n .  I'm grateful to S t e p h e n s o n Y a n g f o r i n t r o d u c i n g m e to the w o r l d o f observational  a s t r o n o m y . A n d o f c o u r s e , I o w e m u c h t o J a n e t J o h n s o n , o u r g r a d u a t e s e c r e t a r y at U B C  for h e r e n d l e s s p a t i e n c e a n d great a d v i c e , as w e l l as to a l l m y f r i e n d s a n d f a m i l y w h o  h e l p e d m e t h r o u g h h a r d times b y p r o v i d i n g g o o d times ( y o u k n o w w h o y o u are!).  T h i s thesis is d e d i c a t e d to m y father, D r . R e g J. Skaret ( 1 9 4 9 - 2 0 0 0 ) . H i s  incredible support, encouragement,  a n d e n t h u s i a s m e n s u r e d that I f o l l o w m y d r e a m s . I  c o u l d n o t h a v e c o m p l e t e d this project w i t h o u t h i s g u i d a n c e . H e w a s a n a m a z i n g m a n , a n d  a n e v e n m o r e a m a z i n g father. A s y o u r e a d this thesis, k n o w that I o w e m u c h o f e v e r y  success i n m y life to h i m . H e will b e truly missed.  L o o k d a d- 1 finished!  1  Chapter 1: Introduction 1.1 Space Weather and Astronomy Astronomers who use space telescopes don't have to cope with the hassles of a cloudy night at the observatory, or poor seeing due to atmospheric interference. At first glance, the space environment would appear to be very calm and 'weather-free'. But on closer inspection, one finds that low-Earth orbit (LEO) is actually quite active, has its own weather, and its own set of problems which space astronomers must be aware of. The chance to peer out at space from outside the atmosphere affords astronomers the ability to analyse wavelength regions opaque through the Earth's atmosphere. For others, the telescope needs to be in orbit about the Earth to escape the scintillation noise associated with a turbulent atmosphere and to have a complete duty cycle. The sensitivity and capacity of micro-electronics such as memory devices, signal processors, and photoelectric detectors has provided astronomers with a new chance to probe regimes not before open for observation. However, the trade-off has repetitively been an increasing sensitivity to the charged particle environment associated with the space environment. The near-Earth radiation environment is complex. A l l variety of atoms, from light protons to uranium nuclei (z=92), are accelerated to high energies by a wide variety of sources, some of which remain mysteries even today. Plasma from the solar wind constantly injects and replenishes the supply of charged particles which bombard satellites. Hence, the number and intensity of the particles varies strongly during the solar cycle. The presence of'cosmic radiation' first discovered in 1912 by Hess (Van Allen 1983) creates adverse conditions in the space orbital environment. During very strong Coronal Mass Ejections (CMEs) from the sun, charged particle interaction with the Earth's magnetosphere can be so severe that charged particles reach northern latitudes, disrupting power supplies, creating dangerous currents through long oil pipelines, and wiping out radio communications. These events are few and isolated to solar maximum here on Earth. However, in the orbital environment with less geomagnetic shielding, satellites experience much higher doses as well as a greater duration of exposure to  2  c h a r g e d particles. T h u s , satellites i n L E O m u s t b e d e s i g n e d to tolerate i s o l a t e d solar e v e n t s a s s o c i a t e d w i t h a l a r g e flux o f c h a r g e d p a r t i c l e s a s w e l l a s t h e a m b i e n t  flux  of  c h a r g e d particles m o s t l y c o n c e n t r a t e d i n the V a n A l l e n R a d i a t i o n belts. T h e r a d i a t i o n e n v i r o n m e n t c a n p r o d u c e a m y r i a d o f hazards to a n o r b i t i n g spacecraft.  T h e m o s t c r i t i c a l effect is a S i n g l e E v e n t F a i l u r e ( S E F ) i n w h i c h a s i n g l e  interaction between a charged particle a n d a n o n b o a r d micro-electronic (usually m e m o r y ) d e v i c e c a u s e s c r i t i c a l f a i l u r e . L u c k i l y , S E F s are v e r y r a r e i n m o d e r n satellites as c r i t i c a l m i c r o e l e c t r o n i c s are n o r m a l l y d u p l i c a t e d to p r o v i d e o n b o a r d r e d u n d a n c y , or b a c k u p . S E F s are just o n e o f a class o f effects c a l l e d S i n g l e E v e n t E f f e c t s ( S E E s ; S e c t i o n  6.2)  w h i c h are c a u s e d b y a s i n g l e p a r t i c l e i n t e r a c t i o n w i t h a m i c r o - e l e c t r o n i c d e v i c e . M o s t S E E s d o not interrupt n o r m a l operations but d o necessitate regular g r o u n d c o m m u n i c a t i o n s w i t h the satellite i n c l u d i n g r e g u l a r u p l i n k o f o p e r a t i n g s e q u e n c e s  to  a v o i d m o r e critical effects. T h e p r e s e n c e o f the V a n A l l e n R a d i a t i o n belts a n d the i m p l i c a t i o n s o f r e g u l a r traverses t h r o u g h their particle r i c h e n v i r o n m e n t s often influences the c h o i c e o f orbit for a satellite ( C h a p t e r 4). I n the heart o f the r a d i a t i o n belts i n L E O (a f e a t u r e c a l l e d the S o u t h A t l a n t i c A n o m a l y ( S A A ) b e c a u s e o f its g e o g r a p h i c a l l o c a t i o n ) it is o f t e n n o t p o s s i b l e to collect g o o d data. C h a r g e d particles m a y hit detectors creating s p u r i o u s s i g n a l ( S e c t i o n 4.2),  o r as i n the c a s e o f the H u b b l e S p a c e T e l e s c o p e ( H S T ) , t h e satellite m a y b e  p o w e r e d d o w n i n o r d e r to r e d u c e l o n g t e r m d a m a g e . F o r space m i s s i o n s s u c h as M O S T  ( S e c t i o n 1.3),  asteroseismology  o n e great a d v a n t a g e o f b e i n g i n orbit is the ability  to o b s e r v e a target f o r a n e x t e n d e d p e r i o d o f time. T h u s , the loss o f data t h r o u g h the S A A c a n b e detrimental to m e e t i n g s c i e n c e  goals.  R e g u l a r passes t h r o u g h the radiation belts also cause c u m u l a t i v e d a m a g e i n sensitive microelectronics. G r a d u a l l y , c h a r g e d particles c a n destroy the p h y s i c a l p r o p e r t i e s o f s p e c i f i c d e v i c e s . I n particular, s i l i c o n lattices are b r o k e n d o w n b y c h a r g e d p a r t i c l e s . C h a r g e c o u p l e d d e v i c e s ( C C D s ; S e c t i o n 1.4) a r e o n e t y p e o f d e v i c e u t i l i s e d r e g u l a r l y b y a s t r o n o m e r s a n d s p a c e a s t r o n o m e r s a l i k e a n d are s u s c e p t i b l e to this e f f e c t space weather. C h a r g e m a y also a c c u m u l a t e w i t h i n circuits o v e r time. If the c u m u l a t i v e c h a r g e b u i l d u p is s u f f i c i e n t l y h i g h to c r e a t e a d i s c h a r g e , c r i t i c a l f a i l u r e is a p o s s i b i l i t y d e p e n d i n g o n the d e s i g n o f the satellite.  of  3  Space astronomers parts s p e c i f i c to telescopes.  m u s t also b e a w a r e o f h o w the orbital e n v i r o n m e n t effects Coatings used o n optical components c o u l d potentially  interact w i t h a t o m i c o x y g e n to c a u s e b r o w n i n g . In order to mitigate the d a m a g e c a u s e d b y s p a c e w e a t h e r a n d a v o i d critical f a i l u r e s , it i s e s s e n t i a l f o r a t h o r o u g h r a d i a t i o n a n a l y s i s t o b e p e r f o r m e d o n a s p a c e c r a f t . A  satellite s h o u l d a l w a y s use r a d i a t i o n h a r d e n e d parts, o n - b o a r d r e d u n d a n c y i n c r i t i c a l  operating devices, a n d regular planned ground communications. Incorporation o f a space 'weather forecast' into operating procedure a n d data reduction guidelines w i l l h e l p ensure all scientific goals w i l l be met. A g o o d k n o w l e d g e o f the a m b i e n t radiation e n v i r o n m e n t o f the satellite s h o u l d also p r o v i d e a m i n i m u m m i s s i o n l i f e t i m e estimate a n d b e u s e d i n d e s i g n i n g the satellite to m a k e sure there is s u f f i c i e n t o n - b o a r d s h i e l d i n g .  1.2 Space Weather and M O S T The  a i m o f this thesis is to p r o v i d e a c o m p l e t e r a d i a t i o n a n a l y s i s f o r the  ( M c r o v a r i a b i l i t y a n d O s c i l l a t i o n s o f Stars) microsatellite i n o r d e r to assist the  M O S T M O S T  instrument t e a m w i t h c r u c i a l p l a n n i n g decisions, essentially p r o v i d i n g the 'weather forecast' f o r the m i s s i o n . A b r i e f i n t r o d u c t i o n to the M O S T m i s s i o n a n d the M O S T C C D i s f o u n d i n S e c t i o n s 1.3 a n d 1.4  respectively.  C h a p t e r 2 is a p r i m e r o n the p h y s i c a l p r o c e s s e s that trap particles i n the magnetosphere.  Earth's  C h a p t e r 3 describes the a p p r o a c h to m o d e l i n g the r a d i a t i o n e n v i r o n m e n t  e m p l o y e d i n this study. I n C h a p t e r 4, the b a s e l i n e o r b i t a l p a r a m e t e r s are e v a l u a t e d f o r the  M O S T  m i c r o s a t e l l i t e m i s s i o n . T h i s analysis has a i d e d the M O S T t e a m to c o n v i n c e the C a n a d i a n S p a c e A g e n c y ( C S A ) to i d e n t i f y a l a u n c h v e h i c l e c a p a b l e o f t a k i n g M O S T to a p o l a r s u n s y n c h r o n o u s orbit. D e v i a t i o n s i n L o c a l T i m e o f A s c e n d i n g N o d e ( L T A N ) w e r e s h o w n to a d v e r s e l y a f f e c t s c i e n c e o p e r a t i o n s a n d h e n c e , l a u n c h o p p o r t u n i t i e s to t h e s e orbits w e r e r u l e d out. C h a p t e r 5 e m p l o y s the t e c h n i q u e s o u t l i n e d i n C h a p t e r 3 to assess the r a d i a t i o n e n v i r o n m e n t f o r the b a s e l i n e orbit. T h e r a d i a t i o n e n v i r o n m e n t o f the b a s e l i n e orbit w a s u s e d to p r o v i d e a forecast f o r M O S T .  4  T h i s weather forecast has p l a y e d important role i n the d e v e l o p m e n t o f the M O S T microsatellite m i s s i o n . A s this w o r k w a s progressing, so w a s the m e c h a n i c a l d e s i g n o f the telescope. B a s e d o n results p r e s e n t e d here, the d e s i g n w a s  fine-tuned  s u c h that  s u f f i c i e n t s h i e l d i n g is present to protect sensitive spacecraft parts, y e t m i n i m u m o f h e a v y materials are u s e d i n order to reduce m a s s constraints.  amounts  Current operating  procedure guidelines w e r e established i n conjunction w i t h this study i n order to m i n i m i s e i m p a c t o n scientific data d u e to c o s m i c rays (Section 4.2) a n d single event effects ( S E E s , S e c t i o n 6.2). A m i n i m u m m i s s i o n l i f e t i m e w a s e s t a b l i s h e d f o r a w o r s e - c a s e r a d i a t i o n environment a n d f o u n d to be o f sufficiently l o n g duration to meet a l l primary scientific objectives. A s the i m p a c t o f the radiation e n v i r o n m e n t ( a n d other orbital e n v i r o n m e n t related factors) is less t h a n critical thresholds, the forecast f o r the M O S T  microsatellite  looks 'sunny a n d warm'.  1.3 The MOST microsatellite: A Space Seismology Pioneer T h e M O S T s p a c e satellite project is u n i q u e i n C a n a d i a n a s t r o n o m y . F u n d e d b y t h e C a n a d i a n S p a c e A g e n c y ( C S A ) , M O S T i s C a n a d a ' s first m i c r o s a t e l l i t e m i s s i o n ( t h e b u s is r o u g h l y the d i m e n s i o n a n d m a s s o f a suitcase). A p i c t u r e o f t h e satellite itself is s h o w n i n F i g u r e 1.1. T h e d r i v i n g s c i e n c e g o a l b e h i n d M O S T i s t o p r o b e t h e i n t e r n a l s t r u c t u r e a n d c e n t r a l c o m p o s i t i o n o f n e a r b y stars b y m e a s u r i n g b r i g h t n e s s  oscillations  w i t h a m p l i t u d e s as s m a l l as a f e w m i c r o magnitudes to apply the techniques o f asteroseismology. Asteroseismology was born through helioseismology, the study o f the five-minute  oscillations o f the sun  ( D e m a r q u e & G u e n t h e r 1 9 9 9 ) . It h a s allowed astronomers  to p e e r into the sun's  interior a n d c o m p a r e results to the Standard Solar M o d e l (Matthews  1.1 Photo of a mock-up of the MOST microsatellite with coffee mug showing scale.  Figure  1990).  Similar to the w a y geophysicists use pressure w a v e s ( p - w a v e s ) created b y  5 earthquakes to infer the thickness a n d c o m p o s i t i o n o f the Earth's internal layers, asteroseismologists  use s o u n d w a v e s , i n d u c e d b y c o n v e c t i v e turbulence o f the Sun's  surface, to p r o b e t h e sun's interior. A s t e r o s e i s m o l o g i s t s u s e s p h e r i c a l h a r m o n i c s to describe the  nonradial pulsations  c r e a t e d b y s o u n d w a v e s as t h e y r e s o n a t e i n a c o u s t i c  cavities beneath the solar surface. A s the b e h a v i o r o f the s o u n d w a v e s is directly related to the m e d i u m i n w h i c h they travel, the m o d e patterns i m p r i n t e d o n the stellar surface b y t h e s e w a v e s c o n t a i n s i n f o r m a t i o n o n t h e i n t e r n a l s t r u c t u r e a n d c o m p o s i t i o n o f t h e star. T h u s , the tools o f a n asteroseismologist are the e i g e n f r e q u e n c i e s ( a n d to a lesser extent, a m p l i t u d e s ) o f the m o d e patterns ( T a s s o u l 1990). T h e d i f f e r e n c e b e t w e e n a s t e r o s e i s m o l o g y a n d h e l i o s e i s m o l o g y is that o n l y s i m p l e n o n r a d i a l patterns (i.e. l o w d e g r e e (/) a n d h i g h o v e r t o n e ( « ) ) a r e d e t e c t a b l e w h e n o b s e r v i n g t h e i n t e g r a t e d l i g h t f r o m a p o i n t source. T h e s u n is r e s o l v a b l e as a d i s k a n d thus, e v e n h i g h spatial f r e q u e n c y m o d e s w h i c h d o n o t result i n variations o f the total d i s c are detectable. The  challenge i n the observation o f seismic oscillations is the relatively l o w  c h a n g e i n e i t h e r D o p p l e r v e l o c i t y o r o v e r a l l b r i g h t n e s s o f t h e star. T h e D o p p l e r v e l o c i t i e s due to vibration o f the S u n i n integrated light are o n l y a f e w c m / s , a n d the overall brightness fluctuations are o n the o r d e r o f a f e w m i c r o m a g n i t u d e s . C u r r e n t g r o u n d - b a s e d detection thresholds are ~ 3 m / s i n D o p p l e r v e l o c i t y a n d 100 m i c r o m a g n i t u d e s i n brightness fluctuations (photometric p r e c i s i o n is l i m i t e d b y noise d u e to atmospheric s c i n t i l l a t i o n ) . R e c e n t reports o f o s c i l l a t i o n s i n a l p h a U r s a e M a j o r i s b y B u s a z i et a l . ( 1 9 9 9 ) , u s i n g t h e starsensor c a m e r a o n b o a r d the f a i l e d i n f r a r e d satellite W I R E , s h o w that it i s p o s s i b l e t o d e t e c t l o w a m p l i t u d e stellar v a r i a b i l i t y f r o m s p a c e . H o w e v e r , t h e variations correspond to periods o f days a n dhours, a n d amplitudes o f hundreds o f m i c r o m a g n i t u d e s , w e l l above the r e g i m e to b e explored b y M O S T . The M O S T  d e s i g n is o p t i m i z e d to detect o s c i l l a t i o n s o f a m p l i t u d e s o f a f e w  m i c r o m a g i n as f e w as t e n d a y s o f m o n i t o r i n g a star b r i g h t e r t h a n V ~ 6 (solar  p-modes  decay i n - 1 0 days). In order to detect solar-type oscillations o f 4 p p m amplitude i n a V = 3 . 0 m a g n i t u d e star w i t h 9 9 % c o n f i d e n c e , t h e r m s n o i s e l e v e l m u s t b e b e l o w ppm,  1.08%  equivalent to a signal to noise ( S / N ) value o f 3.7 (Matthews & K u s c h n i g 2000).  T a b l e 1.1 s h o w s t h e d e t e c t i o n l i m i t s b a s e d o n n u m e r i c a l s i m u l a t i o n s p e r f o r m e d b y K u s c h n i g (2000). M a i n t a i n i n g a h i g h duty c y c l e w i l l reduce relative b a c k g r o u n d noise  6  Target Star Window TSYV  Figure 1.2  Fabry microlens pupil image as projected on the C C D  (after Matthews & K u s c h n i g ,  2000b)  c o n t r i b u t i o n s t o a l l o w M O S T t o r e s o l v e fine s t r u c t u r e i n t h e s t e l l a r e i g e n f r e q u e n c y spectrum, a particularly sensitive tool for measuring core H e fraction. T h u s , w e will be able to date  individual  m a i n s e q u e n c e stars. A s t e r o s e i s m o l o g y o f m e t a l p o o r s u b d w a r f s ,  b e l i e v e d to b e the oldest objects i n the g a l a x y d u e to their p r i m i t i v e c o m p o s i t i o n , w i l l a l l o w us to p l a c e a m e a n i n g f u l l o w e r l i m i t o n the age o f the M i l k y W a y a n d the U n i v e r s e . T h e M O S T t e a m has a d o p t e d a s i m p l e d e s i g n f o r the space p h o t o m e t e r . A  15-cm  M a k s u t o v t e l e s c o p e f e e d s a C h a r g e C o u p l e d D e v i c e ( C C D ) c a m e r a w i t h a set o f F a b r y m i c r o l e n s e s p o s i t i o n e d a b o v e the detector f o c a l plane. T h e starbeam is d i r e c t e d onto o n e o f these lenses, w h i c h projects a n i m a g e o f the telescope p u p i l onto the C C D , c o v e r i n g about 2 0 0 0 p i x e l s i n total. A s the starbeam m o v e s o v e r the lens d u e to the t r a c k i n g errors o f the attitude c o n t r o l s y s t e m ( A C S ) , the p u p i l i m a g e r e m a i n s ( F i g u r e 1.2).  fixed  o n the s a m e pixels  T h i s m i n i m i s e s the M O S T instrumental sensitivity to i m a g e w a n d e r a n d  C C D f l a t f i e l d variations. A s c h e m a t i c o f the F o c a l p l a n e w i t h s c i e n c e C C D a n d A t t i t u d e C o n t r o l S y s t e m ( A C S ) C C D i s s h o w n i n F i g u r e 1.3. T h e m i c r o l e n s a r r a y i s o n l y projected onto a s m a l l area o f the science C C D . S i n c e m a n y o f the M O S T s c i e n c e targets  7  ( A p p e n d i x A ) are v e r y b r i g h t ( > V ~ 2 ) , the l i g h t m u s t b e s p r e a d out o v e r a s u f f i c i e n t l y large n u m b e r o f p i x e l s i n order to r e d u c e the a m o u n t o f saturation i n a g i v e n exposure. H o w e v e r , the light m u s t b e c o n c e n t r a t e d e n o u g h so that w h e n o b s e r v i n g fainter targets ( V < 6 ) , the telescope still operates i n a h i g h s i g n a l r e g i m e . O n e p u p i l i m a g e spans 80 x pixels. M o r e b a c k g r o u n d o n t h e M O S T m i s s i o n a n d its s p e c i f i c s c i e n c e g o a l s c a n b e f o u n d at t h e M O S T w e b s i t e : h t t p : / / w w w . a s t r o . u b c . c a / M O S T /  Figure 1.3 Schematic of the MOST focal plane showing Science and ACS CCDs. The dotted line circles represent the optical axis of the telescope. The Fabry microlens array focuses a pupil image of the star onto the lowerrighthand corner of the science CCD.  80  8  1.4 Charge-Coupled Devices (CCDs) A l t h o u g h t h e y w e r e first d e v e l o p e d as m e m o r y d e v i c e s , C C D s h a v e r e p l a c e d photographic f i l m i n virtually every optical a n d near infra-red astronomical i m a g i n g c a m e r a , s i n c e the s i l i c o n lattice that t h e y are m a d e o f is n a t u r a l l y l i g h t s e n s i t i v e t h r o u g h the p h o t o e l e c t r i c effect. A s t r o n o m e r s c a n m a n i p u l a t e the d i g i t a l i m a g e s w h i c h are p r o d u c e d b y C C D s a n d subtract o f f n o i s e sources s u c h as the b a c k g r o u n d f r o m the sky. T h e f u n d a m e n t a l b u i l d i n g b l o c k o f a C C D is a m e t a l - i n s u l a t o r - s e m i c o n d u c t o r ( M I S ) capacitor. T h e c a p a c i t o r c o l l e c t s a n d stores c h a r g e packets, a n d t h e n transfers  the  c h a r g e p a c k e t to another capacitor i n series w h e n the v o l t a g e g a t i n g the c h a r g e is c h a n g e d . H e n c e , the C C D is d u b b e d a  charge coupled  d e v i c e . E a c h M I S c a p a c i t o r is  a r r a n g e d i n a string or r o w c a l l e d a serial-shift register, d o w n w h i c h c h a r g e packets shift. In a 2 - D i m a g i n g C C D , the serial shift registers are a r r a n g e d r o w b y r o w to f o r m a 2 - D p l a t e o f c a p a c i t o r s , e a c h i n d i v i d u a l l y s e n s i t i v e t o l i g h t f a l l i n g o n it t h r o u g h the p h o t o e l e c t r i c effect. T h e i m a g e is p r o j e c t e d onto this 2 - D plate, a n d electrons  are  g e n e r a t e d i n e a c h c a p a c i t o r i n p r o p o r t i o n t o t h e i n t e n s i t y o f t h e l i g h t f a l l i n g o n it. T h e n the v o l t a g e - g a t e d c h a n n e l s are m a n i p u l a t e d s u c h that a s i n g l e c h a r g e p a c k e t f r o m e a c h serial shift register is transferred d o w n the r o w to a p e r p e n d i c u l a r serial shift register ( F i g u r e 1.4).  T h i s p e r p e n d i c u l a r shift register collects a n u m b e r o f charge packets e q u a l  to the n u m b e r o f r o w s o f serial shift registers i n the 2 - D d e v i c e a n d transfers t h e m o n e b y o n e to a r e c o r d i n g d e v i c e o r o u t p u t a m p l i f i e r . O n c e the p e r p e n d i c u l a r s h i f t register is ' e m p t y ' , the net c h a r g e p a c k e t i n e a c h o f the p a r a l l e l s h i f t registers is r e a d out. C h a r g e s are g e n e r a t e d t h r o u g h the p h o t o e l e c t r i c effect, but m u s t b e stored b e f o r e they are c o l l e c t e d . I n the standard o p e r a t i o n o f a C C D , a p o s i t i v e v o l t a g e is a p p l i e d across the gate o f the M I S capacitor, less t h a n the F e r m i potential (threshold potential a b o v e w h i c h e l e c t r o n s c a n start m o v i n g ) n e e d e d t o attract a s u b s t a n t i a l n u m b e r o f electrons. E l e c t r o n - h o l e pairs are g e n e r a t e d a n d p u s h e d a w a y b y the b u i l d u p o f current i n the c a p a c i t o r , t o w a r d s the e d g e s o f the c a p a c i t o r o r c o l u m n i s o l a t i o n r e g i o n s . T h i s l e a v e s a region d e v o i d o f electron-hole pairs, or a region o f  depletion.  A s electrons  are  generated they c o l l e c t i n the d e p l e t i o n r e g i o n . T o trap electrons i n the center o f the  9  C h a r g e packets are r e a d o u t to a n output a m p l i f i e r .  Figure 1.4 Schematic of CCD function. Buckets of water are representative of capacitors collecting charge. The perpendicular serial shift register is responsible for reading out the rest of the 2D array to the output amplifier in order to record the image. (After Hardy, 1997)  capacitor, or p i x e l , the capacitor has three p o l y s i l i c o n gates, or electrodes  supplying  v o l t a g e t h r o u g h t h e c a p a c i t o r , a s i l l u s t r a t e d b y F i g u r e 1.5. T h e m i d d l e e l e c t r o d e i s b i a s e d s u c h that the potential w e l l created u n d e r n e a t h the surface has a m i n i m u m i n the central r e g i o n o f the p i x e l , w h e r e the e l e c t r o n s gather. C h a n n e l stops o n the sides o f the p i x e l a n d t h e c o l l e c t i o n o f e l e c t r o n h o l e p a i r s at t h e p e r i m e t e r o f t h e C C D f u r t h e r a c t s a s b a r r i e r s , t r a p p i n g the electrons i n the center o f the p i x e l . F o r a m o r e d e t a i l e d history a n d d e s c r i p t i o n o f C C D s , see H a r d y (1997).  10  T h e p r e c i s i o n o f C C D s m a k e s t h e m a n ideal detector for the  M O S T  microsatellite. Tests i n d e m o n s t r a t i o n o f the f u n c t i o n a l i t y o f the K e p l e r m i s s i o n b y J e n k i n s et a l . ( 1 9 9 6 ) s h o w e d that a b a c k - i l l u m i n a t e d R e t i c o n C C D w a s c a p a b l e o f d e t e c t i n g p l a n e t a r y transits, a s i g n a l that is o n l y a b o u t 8 0 p p m o f the target stars b r i g h t n e s s ( o n c e c a l i b r a t e d f o r the effects o f m o t i o n ) . I n fact, t h e tests s h o w e d that t h e C C D w a s shot noise l i m i t e d a n d capable o f detecting a s i g n a l at a l e v e l o f 3 p p m . T h e M O S T C C D i s m a n u f a c t u r e d b y M a r c o n i , a n d is s l i g h t l y d i f f e r e n t f r o m t h e R e t i c o n C C D i n d i m e n s i o n a n d sensitivity. H o w e v e r , n u m e r i c a l simulations o fthe M O S T  photometer  b y K u s c h n i g s h o w that stellar o s c i l l a t i o n signals w i l l b e detected ( M a t t h e w s & K u s c h n i g 2 0 0 0 a ) . I n o b s e r v a t i o n s o f a 4 t h m a g n i t u d e star, t h e n o i s e l e v e l o f ~ 1 p p m w i l l b e d o m i n a n t l y d u e t o p h o t o n s h o t n o i s e ( > 7 0 % ) , a r e d u c e d d u t y c y c l e ( > 7 % , S e c t i o n 4.3), a n d stellar g r a n u l a t i o n n o i s e ( - 5 % ) . 99%  A solar oscillation spectrum w i l l b e detected w i t h  c o n f i d e n c e at 4.1 p p m . T a b l e 1 s h o w s t h e results o f t h e n u m e r i c a l s i m u l a t i o n s f o r  stars o f v a r y i n g m a g n i t u d e .  Pixel outline  Gates  Channel Stop Figure 1.5 Top view of CCD pixel, showing three gates  11  T a r g e t star  exposure  d a t a rate  noise level  signal detection  magnitude ( V )  t i m e (s)  #/min  (ppm)  (ppm)  0.4  1  10  0.72  2.6  10  1.2  2  10  0.72  2.6  10  3  12  5  1.02  3.7  10  4  30  2  1.14  4.1  20  6  60  1  1.92  6.9  40  limit  (99%)  time base (days)  Table 1.1 Noise level and detection limits at a 99% confidence level based on numerical simulations of the MOST microsatellite (after Matthews & Kuschnig 2000a).  1.4.1 The M O S T C C D M O S T will use t w o identical C C D s  i n the f o c a l plane; the devices are c u s t o m  p a c k a g e d v e r s i o n o f the 4 7 - 2 0 type b u i l t b y M a r c o n i ( f o r m e r l y E E V L t d . ) . C C D s p e c i f i c a t i o n s o f t h e s c i e n c e g r a d e M O S T C C D s a r e l i s t e d i n T a b l e 1.2 ( F u r t h e r i n f o r m a t i o n o n the o f f - t h e - s h e l f m o d e l o f C C D 4 7 - 2 0 is i n A p p e n d i x F ) .  N o i s e at 1 5 0 K H z M e a n D a r k S i g n a l at - 3 0 ° C Peak  Signal  6 . 7 r m s e" 16.3  e'/pixel/minute  119ke7pixel  Serial C T E  0.999996 - 1.000002  Parallel C T E  0.999998 - 0.999997  Quantum Efficiency at 4 0 0 n m  44.8%  at 5 0 0 n m  83.0%  at 6 5 0 n m  90.2%  Pixels Pixel Size P e a k charge storage  1024 x 1024 13 x 13 u r n 120,000  e/pixel  Table 1.2 Description of CCD47-20 and resultsfromtesting of the MOST science grade CCD.  12  Gate  N-type silicon  Oxide layer  Channel stop  Depletion region  P-type silicon  Figure 1.6 Schematic of typical buried channel CCD, in inverted mode. In an inverted mode, electron hole pairs accumulate at the silicon oxide interface layer.  C C D 4 7 - 2 0 is a 1024  x 1024,  b a c k i l l u m i n a t e d d e v i c e , w i t h p i x e l s 13 u m w i d e .  T h e C C D structure is b a s e d o n a p - t y p e , e p i t a x i a l l a y e r o f s i l i c o n a b o u t 10-20 thick, c o v e r e d b y a n i n s u l a t i n g layer o f s i l i c o n d i o x i d e about 1000  A n g s t r o m s thick. T h i s  structure is s a n d w i c h e d b e t w e e n the plates o f a M I S ( M e t a l Insulated c a p a c i t o r ( F i g u r e 1.6).  microns  Semiconductor)  T h e C C D 4 7 - 2 0 d e v i c e is b a c k - i l l u m i n a t e d i n o r d e r to  increase  q u a n t u m e f f i c i e n c y ( Q E ) , a m e a s u r e o f t h e r a t e at w h i c h a n e l e c t r o n i s p r o d u c e d b y i n c o m i n g p h o t o n . T h a t is, i n order to prevent p h o t o n s f r o m b e i n g s t o p p e d b y the i n s u l a t i n g o x i d e l a y e r a n d not p e n e t r a t i n g into the p - s i l i c o n w h e r e the p h o t o e l e c t r i c  effect  o c c u r s a n d s i g n a l is g e n e r a t e d , the C C D is b a s i c a l l y f l i p p e d o v e r a n d the p - s i l i c o n substrate is e t c h e d o r t h i n n e d . T h e p - s i l i c o n is t h e n d i r e c t l y i l l u m i n a t e d so that p h o t o n s generate electrons to b e i m m e d i a t e l y c o l l e c t e d . T h u s , b a c k - i l l u m i n a t e d d e v i c e s generate a greater s i g n a l (i.e. h i g h e r Q E ) . C C D 4 7 - 2 0 is o p e r a t e d i n a n i n v e r t e d m o d e ( I M O ) a n d u t i l i z e s M P P ( M u l t i - P i n P h a s e d ) t e c h n o l o g y i n o r d e r to reduce d a r k current. In a n i n v e r t e d m o d e , the s i l i c o n - t o s i l i c o n - d i o x i d e i n t e r f a c e is h e l d i n i n v e r s i o n (the p o t e n t i a l i n the gate h i g h a n d e l e c t r o n h o l e pairs a c c u m u l a t e ) . I f the v o l t a g e a c r o s s the e l e c t r o d e is less t h a n a b o u t -6 V , the b i a s o n the e l e c t r o d e is s u f f i c i e n t l y n e g a t i v e as to attract e l e c t r o n - h o l e p a i r s to t h e o x i d e i n t e r f a c e s u r f a c e . T h e n the s u r f a c e is s a i d to b e e l e c t r o n h o l e pairs is c a l l e d the  inversion layer.  inverted,  a n d the l a y e r c o n t a i n i n g the  A s the h o l e s f l o o d this layer, they fill the  i n t e r f a c e states, b l o c k i n g e l e c t r o n s f r o m the i n t e r f a c e r e g i o n a n d t r a p p i n g t h e m i n the b u r i e d channels. T h i s also serves to r e d u c e the p a t h w a y f o r electrons e x c i t e d t h e r m a l l y  13  f r o m m o v i n g f r o m the v a l e n c e b a n d to the c o n d u c t i o n b a n d , r e d u c i n g or e l i m i n a t i n g d a r k current ( S e c t i o n 6.1.1).  H o w e v e r , i f the entire p i x e l is o p e r a t e d i n i n v e r s i o n , t h e n the  electrons are n o l o n g e r c o n f i n e d to their p i x e l . T o o v e r c o m e this, a n i m p l a n t is p l a c e d b e l o w o n e o f the electrodes. T h e i m p l a n t alters the p o t e n t i a l w e l l i n the d e v i c e s u c h that electrons c a n not penetrate t h r o u g h the r e g i o n u n d e r n e a t h that electrode. T h u s , a p o t e n t i a l b a r r i e r i s set u p b y t h e i m p l a n t a l o n g o n e s i d e o f t h e p i x e l , t r a p p i n g t h e c h a r g e i n s i d e . W i t h this d e v i c e architecture, d a r k current is s i g n i f i c a n t l y s u p p r e s s e d , a n d c h a r g e  packets  are e f f e c t i v e l y stored i n a single p i x e l for a s p e c i f i e d integration time. The  s i l i c o n - s i l i c o n d i o x i d e i n t e r f a c e is a p o o r p l a c e t o store a n d / o r t r a n s f e r c h a r g e  b e c a u s e o f a h i g h d e n s i t y o f t r a p p i n g states, o r p o t e n t i a l w e l l s w h i c h t e n d to h o u s e e l e c t r o n s o r e l e c t r o n - h o l e p a i r s f o r e x t e n d e d p e r i o d s o f t i m e . T h e t r a p p i n g states s i m p l y arise f r o m a d i s r u p t i o n i n the s i l i c o n lattice structure. T h u s , C C D s u s u a l l y store a n d transfer c h a r g e w i t h i n the p-type s i l i c o n layer. In o r d e r to f o r c e the electrons to d e p t h i n the structure, the p surface is c o a t e d w i t h a s i l i c o n lattice r i c h i n n - t y p e i m p u r i t i e s , o r a n n - t y p e s i l i c o n l a y e r . T h e n - t y p e s i l i c o n i s d o p e d s u c h t h a t it i s p o s i t i v e l y b i a s e d w i t h respect to the p - t y p e layer. H e n c e , electrons are c o l l e c t e d a n d transferred i n the p - t y p e layer, r e m o v e d f r o m the s i l i c o n - s i l i c o n d i o x i d e interface. T h u s , this t y p e o f d e v i c e is called a  buried channel device. T h e C C D is the m o s t sensitive c o m p o n e n t o f the M O S T telescope to the orbital  environment.  14  Chapter 2: The Theory behind Space Weather 2.1 History T h e a u r o r a e b o r e a l i s g a v e s c i e n t i s t s t h e first i m p o r t a n t c l u e s a b o u t c h a r g e d particle m o t i o n i n the Earth's magnetic  field.  I n f a c t , " c o s m i c " r a d i a t i o n w a s first d e t e c t e d  b y B i r k e l a n d i n 1895 i na v a c u u m c h a m b e r experiment d e s i g n e d to study the aurorae borealis ( V a n A l l e n 1983).  H o w e v e r , it w a s a n o t h e r 10 y e a r s b e f o r e it w a s r e c o g n i s e d  that t h e s o u r c e o f i o n i s i n g e n e r g y w a s extraterrestrial, w h e n it w a s o b s e r v e d that t h e a m o u n t o f i o n i z a t i o n i n the c h a m b e r s rose w i t h increasing altitude ( c f . K l e c k e r 1996). B u i l d i n g o n B i r k e l a n d ' s w o r k , a n d m o t i v a t e d t o s h o w that t h e aurorae are generated b y c h a r g e d electrons a n d ions trapped i n the geomagnetosphere,  St0rmer  explored thetheory o f charged particle interactions with a dipole magnetic  field.  s h o w e d that there are t w o d y n a m i c a l regions i n a d i p o l e m a g n e t i c  field,  H e  o n e that is  u n b o u n d e d a n d accepts c h a r g e d particles f r o m infinity, a n d another that is b o u n d e d a n d t r a p s c h a r g e d p a r t i c l e s i n d e f i n i t e l y , a radiation belt ( F i g u r e 2 . 1 ) . T h e t w o r e g i o n s h a v e n o o v e r l a p i n t h e i d e a l St0rmerian c a s e ( V a n A l l e n 1 9 8 3 ) . A l t h o u g h S t o r m e r w a s  Figure 2.1 Meridian projection of a trapped charged particle (after Van Allen 1983).  15  unsuccessful i n proving that the aurorae were indeed caused by trapped electrons or ions, these advances laid the theoretical framework for magnetospheric particle motion. In the 1930's a group o f researchers, including Arthur Compton, Robert M i l l i k a n , W i l l i a m Pickering, W i l l m o t Hess, and others, collected ionisation chamber and Geiger counter measurements at various altitudes using balloon-borne instruments. The evidence showed that the radiation emanated from the Sun and had a particulate nature. In 1936 Hess was given the N o b e l Prize for his discovery of 'galactic cosmic rays', which we now know to be particles as well. Although the balloon-borne measurements showed increasing cosmic ray radiation up to 30 k m , it was unclear how to extrapolate the results to even higher altitudes. It was this problem which inspired an early U S Rocketry program to investigate high altitude phenomenon, paving the way for the first American artificial satellites to investigate geophysical parameters o f the earth. The high-altitude measurements o f very early satellites such as Explorer I and III led to the discovery o f the " V a n A l l e n " radiation belts of the Earth, much as Stermer had predicted (Van A l l e n 1959). Explorer  Figure 2.2 Artists conception of the Van Allen radiation belts. Inner and outer radiation belts are both shown.  discovered two radiation belts, and inner and an outer belt, separated by a slot region (Figure 2.2).  2.2 Charged Particle Motion in a Magnetic Field The fundamental motion o f a charged particle i n a magnetic field is described by the Lorentz equation:  F = ^- = q(vxB + E) dt  (2.1)  16 where F is the force on a charged particle due to a magnetic field, p is the momentum of the particle, q is the charge on the particle, v is the velocity of the particle, B is the magnetic field strength, and E is the electric field strength. The momentum p of a charged particle given by:  p = mv + qA  ^'  1 T)  where A is the vector potential of the magnetic field. For temporally uniform magnetic fields with simple geometry, the solution to equation (2.1) is easily integratable. However, for the magnetic field of the Earth, direct integration is not possible. Instead, the solution must be restricted to regions of space which have approximately uniform and simple magneticfieldswhere a direct solution is feasable. Models of the radiation environment are usually semi-empirical (i.e. they utilise a combination of theoretical interpretation with experimental data to make predictions). Three basic motions describe the trajectory of a trapped charged particle (Figure 2.3). First, in the absence of electricfields,it is trivial to show the parallel velocity (i.e. velocity along magnetic field lines) is constant, and the magnitude of the perpendicular velocity is constant but with changing direction. Thus, the particle sweeps out a helical pathway around magnetic field lines with gyroradius p (radius of the circular component of motion) defined by equating the centripetal force to the magnetic force:  _ mv ~Bq  (2.3)  17  S e c o n d , the particle w i l l drift a l o n g m a g n e t i c higher magnetic  field  field  l i n e u n t i l it r e a c h e s a n a r e a o f  i n t e n s i t y w h e r e it is m i r r o r e d o r ' b o u n c e d ' i n the o p p o s i t e d i r e c t i o n .  A n d third, i n h o m o g e n e i t i e s i n the Earth's m a g n e t i c  field  cause a s l o w w e s t w a r d drift o f  p r o t o n s , a n d e a s t w a r d d r i f t o f e l e c t r o n s ( d u e to the o p p o s i t e c h a r g e o n e a c h p a r t i c l e , the forces d u e to the i n h o m o g e n e i t i e s are i n o p p o s i n g directions). T h e s e three m o t i o n s , discussed b e l o w in terms o f  adiabatic invariants,  c o n f i n e t r a p p e d particles to drift shells.  2.2.1 Adiabatic Invariants In o r d e r to m o d e l c h a r g e d particle m o t i o n a r o u n d the E a r t h , three p a r a m e t e r s are c a l c u l a t e d w h i c h q u a n t i f y the three different types o f m o t i o n . In a n y m e c h a n i c a l  system  w i t h p e r i o d i c m o t i o n w h e r e the c h a n g e s i n the f o r c e s a l o n g the paths o f m o t i o n are s l o w , it is p o s s i b l e to c a l c u l a t e v a l u e s w h i c h r e m a i n c o n s t a n t o v e r t h e p a t h w h e n i n t e g r a t e d o v e r c h o s e n p e r i o d i c o r b i t s . T h e s e are k n o w n as  adiabatic invariants.  T h e three adiabatic  i n v a r i a n t s are f o u n d b y integrating o v e r o n e g y r a t i o n orbit, o n e b o u n c e p e r i o d , a n d o n e periodic trajectory respectively.  M i r r o r Point  Figure 2.3 Schematic of charged particle motion in the geomagnetic field depicting the three motions of gyration, bouncing, and drift. (After Hess 1968)  T h e first a d i a b a t i c i n v a r i a n t i s g i v e n b y t h e f o l l o w i n g s u r f a c e i n t e g r a l o v e r t h e g y r o p e r i o d a n d a l o n g the h e l i c a l particle trajectory:  18  J = §(p + qA)dl x  _ Tip  2  (2.4)  ~~qB 2mB  (2.5)  p± i s t h e p e r p e n d i c u l a r c o m p o n e n t o f t h e m o m e n t u m v e c t o r . J j , o r fi,  is c a l l e d the  m a g n e t i c m o m e n t , a n d determines the b o u n c e m o t i o n o f trapped particles i n the m o d e l s . T h e m a g n e t i c f o r c e o n a c h a r g e d p a r t i c l e is p e r p e n d i c u l a r t o the f i e l d l i n e d i r e c t i o n . T h u s , the m a g n e t i c force i n a r e g i o n o f h i g h e r m a g n e t i c  field  strength serves to  impart  m o m e n t u m to the particle p e r p e n d i c u l a r to the  field  (i.e., t o p ± ) . T h e r e f o r e , the  p e r p e n d i c u l a r m o m e n t u m s q a u r e d to m a g n e t i c  field  ratio is constant. H o w e v e r , i n a  quiescent  field  (a  field  particle's  that is t e m p o r a l l y stable) total m o m e n t u m is still c o n s e r v e d , so the  p a r a l l e l m o m e n t u m o f the particle d r o p s to zero. T h e p o i n t w h e r e the p a r a l l e l m o m e n t u m is z e r o is d e f i n e d as the m i r r o r p o i n t ( B ) , b e c a u s e the p a r t i c l e is t h e n r e f l e c t e d o u t o f the m  r e g i o n o f h i g h e r m a g n e t i c i n t e n s i t y a n d ' m i r r o r s ' its m o t i o n to the o t h e r m a g n e t i c p o l e .  B  f o r a p a r t i c l e at a n y p o i n t a l o n g its t r a j e c t o r y c a n b e f o u n d b y c o n s i d e r i n g t h e p i t c h (a)  of  the  m  particle.  A s the particle m o v e s to h i g h e r m a g n e t i c intensity, the p i t c h o f the particle w i l l i n c r e a s e to 9 0 ° . T h u s , B  m  c a n b e f o u n d i f the p i t c h a n d m a g n e t i c  field  is k n o w n f o r a n y o t h e r p o i n t  a l o n g t h e p a r t i c l e ' s p a t h , a n d b y u s i n g t h e first a d i a b a t i c i n v a r i a n t a s f o l l o w s :  2  p,  2  p  • 2/  sin  ± ± =  B  \  (a) —  B  oc J,  (2.7)  19  T h e first adiabatic invariant also relates to the gyroradius. Substituting e q u a t i o n (2.2) i n (2.5) yields:  P =  (2.9)  T h u s , stronger magnetic fields trap particles w i t h a higher perpendicular m o m e n t u m a n d smaller  gyroradius. T h e first a d i a b a t i c i n v a r i a n t i n t r o d u c e s a s e c o n d p e r i o d i c m o t i o n o f t h e  particle,  t h e b o u n c e p e r i o d ( t i m e it t a k e s f o r t h e p a r t i c l e t o g o f r o m a m i r r o r p o i n t i n t h e n o r t h t o a m i r r o n p o i n t i n t h e s o u t h ) . B y i n t e g r a t i n g o v e r t h e b o u n c e p e r i o d a n d a l o n g ds ( t h e s u r f a c e d e f i n e d b y t h e field l i n e ) t h e s e c o n d a d i a b a t i c i n v a r i a n t i s f o u n d . It i s e q u i v a l e n t t o t h e i n t e g r a l o f t h e p a r a l l e l m o m e n t u m o v e r a field l i n e b e t w e e n t h e t w o m i r r o r p o i n t s , ± B  m  , and definedby:  (2.10) (2.11)  T h e s e c o n d a d i a b a t i c i n v a r i a n t , a l s o c a l l e d t h e i n t e g r a l i n v a r i a n t (I), d e f i n e s d r i f t s h e l l s i n the assymetric geomagnetic lines o f constant magnetic  field.  field  A s particles m i r r o r b a c k a n d forth, they c a n drift a l o n g  strength, o r drift i n longitude i n the d i r e c t i o n s p e c i f i e d b y  their charge. W h e n magnetic higher magnetic  field  field  strength i n c r e a s e s w i t h t i m e (as i n a g e o m a g n e t i c s t o r m ) ,  strengths increase t h e m o m e n t u m o f the t r a p p e d particles. T h e  m i r r o r points rise to higher elevations i n order to k e e p the integral invariant constant. C o n v e r s e l y , w h e n the magnetic  field  strength d e c r e a s e s , t h e particles w i l l m i r r o r at p o i n t s  c l o s e r t o the E a r t h , a n d m a y b e r e m o v e d f r o m t h e r a d i a t i o n belts i f the m i r r o r p o i n t is l o w enough to include significant  atmosphere.  T h e third adiabatic invariant J?  is f o u n d b y integrating over a third fundamental  p e r i o d o f m o t i o n , t h e t i m e it takes f o r t h e particle to drift a r o u n d the e a r t h i n t h e drift s h e l l s d e f i n e d b y J2. J3 i s g i v e n b y :  20  ^3 = i(p + q4)di = q§B-dS = q<&  (2.12)  w h e r e dl i s t h e t h e p a t h a l o n g t h e d r i f t s h e l l , dS i s a n e l e m e n t o f t h e s u r f a c e e n c l o s e d b y t h e d r i f t p a t h a n d <D i s t h e c o n s t a n t m a g n e t i c f l u x e n c l o s e d b y t h e d r i f t p a t h . A s l o n g a s the g e o m a g n e t o s p h e r e is stable, particles w i l l return to the starting p o i n t o f their drift path. T h i s adiabatic invariant is not c o n s e r v e d d u r i n g m a g n e t i c  storms.  2.2.2 B and L coordinates W i t h s u c h a c o m p l i c a t e d pattern o f m o t i o n a n d so m a n y degrees o f f r e e d o m i n the variables o f the trapped particles (species, energy, pitch, altitude, latitude a n d longitude), it h a s p r o v e n a d v a n t a g e o u s t o p a r a m e t e r i z e t h e s p e c i e s p o s i t i o n . T h e m o s t p o p u l a r s c h e m e is b a s e d o n M c l l w a i n ' s d i p o l e s h e l l p a r a m e t e r L ( M c l l w a i n 1961). L d e s c r i b e s the p o s i t i o n o f the trapped particle i n terms o f the scalar m a g n e t i c field strength ( B ) , a n d i n t e g r a l a d i a b a t i c i n v a r i a n t (I). I f t w o p a r t i c l e s h a v e t h e s a m e B a n d I v a l u e s , t h e y experience the s a m e forces f r o m the m a g n e t i c f i e l d a n d are constrained to the s a m e drift shell a b o u t the earth. H e n c e , L is w r i t t e n as a f u n c t i o n o f B a n d I ( e q u a t i o n 2.13) a n d d e s c r i b e s the shells that particles are c o n f i n e d t o b y l a b e l l i n g e a c h s h e l l w i t h a u n i q u e number.  I B\.  r  B  3  (2.13)  M j  K  T h e f u n c t i o n F is a p p r o x i m a t e d n u m e r i c a l l y f o r the c o m p l e x m a g n e t i c f i e l d o f the E a r t h ( M c l l w a i n 1961), a n d M is the d i p o l e m o m e n t o f the Earth's m a g n e t i c 10  field  ( M = 8.06 x  g a u s s c m ). T h e p o s i t i o n o f a p a r t i c l e i s d e r i v e d e x p l i c i t l y b y k n o w i n g b o t h B a n d L  for the particle.  21  "N^ Collisionless Shock Fronf  Magnetosheath boundary  Figure 2.4 Schematic of the Earth's geomagnetosphere. (After Hess 1968)  2.3 The Geomagnetosphere T h e b o u n d a r y o f the Earth's m a g n e t o s p h e r e ( c a l l e d the  magnetopause)  is f o r m e d  w h e r e the Earth's m a g n e t i c f i e l d m e e t s a n d interacts w i t h the solar w i n d ( F i g u r e 2.4). A s t h e c h a r g e d p a r t i c l e s o f t h e s o l a r p l a s m a b o m b a r d t h e E a r t h ' s m a g n e t i c f i e l d at t h e m a g n e t o p a u s e , 9 9 . 9 % o f the particles are d e f l e c t e d a r o u n d the E a r t h ( B a r t h 1997). T h e l e a d i n g edge o f the Earth's m a g n e t i c  field  is c o m p r e s s e d against the collisionless s h o c k o f  the solar w i n d , a n d the streaming particles s w e e p the m a g n e t i c outwards f r o m the sun, significantly distorting the magnetic  field  field  lines o f the E a r t h  f r o m the s i m p l e d i p o l e  c o n f i g u r a t i o n set b y t h e g e o d y n a m o . T h e outer magnetic  field  a n d transition r e g i o n b e t w e e n the t w o areas has a  c o m p l i c a t e d a n d d y n a m i c structure d u e to external m a g n e t i c  field  interactions with the  s o l a r w i n d . L u c k i l y , s i n c e t h e M O S T m i c r o s a t e l l i t e w i l l b e i n L E O , it i s n o t n e c e s s a r y this w o r k to f a c e the c h a l l e n g e o f c h o o s i n g a m o d e l to represent the external  in  field.  W i t h i n a p p r o x i m a t e l y 5 E a r t h r a d i i , the m a g n e t o s p h e r e is s h i e l d e d f r o m the upsetting effects o f the solar w i n d a n d is m u c h m o r e stable. T h i s i n n e r r e g i o n is dominated b y the magnetic  field  originating f r o m the core d y n a m o within the Earth. T h e  22  current  field  is m o s t s i m p l y d e s c r i b e d b y a d i p o l e w i t h m a g n e t i c m o m e n t offset f r o m the  r o t a t i o n a l a x i s o f t h e E a r t h b y ~ 1 1 ° , a l t h o u g h t h e field i s o n l y q u a s i - d i p o l a r ( w i t h a b o u t 10% o f the  field  energy i n h i g h e r order configurations). F i e l d strengths range f r o m a f e w  n a n o t e s l a s ( n T ) a t h i g h a l t i t u d e s t o 5 0 , 0 0 0 n T at l o w a l t i t u d e s a n d h i g h l a t i t u d e . T h e Earth's inner magnetic field  field  is neither spatially n o r t e m p o r a l l y stable. T h e  s t r e n g t h is d e c r e a s i n g at a n a p p r o x i m a t e rate o f 6 % e v e r y 1 0 0 y e a r s , e q u i v a l e n t t o  2 0 n T r e d u c t i o n i n the m a g n e t i c m o m e n t p e r year ( B a r t h 1997). T h i s is a substantial change, b u t s m a l l c o m p a r e d to the instability o f the outer magnetosphere w h e r e p e r i o d i c g e o m a g n e t i c s t o r m s u p s e t t h e field l i n e s o n a m u c h s h o r t e r t i m e s c a l e . Since the inner magnetic  field  i s n o n - s t a t i c a n d c h a n g e s i n t h e field a r e c u r r e n t l y  u n p r e d i c t a b l e , static m o d e l s a r e e m p l o y e d w i t h u p d a t e s r e l e a s e d e v e r y 5 y e a r s b y a n International A s s o c i a t i o n o f G e o m a g n e t i s m a n dA e r o n o m y ( I A G A ) w o r k i n g g r o u p to r e f l e c t c h a n g i n g c o n d i t i o n s n o t e d b y e x p e r i m e n t a l d a t a (e.g., M a n d e a et a l . 2 0 0 0 ) . T h e standard reference m o d e l s are b a s e d o n a spherical h a r m o n i c e x p a n s i o n o f the geomagnetic potential i n the f o r m :  V = at £ ( a / r ) " [ g » c o s ( m 0 + O +1  ( 21 4 )  H=1TW=0 w h e r e V i s t h e g e o m a g n e t i c p o t e n t i a l , g„  m  a n d h„  m  are m o d e l coefficients, a is the m e a n  r a d i u s o f t h e e a r t h ( 6 3 7 1 . 2 k m ) , r i s t h e r a d i a l d i s t a n c e f r o m t h e c e n t e r o f t h e E a r t h , <j> i s t h e e a s t l o n g i t u d e , 9 i s t h e g e o c e n t r i c c o l a t i t u d e , a n d P„  m  cos(fT) i s t h e a s s o c i a t e d  L e g e n d r e f u n c t i o n o f d e g r e e n a n d o r d e r m. T h e I n t e r n a t i o n a l G e o m a g n e t i c R e f e r e n c e F i e l d ( I G R F ) m o d e l p r o v i d e s a s e t o f c o e f f i c i e n t s g„  m  o n a static m a g n e t i c  field  'definitive' reference  fields  a n d h„  m  f o r experimental data b a s e d  i n a g i v e n e p o c h ( M a n d e a et a l . 2 0 0 0 ) . T h u s , t h e r e n o w a set o f (DRGF45, DRGF50, DRGF55, DRGF60, DRGF65,  DRGF70,  D R G F 7 5 , D R G F 8 0 , D R G F 8 5 ) f o r w h i c h the data is d e f i n i t i v e o n l y i n that n o m o r e c a n be collected because w e c a n not travel b a c k i n time. F i e l d m o d e l s f o r the times b e t w e e n the reference e p o c h s c a n b e linearly interpolated f r o m the existing data. A p p e n d i x B a n d F i g u r e 3.3 s h o w s t h e r e s u l t i n g B v a l u e s f o r t h e M O S T b a s e l i n e orbit.  23  2.3.1 The South Atlantic Anomaly (SAA) T h e offset o f the magnetic dipole a n d the presence o f higher order terms i n  Geographic axis  the s p h e r i c a l h a r m o n i c representation o f the m a g n e t i c f i e l d causes the V a n A l l e n radiation belts to b e a s y m m e t r i c about the E a r t h ( F i g u r e 2.5). T h e belts e x t e n d to m u c h l o w e r altitudes o v e r a large region centered o n the S o u t h P a c i f i c , called the South Atlantic A n o m a l y ( S A A ) . T h e S A A is the m o s t significant feature o f radiation e n v i r o n m e n t i n L E O . T h e S A A is a d i pi n the field strength o f the Earth's m a g n e t i c field over the S o u t h A t l a n t i c O c e a n o f fo f the  Figure 2.5 Schematic slice through the Earth showing the offset in magnetic axis and resulting South Atlantic Anomaly (SAA).  coast o f B r a z i l . T h i s is d u e to the physical offset o f the m a g n e t i c axis o f the d i p o l e m o m e n t o f the Earth's axis  f r o m t h e g e o g r a p h i c a x i s b y 2 8 0 m i l e s , as w e l l as t h e i n c l i n a t i o n o f t h e a x i s b y ~ 1 1 ° T h e m a g n e t i c f i e l d strength i n t h e S A A d r o p s t o b e l o w 0.2 G a u s s at 8 0 0 k m , c r e a t i n g a natural funnel f o r trapped magnetospheric particles. T h e r e h a s b e e n a ( p r i m a r i l y ) n o r t h w e s t w a r d ' d r i f t ' o f t h e S A A ( D y e r et a l . 1 9 9 9 ) . T h e drift is d u e to s e c u l a r decrease i n the d i p o l e t e r m o f the Earth's m a g n e t i c f i e l d  Location  Year  Surface Surface 1336 km 1336 km  1970 1993 1970 1993  Longitude  Latitude of  of Centroid  Centroid  -26.2 -27.4 -18.8 -18.7  -49.9 -54.1 -45.1 -50.1  Table 2.1 Location of centroid of minimum of SAA between 1970 and 1993 (Lauriente etal., 1996)  ( L a u r i e n t e et a l . 1996). T h e drift is n o t a m o t i o n o f the entire m a g n e t i c f i e l d as a w h o l e , but a change i n the location o f the broad irregularly shaped centroid o f m i n u m u m field intensity associated with the S A A , a n d varies w i t h altitude ( T a b l e 2.1). H o w e v e r , the d e f i n i t i v e b o u n d a r i e s o f  24  the S A A are k n o w n to b e d i f f e r e n t t h a n o b s e r v e d i n the past as i n d i c a t e d b y r e c e n t m a p p i n g s o f the S A A b y satellite m i s s i o n s s u c h as t h e H u b b l e S p a c e T e l e s c o p e ( H S T ) and the F a r - Ultraviolet Spectroscopic E x p l o r e r ( F U S E ) (Fullerton, private c o m m u n i c a t i o n 2000).  2.3.2 Geomagnetospheric Shielding T h e g e o m a g n e t o s p h e r e serves as a natural r a d i a t i o n s h i e l d f o r spacecraft i n L E O . T h e degree to w h i c h the magnetosphere w i l l b e able to stop a n i n c o m i n g particle f r o m entering the t r a p p i n g regions o f the m a g n e t i c field w i l l d e p e n d o n b o t h the m o m e n t u m a n d c h a r g e o f the i n c o m i n g p a r t i c l e , a n d its a r r i v a l d i r e c t i o n . T h e d e g r e e o f p e n e t r a t i o n o f  a n y g i v e n p a r t i c l e i s d e s c r i b e d b y t h e magnetic rigidity o f t h e p a r t i c l e r, a n d t h e cutoff rigidity ( o r S t d r m e r r i g i d i t y ) o f t h e m a g n e t i c f i e l d r . s  In a dipole field, the magnetic rigidity ( i ngigavolts, o r G V ) is a property o f the p a r t i c l e ' s e n e r g y E, a t o m i c m a s s A ( i n a m u ) , a n d c h a r g e z:  (2.15)  z M o is e q u a l to 931 M e V . Intuitively, it is easy to see that i f the particle's ratio o f m a s s t o c h a r g e is l o w , t h e n it w i l l b e d e f l e c t e d m o r e easily. E l e c t r o n s h a v e the l o w e s t m a s s to charge ratio, f o l l o w e d b y protons.  T h u s , f o r a g i v e n energy, h e a v y ions w i l l penetrate the  g e o m a g n e t i c f i e l d the furthest, a n d electrons w i l l b e d e f l e c t e d the most. H o w e v e r , i fthe particle h a s s u f f i c i e n t l y h i g h e n e r g y it w i l l still penetrate t h e s h i e l d as h i g h e n e r g y particles have h i g h magnetic rigidity. Stdrmer described  cutoff  rigidity  i n his early w o r k o n the aurorae f o r a s i m p l e  d i p o l e m a g n e t i c f i e l d ( B a r t h , 1 9 9 7 ) . A l t h o u g h t h e c a s e i s o v e r s i m p l i f i e d , it i s a g o o d s t a r t i n g p o i n t . U s i n g g e o m a g n e t i c l a t i t u d e X, z e n i t h a n g l e e, a n d a z i m u t h a l a n g l e f r o m t h e m a g n e t i c n o r t h p o l e <j> t o d e s c r i b e a r r i v a l d i r e c t i o n o f t h e p a r t i c l e , t h e c u t t o f f r i g i d i t y r ( i n s  G V ) is g i v e n b y :  M  cos X  R  (l + ^ l - s i n ^ sin^cos X)  4  3  1  (2.16)  25  w h e r e M i s the magnetic dipole m o m e n t o f the field, a n d R is the distance f r o m the dipole center o f the E a r t h i n E a r t h radii. I f the m a g n e t i c rigidity o f the particle (equation 2.15) is less t h a n the c u t o f f rigidity o f the g e o m a g n e t i c field, t h e n the particle w i l l b e deflected a w a y f r o m t h e E a r t h . A g a i n , i n t u i t i v e l y it's c l e a r t h a t a s t r o n g e r m a g n e t i c f i e l d w i l l deflect m o r e particles. L e s s o b v i o u s l y , equation (2.16) demonstrates the rigidity d e c r e a s e s w i t h i n c r e a s i n g g e o m a g n e t i c l a t i t u d e . It i s f o r t h i s r e a s o n t h a t c h a r g e d p a r t i c l e s f r o m large solar events are better able to penetrate into southern a n d n o r t h e r n g e o g r a p h i c latitudes to cause the aurorae. In reality, the d i p o l e a p p r o x i m a t i o n is n o t sufficient to calculate cutoff rigidities for the Earth's magnetic field, a n dm o r e c o m p l i c a t e d m o d e l s are u s e d .  2.3.3 Geomagnetic Storms T h e geomagnetosphere is non-static, w i t h l o n g t e r m secular variations d u e to the g e o d y n a m o but m u c h larger rapid variations d u e to the solar w i n d . S o l a r - m o d u l a t e d changes i n the Earth's m a g n e t i c field are d u b b e d m a g n e t i c storms. Solar w i n d variations are d u e p r i m a r i l y to solar flares a n d c o r o n a l m a s s (CMEs). C M E s  ejections  are large e r u p t i o n s f r o m t h e c h r o m o s p h e r e o f the s u n that eject u p to 1  b i l l i o n m e t r i c t o n s o f m a t e r i a l at s p e e d s a v e r a g i n g 4 0 0 - 7 0 0 k m / s ( Z i r i n 1 9 8 8 ) , b u t a s h i g h as 2 0 0 0 k m / s ( A l p e r t 2 0 0 0 ) . T h e y s t e m f r o m a n i m b a l a n c e i n m a g n e t o h y d r o s t a t i c equilibrium i n the sun, where magnetic field loops a n darches b e c o m e tangled i n a n i n c r e a s i n g m a g n e t i c f i e l d b a c k g r o u n d . T h e m a g n e t i c structures e x p a n d a n d act like pistons o n the coronal p l a s m a p r o d u c i n g flows a n d shock waves (Stepanova & K o s o v i c h e v 2000).  T h e s h o c k front hits t h e m a g n e t o p a u s e o f the earth a b o u t 2 d a y s later,  d r a g g i n g the m a g n e t i c f i e l d lines o f the E a r t h a n dc o m p r e s s i n g the front e n d o f the m a g n e t o s p h e r e . I n o b s e r v a t i o n s m a d e b y L u i et a l . ( 2 0 0 0 ) , t h e m a g n e t o p a u s e w a s c o m p r e s s e d to w i t h i n geostationary orbits. W h i l e C M E ' s increase the v e l o c i t y o f the solar w i n d , solar flares  increase the  density o f the solar w i n d . Flares are created i n the solar photosphere (interior to the c h r o m o s p h e r e ) d u r i n g m a g n e t i c b r e a k i n g ( w h e n m a g n e t i c field lines o f the s u nare twisted, break open, a n dre-connect i n a lower energy configuration). T h e energy f r o m m a g n e t i c b r e a k i n g increases the energy o f particles i n the solar w i n d . W h i l e flares m a y  26  upset radio c o m m u n i c a t i o n s a n d are a n important m o d u l a t o r to atmospheric drag,  C M E s  h a v e a h i g h e r correlation w i t h large g e o m a g n e t i c disturbances. C M E s a n d flares o c c u r s i m u l t a n e o u s l y d u r i n g the largest solar events. T h e intensity o f the g e o m a g n e t i c storm d e p e n d s o n the orientation a n d strength o f the C M E . T h e r e w e r e o n average 0.9 C M E s p e r d a y d u r i n g the 1974 solar m a x i m u m , a n d 0.74 p e r d a y d u r i n g the subsequent solar m i n i m u m i n 1980, b u t o n l y as m a n y as  -70%  o f these are associated w i t h interplanetary s h o c k fronts ( Z i r i n 1988). T h e actual n u m b e r o f C M E s d e c r e a s e s as the solar c y c l e decreases, b u t the f r a c t i o n o f those c r e a t i n g interplanetary shock fronts reaches a m a x i m u m i m m e d i a t e l y f o l l o w i n g solar m a x i m u m ( L i n d s a y et a l . 1 9 9 5 ) . F o r t u n a t e l y , m o s t o f C M E s a r e d i r e c t e d i n t o e m p t y s p a c e a n d n o t t o w a r d s E a r t h . T h e largest g e o m a g n e t i c storms are f r o m a direct, f a c e - o n i m p a c t o f a C M E ( L u i 2000). Furthermore, since C M E s are m a d e o f charged particles a n d are not e l e c t r i c a l l y n e u t r a l , e a c h C M E h a s a d i f f e r e n t m a g n e t i c o r i e n t a t i o n . I f it is a s o u t h w a r d orientation, then the m a g n e t i c f i e l d o f the C M E is m o r e easily c o u p l e d to the m a g n e t i c f i e l d o f the earth a n d a v e r y large d i s t u r b a n c e results ( B a r t h , 1997). T h u s ,  geomagnetic  storms are very hard to predict. E v e n w i t h early w a r n i n g detections o f C M E s b y orbiting spacecraft, the intensity o f the s t o r m cannot b e k n o w n i n advance. Satellites i n geostationary o r high-altitude orbits are m o s t h e a v i l y i n f l u e n c e d b y g e o m a g n e t i c storms. H o w e v e r , it is d u r i n g s u c h s t o r m s that particles are i n j e c t e d i n t o t h e i n n e r r a d i a t i o n belts. T h u s , g e o m a g n e t i c storms f o r a L E O are associated w i t h a s m a l l increase i n c h a r g e d particle b o m b a r d m e n t d u e to a decrease i n c u t o f f rigidity (i.e., a n increase i n g e o m a g n e t i c t r a n s m i s s i o n f u n c t i o n , section 3.1.3).  2.4 Charged Particle Populations T h i s description o f charged particle m o t i o n has thus far neglected to consider i n a n y detail the o r i g i n a l sources o f the c h a r g e d particles. T h e r e are f o u r p o p u l a t i o n s o f c h a r g e d particles that c a n interact w i t h a spacecraft: (a) r e s i d u a l m a g n e t o s p h e r i c t r a p p e d particles, (b) solar energetic particles ( S E P ) , (c) G a l a c t i c c o s m i c rays ( G C R s ) , a n d (d) a n a n o m a l o u s c o s m i c ray ( A C R ) component. T h e m a i n properties distinguishing the p o p u l a t i o n s are s u m m a r i z e d i n T a b l e 2.2. T h e S u n turns o u t to b e the m o s t i m p o r t a n t factor, b o t h as a s o u r c e a n d as a m o d u l a t o r o f these p o p u l a t i o n s .  27  Properties o f the  (a)  (b) S o l a r E n e r g e t i c  C h a r g e d Particle  Magnetospheric  Particles ( C M E s )  Populations  Particles  Energy Range  0.04  -300 M e V  (c)  Galactic  Cosmic Rays  (d) A n o m a l o u s Cosmic Rays  v a r i e s , l a r g e events n o l i m i t , u p t o 100s u p t o 100 M e V / n  ( p r o t o n s ) ; .04 ~ 7  >430MeV  GeV/n  protons a n d  dominated b y  8 3 % p r o t o n s , 13 %  enriched i n  electrons  protons, coronal  H e ions, 3 %  elements w i t h  abundance o f heavy  electrons, a n d 1%  l a r g e 1st  ions  heavier nuclei  ionisation  M e V (electrons) Composition  potential ( H , N , O, Ne) C h a r g e state o f  N/A  heavy ions  Intermediately  Fully charged  Singly charged  charged heavy ions  heavy ions  heavy ions  Solar C y c l e  Protons increase  Increase n u m b e r o f  Increase d u r i n g  Increase d u r i n g  Modulation  d u r i n g solar m a x ,  events d u r i n g s o l a r  solar m a x  solar m a x  CREME  CREME  electrons decrease d u r i n g solar m a x  max,  increase  number o f C M E s w h i c h cause geomagnetic  storms  i n declining phase Models  AP8/AE8  JPL91,  CREME  Table 2.2 Comparison of trapped particle populations.  2.4.1 Magnetospheric Particles T h e m a g n e t o s p h e r i c particles are c o n s i d e r e d as a separate p o p u l a t i o n because o f their l o n g lifetime. O b s e r v a t i o n s o f a n e w p r o t o n belt f o r m e d i n the w a k e o f a v e r y large solar f l a r e i n M a r c h 1991 s h o w e d that particles w e r e t r a p p e d a n y w h e r e f r o m 8 m o n t h s to 2 y e a r s ( D y e r et a l . 1 9 9 6 ) . T h i s d i s c u s s i o n w i l l c o n c e n t r a t e  o n the d y n a m i c s o f  m a g n e t o s p h e r i c particles d u e to v i o l a t i o n o f adiabatic invariants (section 2.2.1), i.e., h o w particles c a n seep into a n d o u t o f the r a d i a t i o n belts.  28 W i t h o u t a source o f replenishment, trapped charged particles i n the radiation belts w o u l d eventually ionise molecular species i n the upper atmosphere a n d be r e m o v e d f r o m the m a g n e t i c f i e l d o f the earth. C o n v e r s e l y , w i t h o u t the s i n k o f the u p p e r a t m o s p h e r e , trapped particle a b u n d a n c e w o u l d increase continually as c h a r g e d particles f r o m the solar w i n d g r a d u a l l y l e a k into the t r a p p i n g r e g i o n s o f the f i e l d . H e n c e , the static f l u x o f p a r t i c l e s at a g i v e n e n e r g y r e p r e s e n t s a n e q u i l i b r i u m b e t w e e n f o u r c o m p e t i n g p r o c e s s e s : particle loss, infusion, acceleration (an increase i n energy o f the particle), a n d diffusion. T r a p p e d m a g n e t o s p h e r i c particles are i n f u s e d into the r a d i a t i o n belts f r o m the solar w i n d , Galactic c o s m i c radiation, and/or f r o m c o s m i c ray albedo neutron decay ( C R A N D ) ( G a s s e r 1990). I n the inner z o n e , C R A N D turns out to b e the d o m i n a n t source o f trapped particles. A s c o s m i c rays hit the upper atmosphere, high-energy neutrons are p r o d u c e d . T h e neutron subsequently d e c a y s after a half-life o f 630s into a p r o t o n a n d electron, w h i c h r e m a i n i n the trapped i n the radiation belts unless the particle trajectory a n d energy is s u c h that they c a n b e carried out o f the magnetosphere. T r a p p e d particles also diffuse f r o m the outer magnetosphere into the inner trapping regions d u r i n g periods o f magnetic storms. P a r t i c l e a c c e l e r a t i o n is n o t a w e l l - u n d e r s t o o d p h e n o m e n o n . T h e r e a s o n that a c c e l e r a t i o n is c i t e d as a n i m p o r t a n t p r o c e s s i n the d i s t r i b u t i o n o f m a g n e t o s p h e r i c p a r t i c l e s is that a n u n s t a b l e r a d i a t i o n belt i n b e t w e e n t h e i n n e r a n d outer r a d i a t i o n belts w a s o b s e r v e d i n 1991 b y t h e C R R E S satellite ( B e a u j e a n et a l . 1996). W i t h i n this n e w r a d i a t i o n belt, electrons w i t h e n e r g i e s e x c e e d i n g 10 M e V a n d p r o t o n s w i t h energies e x c e e d i n g 50 M e V w e r e detected ( W a l t 1996). S o m e process m u s t b e r e s p o n s i b l e f o r accelerating the trapped particles to higher energies, p r o b a b l y l i n k e d to geomagnetic storms since a large C M E i m p a c t e d the E a r t h just prior to f o r m a t i o n o f the n e w radiation belts. D i f f u s i o n is p r o b a b l y t h e m o s t i m p o r t a n t o f the f o u r c o n t r o l l i n g p r o c e s s e s as it is directly t i e d to the others. T r a p p e d particle d i f f u s i o n m u s t b e cast i n a different f o r m t h a n the standard d i f f u s i o n e q u a t i o n (e.g., gas d i f f u s i n g d o w n a c o l u m n ) b e c a u s e the particles have three n o r m a l m o t i o n s (gyration, b o u n c e , a n d drift). Instead, a F o k k e r - P l a n c k p r e s c r i p t i o n is a d o p t e d .  I n a F o k k e r - P l a n c k d e r i v a t i o n , d i f f u s i o n is d e s c r i b e d i n t e r m s o f  t h e r a t e o f c h a n g e i n c o - o r d i n a t e s o f t h e p a r t i c l e s ( W a l t 1 9 9 4 ) . It i s u s e f u l b e c a u s e t h e  29  c h o i c e o f co-ordinates is arbitrary, C h o i c e o f co-ordinates, i n general, i n v o l v e s the a d i a b a t i c i n v a r i a n t s as they r e d u c e the d i m e n s i o n a l i t y o f the p r o b l e m f r o m s i x d i m e n s i o n s to three. R a d i a l diffusion is particularly important i n t h e i n n e r r a d i a t i o n b e l t s a s it g o v e r n s t h e t r a n s f e r o f particles f r o m the outer z o n e to the inner zone. S i n c e radial d i f f u s i o n describes m o t i o n f r o m o n e drift shell t o a n o t h e r , i t m a k e s s e n s e t h a t it i s r e l a t e d t o f l u c t u a t i o n s i n t h e t h i r d a d i a b a t i c i n v a r i a n t (J3 = q<J>; equation 2.12) f o u n d b y integrating over a drift p e r i o d . I n o r d e r f o r J3 t o b e v i o l a t e d , c h a n g e s i n t h e magnetic field or electric potential fields must occur o v e r t i m e p e r i o d s m u c h m o r e r a p i d t h a n the drift p e r i o d . D r i f t p e r i o d s range f r o m about 1 s e c o n d to 1 d a y ( W a l t 1994), so this type o f d i f f u s i o n o c c u r s i n a variety o f scales. T h e most c o m m o n m e c h a n i s m f o r v i o l a t i n g the t h i r d invariant is a g e o m a g n e t i c  storm  (see s e c t i o n 2.3.2). A s a n i l l u s t r a t i o n o f r a d i a l diffusion, consider a concentric shell o f equatorial t r a p p e d particles i n the E a r t h ' s m a g n e t i c f i e l d as s h o w n i n figure 2.6a. N o w consider a C M E shock front s t r i k i n g that shell o f particles. T h e s h o c k  Figure 2.6 Schematic diagram of radial diffusion in response to a compressed magnetic field. A.) Shell of particles prior to impact with CME. B.) The dotted line represents the position of the old shell and the solid line represents the position after magnetic field compression. C.) Following magnetic relaxation, the particles spread in velocity into a more diffuse shell. (After Walt 1994)  compresses the magnetic  field  front  towards the earth,  m o s t n o t i c e a b l y a l o n g o n the s h o c k front itself. In response to this alteration o f the m a g n e t i c  field,  particles m o v e towards the E a r t h (Figure 2.6b), changing the value o f O , a n d conserving the other t w o a d i a b a t i c i n v a r i a n t s , ju a n d J2. O n c e t h e C M E h a s d i s s i p a t e d , t h e p a r t i c l e s d r i f t a l o n g c o n s t a n t p, J\, a n d <J> a n d g r a d u a l l y f o l l o w t h e r e l a x i n g m a g n e t i c b a c k to their original positions. T h i s causes the  field  30  particles to spread into diffuse bands d e p i c t e d i n 2.6c. A l t h o u g h this scenario is h i g h l y idealized, the m e c h a n i s m f o r transporting trapped particles into the inner trapping region is essentially the s a m e . P i t c h angle d i f f u s i o n also plays a m a j o r role i n the transport o f electrons out o f the r a d i a t i o n b e l t s . It i s c a u s e d b y t h e i n t e r a c t i o n o f e l e c t r o n s w i t h p a r t i c l e s i n t h e E a r t h ' s a t m o s p h e r e , o r b y interactions w i t h e l e c t r o m a g n e t i c w a v e s . T h e latter m e c h a n i s m o f loss is i m p o r t a n t o n l y i n the outer m a g n e t o s p h e r e w h e r e interactions b e t w e e n the magnetosphere a n d the solar w i n d create h i g h - e n e r g y electro-magnetic waves. H o w e v e r , i n the inner magnetosphere, atmospheric particles frequently c o l l i d e w i t h trapped particles. I n d i v i d u a l interactions w i t h electrons d o n o t s i g n i f i c a n t l y alter the p a t h o f the electron, b u t c u m u l a t i v e scattering w i t h a t m o s p h e r i c particles causes a statistical  change  i n the p i t c h angles. T h i s r a n d o m process c a n either s e n d the electrons deeper into the atmosphere w h e r e they are essentially r e m o v e d f r o m the radiation belts, o r to higher altitudes ( W a l t  1994).  Scattering is not applicable to protons o r heavier ions because o f their substantial mass. H o w e v e r , the atmosphere is still the p r i m a r y sink f o r trapped protons. A s h i g h energy protons traverse the atmosphere, inelastic nuclear collisions effectively reduce their energy a n d s l o w t h e m d o w n . A 100 M e V p r o t o n c o o l s to about 100 k e V after  2  2  t r a v e r s i n g 8.6 g m / c m , w h i l e a 1 M e V p r o t o n c o o l s after o n l y " s e e i n g " 0 . 0 0 3 g m / c m  of  o x y g e n . B e l o w 100 k e V , protons are lost i n charge e x c h a n g e reactions w i t h a t o m i c h y d r o g e n ( H e s s 1 9 6 8 ) . T h i s p r o c e s s i s e v e n m o r e e f f i c i e n t at a l o w e r m i r r o r p o i n t o r i n the case o f a t m o s p h e r i c inflation.  2.4.2 Solar Energetic Particles C M E s w e r e d i s c u s s e d i n section 2.3.2 i n the context o f g e o m a g n e t i c  disturbances.  B u t since a C M E is a large mass o f c h a r g e d particles, solar energetic particles ( S E P ) are c o n s i d e r e d as a separate p o p u l a t i o n w i t h i n the r a d i a t i o n e n v i r o n m e n t . U n t i l t h e m i d - 9 0 ' s , S E P s w e r e t h o u g h t to o r i g i n a t e f r o m solar flares, as there is correlation b e t w e e n flare events a n d geomagnetic storms. G o s l i n g (1993) dispels this n o t i o n as t h e 'solar flare' m y t h a n d p o i n t s to C M E s as t h e r e a l h a z a r d i n t h e r a d i a t i o n environment.  31  T h e r e are n o w t w o types o f solar events d e s c r i b e d i n the literature:  impulsive,  gradual  and  n a m e d f o r the duration o f x-ray bursts associated w i t h the events. T h e  i m p u l s i v e events are t y p i c a l l y associated w i t h solar flares a n d are a c c o m p a n i e d b y a n increase i n particle flux. T y p i c a l l y , the i m p u l s i v e events have a n enhancement i n h e a v y i o n s , a n d are d o m i n a t e d b y electrons. T h e i r d u r a t i o n is o n a v e r a g e a f e w h o u r s l o n g (Klecker  1996).  T h e gradual events are strongly associated w i t h C M E s . T h e C M E particle p o p u l a t i o n is v e r y s i m i l a r to that o f the solar c o r o n a l a b u n d a n c e , a n d is m u c h m o r e p r o t o n r i c h t h a n the i m p u l s i v e e v e n t ( K l e c k e r 1996). T h e e v e n t s last s e v e r a l d a y s . S i n c e the C M E events are also associated w i t h g e o m a g n e t i c disturbances, their effects are m o r e severe t h a n the i m p u l s i v e events. C h a r g e d particles i n the ejection c a n diffuse into the i n n e r r a d i a t i o n belts d u r i n g the p e r i o d o f the storm. H e n c e , e v e n spacecraft i n a L E O that are substantially s h i e l d e d b y the g e o m a g n e t o s p h e r e are susceptible to these larger events. L u c k i l y , t h e r e are o n l y a b o u t 10 p e r y e a r d u r i n g s o l a r m a x i m u m ( B a r t h  1997).  2.4.3 Galactic Cosmic Radiation It w a s G a l a c t i c C o s m i c R a d i a t i o n ( G C R ) t h a t H e s s d e t e c t e d i n h i s e a r l y b a l l o o n b o r n e e x p e r i m e n t s . It i s d e f i n i t e l y e x t r a t e r r e s t r i a l  i n n a t u r e (as h e o r i g i n a l l y p r o p o s e d )  a n d is n o w t h o u g h t to e m a n a t e f r o m o u t s i d e the S o l a r S y s t e m , t h o u g h there is still c o n s i d e r a b l e d e b a t e a s t o t h e s o u r c e o f t h e r a d i a t i o n ( C r o n i n et a l . 1 9 9 7 ) . C a s t i n g s u s p i c i o n o n a n i n t e r p l a n e t a r y s o u r c e , G C R e l e m e n t a l a b u n d a n c e p a t t e r n i s e q u a l (to f i r s t o r d e r ) t o t h a t f o u n d i n t h e S o l a r S y s t e m ( T r i b b l e et a l . 1 9 9 9 ) . H o w e v e r , i t h a s i s o t r o p i c arrival directions, a n dthus, p r o b a b l y penetrates t h r o u g h a l l o f interstellar space. Furthermore, the energy spectrum extends to very h i g h energies (>100  GeV/nucleon) and  it is h a r d t o f i n d a s o u r c e t o a c c e l e r a t e p a r t i c l e s t o s u c h h i g h e n e r g i e s w i t h i n t h e S o l a r S y s t e m . I f G C R is i n d e e d g a l a c t i c , it m u s t t r a v e l t h r o u g h ~ 7 g / c m o f interstellar  space;  thus, the h e a v y i o np o p u l a t i o n i n the G C R is thought to b e fully i o n i z e d ( c f . B a r t h 1997). T h e majority o f experiments c o n d u c t e d to study the G C R are i n " n e a r - E a r t h interplanetary space", s u c h as e x p e r i m e n t s f l o w n o n b o a r d the space shuttle ( B a d h w a r 1996). F o r e x a m p l e , the U n i v e r s i t y o f C h i c a g o ' s C o s m i c R a y T e l e s c o p e w a s f l o w n o n f M P - 8 ( i n L E O ) f r o m 1976-1996 to p r o v i d e complete coverage o f the G C R spectra over  32  a f u l l s o l a r a c t i v i t y c y c l e . V o y a g e r h a s a l s o d e t e c t e d b o t h G C R a n d A C R i n its interplanetary travel ( R e a m e s S u n is n o t e d (Barth  1999). I n fact, a decrease i n G C R w i t h distance f r o m the  1997).  T h e major difference b e t w e e n G C R a n d the solar w i n d is the energies o f the particles; G C R has a n extremely h i g h u p p e r energy limit. T h e energy range is tens o f M e V / n to hundreds o f G e V / n u c l e o n . H e n c e , this c o m p o n e n t has h i g h magnetic rigidity a n d penetrates d e e p into the magnetosphere. C o s m i c rays detected b y astronomers u s i n g g r o u n d - b a s e d telescopes are f r o m the G C R p o p u l a t i o n . T h e total f l u x o f G C R is significantly l o w e r t h a n m a g n e t o s p h e r i c particles. Still, G C R is a n e x t r e m e l y important c h a r g e d particle p o p u l a t i o n because o f the h i g h energies o f s o m e o f the i o n s , a n d b e c a u s e o f their ease i n d e p o s i t i n g that e n e r g y into microelectronics a n dother sensitive o n b o a r d spacecraft  components.  2.4.4 The Anomalous Component of Galactic Cosmic Radiation T h e A n o m a l o u s C o m p o n e n t o f R a d i a t i o n ( A C R ) also comes f r o m outside o u r S o l a r S y s t e m . H o w e v e r , its e l e m e n t a l c o m p o s i t i o n a n d c h a r g e is d i f f e r e n t f r o m G C R . A l l elements w i t h a large first i o n i s a t i o n potential ( H , N , O , N e ) s h o w a m a r k e d increase i n a b u n d a n c e o v e r the G C R ( K l e c k e r , 1996). T h e charges o n the h e a v y ions are also different f r o m b o t h G C R a n d the solar w i n d . A C R is s i n g l y i o n i s e d w h i l e G C R is f u l l y c h a r g e d a n d solar w i n d is i n t e r m e d i a t e l y c h a r g e d . D u e to its p r o p e r t i e s , A C R is t h o u g h t to b e t h e result o f r e c y c l i n g o f G C R b y t h e s u n . G C R d i f f u s e s i n t o t h e h e l i o s p h e r e i n t h e S u n w h e r e it is s i n g l y i o n i s e d b y U V r a d i a t i o n a n d interactions w i t h c h a r g e d s o l a r w i n d s . T h e n it is a c c e l e r a t e d i n t h e h e l i o s p h e r e o r at t h e o u t e r t e r m i n a t i o n s h o c k a n d r e l e a s e d b a c k i n t o s p a c e . T h i s p o p u l a t i o n h a s e v e n h i g h e r m a g n e t i c r i g i d i t i e s t h a n G C R d u e to its l o w c h a r g e t o m o m e n t u m r a t i o . B e a u j e a n et a l . ( 1 9 9 6 ) h a v e d e t e c t e d t h e A C R a s l o w a s L 1 . 4 - 1 . 6 ( a b o u t 4 0 0 k m a l t i t u d e at t h e e q u a t o r ) i n t h e S A A .  =  33  2.5 Solar Cycle Modulation N o t o n e o f the c h a r g e d particle populations is unaffected b y the influence o f the solar cycle. S c h w a b e a n n o u n c e d the d i s c o v e r y o f a solar 'sunspot' c y c l e i n 1849 w h e n h e n o t i c e d a g r a d u a l rise a n d fall i n the n u m b e r o f s u n s p o t s o v e r t i m e , as is s h o w n f o r d a t a since 1750 i n figure 2.7 ( c f . Z i r i n 1983). Sunspots are optically dark areas o f the S u n associated with magnetic flux ropes entering a n d exiting the solar photosphere, a n d hence appear i n pairs o r groups. T h e polarity o f e a c h o f the i n d i v i d u a l spots c a n b e o b s e r v e d t h r o u g h the Z e e m a n effect ( H a l e m a d e s u c h observations i n 1912). E a c h pair i n a g r o u p o f sunspots has o p p o s i n g polarity ( Z i r i n 1983). C o n t i n u o u s m e a s u r e m e n t o f the polarity o f t h e s u n s p o t s o v e r t h e c o u r s e o f t h e n o t e d 11 y e a r c y c l e s h o w e d t h a t c o n s e c u t i v e  cycles  d e m o n s t r a t e d a p o l a r i t y reversal. H e n c e , the 11-year solar c y c l e is a c t u a l l y is s u b - c y c l e o f the sun's 2 2 - y e a r m a g n e t i c p o l a r i t y reversal p e r i o d .  34  Figure 2.7 Sunspot number as a function of date, clearly showing an 11 year periodic solar activity cycle. (Plot is courtesy of David Hathaway, NASA MSFC)  A l t h o u g h t h e c o l l o q u i a l q u o t e d s o l a r a c t i v i t y p e r i o d i s 11 y e a r s , t h e a c t u a l d u r a t i o n o f the c y c l e s lasts a n y w h e r e f r o m 9-13  y e a r s w i t h a n a v e r a g e o f 11.5  years over  the past 4 0 y e a r s ( B a r t h 1997). H e n c e , the c y c l e is u s u a l l y best d e s c r i b e d i n t e r m s o f a 7 year m a x i m u m , interrupted b y a 4-year m i n i m u m . T h e duration o f solar m a x i m a differs s i g n i f i c a n t l y f r o m c y c l e to c y c l e a n d n o m e a n s o f p r e d i c t i n g the d u r a t i o n has b e e n f o u n d . Solar c y c l e activity has opposite effects o n electrons a n d protons. D u r i n g solar m a x i m u m , the s o l a r w i n d is d e n s e r a n d m o r e p a r t i c l e s i o n i s e the E a r t h ' s i o n o s p h e r e . T h i s  35  leads to a slight, b u t noticeable e x p a n s i o n o f the atmosphere ( w h i c h creates greater a t m o s p h e r i c d r a g o n o r b i t i n g satellites!). T h u s , the loss o f t r a p p e d protons t h r o u g h atmospheric collisions is increased d u r i n g solar m a x i m u m . P e a k p r o t o n fluxes o c c u r 1 to 2 years f o l l o w i n g solar m a x i m u m a n dthe degree o f variation ranges f r o m 5-50% b e t w e e n s o l a r m i n a n d s o l a r m a x d e p e n d i n g o n L v a l u e ( H u s t o n et a l . 1 9 9 8 ) . W h i l e t h e loss o f electrons f r o m the inner belts also increases relative to solar m i n i m u m d u e to atmospheric expansion, the injection o f electrons denser i n electrons generated b y  flares)  from  the denser solar w i n d (specifically  a l s o o c c u r s at a s i g n i f i c a n t l y h i g h e r rate. T h u s ,  the trapped electron p o p u l a t i o n increases d u r i n g m a x i m u m solar activity. S i n c e the solar w i n d is stronger d u r i n g solar m a x i m u m , G C R is deflected out o f the S o l a r S y s t e m m o r e readily. H e n c e , solar m a x i m u m also sees a r e d u c t i o n i n b o t h the G C R a n d A C R p o p u l a t i o n s . G C R is d e c r e a s e d b y a f a c t o r o f t h r e e at h i g h latitudes d u e t o solar m o d u l a t i o n ( D y e r 1999). T h e A C R p o p u l a t i o n is r e d u c e d m o r e , b y a factor o f 100 d u r i n g solar m i n i m u m ( K l e c k e r 1996). - H o w e v e r , S E P flux i n c r e a s e s . S o l a r m a x i m u m m a r k s t h e p e a k n u m b e r o f i m p u l s i v e a n d gradual events. T h e r e are u p to 1000 i m p u l s i v e events p e r year d u r i n g solar m a x i m u m a n d o n l y a f e w d u r i n g solar m i n i m u m ( K l e c k e r 1996). T h e r e are o n l y a b o u t 10 g r a d u a l e v e n t s d u r i n g s o l a r m a x i m u m ( u s u a l l y i n t h e d e c l i n i n g p h a s e ) a n d there is n o e v i d e n c e f o r o n e o c c u r r i n g d u r i n g s o l a r m i n i m u m ( a l t h o u g h it m i g h t h a p p e n i n t h e future). S i n c e S E P events cause g e o m a g n e t i c storms, g e o m a g n e t i c activity h i g h l y c o u p l e d to the solar cycle.  36  Currently, the Sun is currently entering solar maximum. SOHO has seen an increase in C M E events. Current sunspot number and predicted sunspot number for the next 7 years are shown in Figure 2.8.  •  f  1  1  1  1  1  1  1  1  1  1  1  1  1898  1  1  1  p  1  1  1  1  1  r  1  NASA/Marshall Space Flight Center 1  Updated October 1999  1998  1  2908  2002  2004  2008  YEAR Figure 2.8 Recent sunspot data and the 'forecast' of the solar activity cycle for the next decade, (http://science.nasa.gov/newhome/headlines/astl4oct99_l.htm)  37  Chapter 3: Modeling the Radiation Environment 3.1 Approach T h e standard a p p r o a c h to m o d e l i n g the space radiation e n v i r o n m e n t is o u t l i n e d i n F i g u r e 3.1. T h i s a p p r o a c h is r e c o m m e n d e d b y the R a d i a t i o n P h y s i c s O f f i c e ( R P O ) , a division o f the N A S A G o d d a r d Space Flight Center ( G S F C ) amongst (http://radhome.gsfc.nasa.gov/radhome/rpo.htm,  others  L a B e l 1996). T h e first step is t o  d e t e r m i n e the a p p r o p r i a t e o r b i t a l p a r a m e t e r s f o r the m i s s i o n s u c h that its s c i e n t i f i c g o a l s c a n b e met. T h e orbital e n v i r o n m e n t f o r the b a s e l i n e parameters is e v a l u a t e d u s i n g a suite o f n u m e r i c a l programs, each designed to calculate a v e r y specific c o m p o n e n t o f the radiation e n v i r o n m e n t o r other e n v i r o n m e n t a l effect, b a s e d o n the theoretical  framework  d e v e l o p e d i n C h a p t e r 2. T h i s c h a p t e r p r e s e n t s a d e s c r i p t i o n o f t h e n u m e r i c a l m o d e l s i n c o r p o r a t e d i n S P A C E R A D I A T I O N 4.00*. C h a p t e r 4 presents the r a d i a t i o n e n v i r o n m e n t o f the M O S T microsatellite a n d interpretation o f the e n v i r o n m e n t a l effects o n the M O S T microsatellite design.  * SPACE RADIATION 4.00 provided courtesy of Alfred Ng, Canadian Space Agency  38  S p a c e P r o j e c t Input  1GRJ-  J  O r b i t G e n e r a t i omn  I  C a l c u l a t e B & L V alues alues I r  CREME J P L 1991  C o s m i c R a y Ions Solar Flare Protons  T  A p p l y geomagnetic shielding  ALLMAG GDALMG LINTRA  4  J  Trapped Particles  1  1  A P S A 1:8  J  Spacecraft I n c i d e n t Fluence  1  J  Transport through shielding S H I F T , D O S E  Simple shielding geometries  1  Complex 3-D m o d e l o f spacecraft  J  D i s p l a c e m e n t dose Total Ionising Dose  *  L E T spectra Single Event Effects  Assessment o f Effects Recommendations for flight  J  Figure 3.1 Schematic of approach to modeling radiation environment for the MOST microsatellite. Routines for specific calculations are indicated in light blue.  39  3.2 AP8/AE8 Trapped Particle Models The AE8  m o s t w i d e l y u s e d m o d e l s f o r evaluating the trapped particle e n v i r o n m e n t s are  a n dA P 8 f o r electrons a n dprotons respectively. T h e s e m o d e l s were developed b y  James Vette i n a joint p r o g r a m sponsored b y N A S A a n dthe U S A i r F o r c e ( U S A F ) , a n d in co-operation w i t h various university teams a n d corporations w i t h data o n the trapped particle r a d i a t i o n e n v i r o n m e n t ( V e t t e 1956). T h e first m o d e l s ( A E 1 a n d A P I ) w e r e released i n the early 60's, b u t i n c l u d e d o n l y data  from  solar m i n i m u m a n dthus were not  p r a c t i c a l f o r m o d e l i n g t h e w o r s t c a s e e n v i r o n m e n t s a satellite w o u l d face*. T h e c u r r e n t l y u s e d versions o f A E 8 a n d A P 8 (released i n 1983 a n d 1976) incorporate data f r o m 43 satellites a n d a r e a p p l i c a b l e t o b o t h t h e m a x i m u m a n d m i n i m u m states o f s o l a r a c t i v i t y . In  1976, f u n d i n g f o r further m e a s u r e m e n t s w a s r e d u c e d so the m o d e l s that  builders rely o n today are b a s e d o n data f r o m 1958-1968 ( P a n a s y u k The  spacecraft  1996).  m o d e l s are e m p i r i c a l m o d e l s f o r static c o n d i t i o n s . B a s e d o n data f r o m the  a b o v e - m e n t i o n e d p e r i o d , the flux o f particles o f a g i v e n energy a n d L value are k n o w n e v e r y w h e r e a l o n g the g e o m a g n e t i c equator. B a n d L values f o r a c h o s e n m a g n e t i c are e x t r a p o l a t e d f r o m t h e s p h e r i c a l e x p a n s i o n c o e f f i c i e n t s o f the g e o m a g n e t i c  field  potential  b y a suite o f three integrated p r o g r a m s d e v e l o p e d b y A l V a m p o l a ( V a m p o l a 1996): A L L M A G ,  G D A L M G ,  a n d L I N T R A ( F i g u r e 3.3, A p p e n d i x B ) . T h e ratio o f the  g e o m a g n e t i c f i e l d s t r e n g t h t o that at t h e g e o m a g n e t i c e q u a t o r , B / B  0 j  is calculated f o r the  orbital trajectory specified. T h e n , f r o m the g e o m a g n e t i c equatorial f l u x values, the g e o m a g n e t i c f l u x values f o r trapped protons a n d electrons are interpolated into B / B L  0  and  space a l o n g the orbital trajectory, a n d integrated o v e r m i s s i o n lifetime to p r o d u c e  p r o t o n o r electron f l u e n c e spectra. In these m o d e l s , the data f r o m o v e r 9 0 e x p e r i m e n t s w a s n o r m a l i z e d to the 1976 standard i n geomagnetospheric field models developed b y Jensen a n d C a i n  (GSFC-12/66  * Intermediate stages of the models also include results from an artificially created electron belt from the Starfish program of high atmosphere nuclear weapon testing.  40  d u r i n g solar m a x i m u m , a n d Jensen a n d C a i n 1960 ( J C 6 0 ) f o r solar m i n i m u m ) i n order to calibrate the data. T h u s , m a n y authors suggest that m o r e current reference m a g n e t i c f i e l d m o d e l s s h o u l d n o t b e u s e d w i t h A E 8 a n d A P 8 ( H e y n d e r i c k x et a l . , 1 9 9 6 , P a n a s y u k , 1 9 9 6 , B a r t h 1 9 9 7 , H u s t o n et a l . , 1998). T h i s r e c o m m e n d a t i o n w a s e x p l o r e d b y t e s t i n g d i f f e r e n t f i e l d m o d e l s f o r the M O S T b a s e l i n e orbit. R e s u l t s are presented i n S e c t i o n 4.2. A P 8 a n d A E 8 also include positional information, parameterized i n M c l l w a i n ' s B a n d L values (Mcllwain  1961).  3.3 Geomagnetic Shielding models S P A C E R A D I A T I O N calculates a geomagnetic transmission function based o n the S t d r m e r i a n i d e a l d i p o l e t h e o r y (see S e c t i o n 2.3). N u m e r i c a l integration o f particles a l o n g their trajectories i n a n I G R F gives isorigidity contours f o r the vertical cutoff, i.e. independent o f a z i m u t h a n d z e n i t h angle ( S h e a a n d Smart, 1983). T h e s e results are extended to o m n i d i r e c t i o n a l f l u x b y a s s u m i n g a S t d r m e r i a n dipole field (Barth,  1997).  T h e resulting f u n c t i o n describes the fraction o f particles w h i c h c a n penetrate the Earth's m a g n e t i c field, or the fraction o f particles w i t h magnetic rigidity e x c e e d i n g the c u t o f f r i g i d i t y , as a f u n c t i o n o f c u t o f f r i g i d i t y f o r e a c h p o i n t a l o n g t h e o r b i t a l trajectory. A l s o i n c l u d e d i n the calculation is the effect o f the Earth's shadow. A s particles stream f r o m the s u n t o w a r d s the E a r t h , the d a r k side o f the E a r t h is n o t o n l y p r o t e c t e d f r o m the r a d i a t i o n b y the Earth's m a g n e t i c f i e l d , b u t also b y the E a r t h itself. T h u s , the c u t o f f rigidity is a s y m m e t r i c a r o u n d the E a r t h i f the E a r t h is c o n s i d e r e d as a n obstacle. D u r i n g a storm, the magnetic rigidty drops off. A larger fraction o f l o w energy particles are transmitted t h r o u g h the geomagnetospheric shielding. H i g h energy particles are attenuated b y the s a m e a m o u n t i n both cases since the m a g n e t i c rigidity o f a h i g h energy particle is a l w a y s greater t h a n the c u t o f f rigidity.  3.4 C R E M E C o s m i c R a y Effects o n Micro-Electronics ( C R E M E ) was developed by James A d a m s a n d others f o r the N a t i o n R e s e a r c h L a b o r a t o r i e s ( N R L ) i n the U n i t e d States a n d r e l e a s e d i n 1 9 8 3 ( T y l k a e t a l . 1 9 9 6 ) . It w a s t h e f i r s t c o m p r e h e n s i v e n u m e r i c a l c o d e t o calculate heavier c h a r g e d particle p o p u l a t i o n s i n the n e a r - E a r t h e n v i r o n m e n t a n d assess  41  t h e i r i m p a c t o n s p a c e c r a f t e l e c t r o n i c s . A s it is i n t e g r a t e d i n t o the S p a c e R a d i a t i o n software, C R E M E  is u s e d to calculate the f o l l o w i n g c h a r g e d particle spectra b e h i n d  geomagnetic a n d spacecraft shielding: (a) g a l a c t i c c o s m i c r a y p o p u l a t i o n e n e r g y s p e c t r a ; (b) a n o m a l o u s c o m p o n e n t e n e r g y spectra; (c) a n d solar energetic particle events  spectra.  L i k e A P 8 a n d A E 8 , C R E M E i s s e m i - e m p i r i c a l . It u t i l i s e s m e a s u r e d d i f f e r e n t i a l f l u x v a l u e s f o r d i f f e r e n t c h a r g e d h e a v y s p e c i e s ( H - N i ) a n d fits the m e a s u r e m e n t s as a f u n c t i o n o f energy a n d a sinusoidal solar m o d u l a t i o n parameter. T h e output is the integral o r d i f f e r e n t i a l f l u x o f p a r t i c l e s i n s i d e t h e s p a c e c r a f t at t h e s e n s i t i v e e l e c t r o n i c c o m p o n e n t b e i n g s t u d i e d . T h e d i f f e r e n t i a l flux i s t h e f r a c t i o n o f e n e r g e t i c p a r t i c l e s i n a g i v e n e n e r g y range ( E + d E ) d i v i d e d b y the energy b i n size ( d E ) . T h e integral o f the differential f l u x yields the n u m b e r o f particles a b o v e a g i v e n energy ( E ) , the integral  flux.  C R E M E also evaluates A C R (the v e r y penetrating s i n g l y i o n i s e d c o m p o n e n t ) . S i n c e this c o m p o n e n t c o m e s f r o m the r e c y c l i n g t h r o u g h the s u n ,o n l y elements w i t h large first i o n i s a t i o n potentials ( H e , N , O , a n d N e ) are present i n this type o f radiation.  3.5 Solar Energetic Particles C R E M E  includes four m o d e l s to describe large solar energetic particle ( S E P )  events s t e m m i n g f r o m C M E s . A n alternate m o d e l , J P L 1991, w a s also e m p l o y e d i n this study ( F e y n m a n n 1993). T h e m o s t e n e r g e t i c o f t h e flare m o d e l s a r e t h e ' C o m p o s i t e W o r s t C a s e S c e n a r i o ' ( C W C S ) a n d the 1972 m o d e l w h i c h is m o d e l e d after observations o f a v e r y large S o l a r E n e r g e t i c Particle ( S E P ) event w h i c h o c c u r r e d i n A u g u s t o f 1972 ( A U G 7 2 ) . S i n c e the h i g h - e n e r g y c h a n n e l (>60 M e V ) observations o f the 1972 event w e r e unreliable d u e to a n e x c e s s i v e l y h i g h e l e c t r o n b a c k g r o u n d a s s o c i a t e d w i t h t h e e v e n t ( M a j e w s k i et a l . , 1 9 9 5 ) , the c o m p o s i t e worst case scenario has a n a d d e d h i g h - e n e r g y tail. A t the t i m e , this w a s d o n e i n o r d e r to create a 'worst case' S E P w h i c h h a d a 9 9 % c o n f i d e n c e l e v e l ( C L ) that the flux  d i d n o t e x c e e d a g i v e n v a l u e . A n o t h e r v e r y l a r g e flare e v e n t w a s o b s e r v e d i n g r e a t e r  detail i n O c t o b e r 1989, a n d d i d not s h o w the high-energy tail o f the C W C S ,  although  42  p e a k p r o t o n f l u x d i d e x c e e d t h e 1 9 7 2 p e a k p r o t o n f l u x b y a f a c t o r o f 2. T h u s , t h e C W C S is n o t a u s e f u l w o r s t - c a s e s c e n a r i o m o d e l . T h e A U G 7 2 event w a s actually a succession o f 4 rapid C M E s o f 'normal' c h a r a c t e r . It a c c o u n t e d f o r 8 4 % o f t h e p r o t o n f l u e n c e o f h i g h - e n e r g y p a r t i c l e s t h a t y e a r ( K i n g 1974). T h i s l e d to the early classification o f ' a n o m a l o u s l y large' events, a n d ' o r d i n a r y ' e v e n t s b y K i n g ( 1 9 7 4 ) . H o w e v e r , F e y n m a n et a l . ( 1 9 9 0 ) r e v i e w e d s o l a r e v e n t databases a n d c o n c l u d e d that there is a n i n c r e d i b l e r a n g e o f e n e r g y s p e c t r a with C M E s , f o r m i n g a c o n t i n u u m i n energy  from  associated  large to s m a l l . T h u s , there is n o s u c h  t h i n g as a ' t y p i c a l ' S E P a n d s p a c e c r a f t m u s t b e r e a d y f o r t h e worst. T y l k a et a l . ( 1 9 9 7 ) suggest that t h e t w o m o s t e n e r g e t i c e v e n t s i n c l u d e d w i t h C R E M E are u n r e a l i s t i c a l l y severe. I n d e e d the h i g h - e n e r g y tail o f the C W C S is unrealistic i n that o n l y t h r e e f l a r e e v e n t s o b s e r v e d t o d a t e h a v e its e n e r g y d i s t r i b u t i o n . H o w e v e r , s i n c e a larger S E P t h a n the A U G 7 2 e v e n t w a s o b s e r v e d i n 1 9 8 9 , it is a p p r o p r i a t e t o c o n s i d e r that e v e n t as the 'worst-case' m o d e l f o r t h e M O S T m i c r o s a t e l l i t e . N o other w o r s t case m o d e l s are i n c o r p o r a t e d into S p a c e R a d i a t i o n 4.0. T h e other three solar flare m o d e l s are s i g n i f i c a n t l y m o r e realistic; they are representative o f average S E P events during solar m a x . T h e 9 0 % worst case  C R E M E  m o d e l is a s c a l e d d o w n v e r s i o n o f the A u g u s t 1972 event, w h i l e the o r d i n a r y m o d e l is m e a n t to reflect a n average energy s p e c t r u m o f m a n y flares.  T h eJ P L m o d e l approaches  S E P events w i t h a slightly different perspective. Instead o f isolating single events, the J P L m o d e l attempts to statistically predict the l o n g t e r m proton, H e l i u m , a n d h e a v y i o n doses f o r a g i v e n m i s s i o n , w i t h i n a g i v e n c o n f i d e n c e l e v e l ( C I , o r l e v e l o f c o n f i d e n c e that t h e s o l a r p r o t o n f l u x w i l l n o t e x c e e d t h e m o d e l v a l u e s ) ( F e y n m a n n et a l . 1 9 9 3 ) . T h e J P L 1991 m o d e l w a s u s e d i n this s t u d y as t h e s t a n d a r d , realistic s o l a r p r o t o n m o d e l . O v e r t h e course o f a 1-year m i s s i o n , the C L f o r J P L 9 1 is 9 7 % . H o w e v e r , the other 4 solar p r o t o n m o d e l s w e r e also evaluated to a l l o w f o r o n e large event d u r i n g the M O S T m i s s i o n . T h e dose resulting f r o m o n e large S E P event over o n e d a y exceeds the yearly doses resulting f r o m using the J P L 9 1  model.  43  3.6 Uncertainties D u e to a p o o r u n d e r s t a n d i n g o f the solar activity c y c l e , a n d the l i m i t e d predictive p o w e r s o f large solar flares, C M E ' s a n d hence, geomagnetic events, evaluating the uncertainty i n the a b o v e m o d e l s is c h a l l e n g i n g . U n t i l accurate predictions are m a d e as to the t i m i n g , severity, a n d d u r a t i o n o f large solar events, the a c c u r a c y o f the i n d i v i d u a l m o d e l s w i l l b e orders o f m a g n i t u d e better than the a c c u r a c y i n p r e d i c t i n g the n u m b e r o f large S E P events a satellite w i l l face. H e n c e , the spacecraft designer is f o r c e d to take a pessimistic v i e w , consider the 'worst-case'  s c e n a r i o , a n d e n s u r e that the satellite c a n  w i t h s t a n d it ( a n d t h e n c r o s s h e r f i n g e r s that it w o n ' t a c t u a l l y h a p p e n ! ) . H o w e v e r , the existing m o d e l s have b e e n i n use f o r a sufficient a m o u n t o f time to c o m p a r e predicted dose to measured dose. G u s s e n h o v e n a n d M u l l e n (1993) indicate ' g o o d ' a g r e e m e n t b e t w e e n the A P 8 / A E 8 m o d e l s a n d o r b i t to orbit d o s e s as m e a s u r e d b y the C o m b i n e d R e l e a s e a n d R a d i a t i o n E f f e c t s Satellite ( C R R E S ) . H o w e v e r , they note that predicted f l u x o f high-energy electrons ( l - 5 M e V ) f r o m A E 8 c a n b e u p to 2 orders o f magnitude too high. O n t h e o t h e r h a n d , c o m p a r i s o n s m a d e b y H u s t o n e t a l . ( 1 9 9 8 ) w i t h d a t a from t h e T I R O S / N O A A l o waltitude p o l a r o r b i t i n g spacecraft indicate that A P 8 M I N a n d A P 8 M A X  under-predict  the d o s e e x p e r i e n c e b y a factor o f 1.7-2.0 c o n s i s t e n t l y o v e r 2  solar cycles. F a v o r a b l e agreement between A P 8 M I N a n d data o n b o a r d the A d v a n c e d Photovoltaic and Electronics E x p e r i m e n t Spacecraft ( A P E X ) (in a highly eccentric 7 0 ° i n c l i n a t i o n orbit) as w e l l as w i t h results f l o w n o n b o a r d P o S A T - 1 ( i n a s u n - s y n c h r o n o u s p o l a r o r b i t v e r y s i m i l a r t o t h e M O S T b a s e l i n e o r b i t ) h a s b e e n f o u n d ( W a t s o n et a l 1 9 9 8 ) . T h e p r e d i c t i o n s f r o m A P 8 M I N o v e r - p r e d i c t the A P E X dose rates b y 1 0 % , T h e p r i m a r y factor i nthe disagreement between the m o d e l s a n d the observed doses is m o s t likely a n overestimation o f l o w energy protons. H o w e v e r , the m o d e l s under-predict the d o s e rates w i t h P o S A T , t h e l o w e r altitude satellite, b y 4 0 % ( D y e r et a l . 1998).  Thus,  t h o u g h the c o l l o q u i a l statement r e g a r d i n g the uncertainty i n the A P 8 m o d e l s is that they over-predict the environment, i n the l o w altitude M O S T baseline orbit (Section 4.1), they m o s t l i k e l y u n d e r - p r e d i c t t h e a c t u a l e n v i r o n m e n t b y a f a c t o r o f a b o u t 2.  44  A c c o r d i n g to the authors o f the C R E M E c o d e , the uncertainty i n the C R E M E spectra ranges f r o m 2-5 d e p e n d i n g o n the e n e r g y p e r n u c l e o n ( c f . B a d h w a r a n d O ' N e i l l 1996).  R e c e n t c o m p a r i s o n s o f C R E M E w i t h n e w e r experimental data a n d exercises  associated w i t h the d e v e l o p m e n t o f a n e w v e r s i o n o f C R E M E ( C R E M E 9 6 ) have s h o w n that the errors i n the o r i g i n a l m o d e l are o n the o r d e r o f 4 0 % . T h e errors arise m a i n l y f r o m the a s s u m p t i o n that t h e s o l a r a c t i v i t y c y c l e is s i n u s o i d a l ( T y l k a et a l . 1997). T h e s o l a r polarity reversal is not a sinusoid, a n d so current m o d e l s ( C R E M E 9 6 ) have incorporated a m o r e realistic solar m o d u l a t i o n factor d e v e l o p e d b y N y m m i k (1996). C o m p a r i s o n w i t h U o S A T - 3 w h i c h a l s o w a s f l o w n i n t h e s a m e o r b i t as t h e M O S T b a s e l i n e o r b i t s h o w s that C R E M E  fails to predict a n elevated level o f h i g h latitude c o s m i c rays d u r i n g solar  m i n i m u m ( D y e r e t a l . 1999). S i n c e M O S T is s c h e d u l e d to l a u n c h d u r i n g a n active solar p h a s e w h e n this e l e v a t i o n i n c o s m i c rays i s n o t a n o b s e r v e d p h e n o m e n o n , C R E M E is satisfactory f o r m o d e l i n g the M O S T radiation environment. T h e Solar A n o m a l o u s a n d Magnetospheric Particle E x p l o r e r ( S A M P E X )  satellite  h a s b r o u g h t t o l i g h t a n o v e r e s t i m a t i o n i n t h e C R E M E p r e d i c t i o n s o f t h e A C R ( T y l k a et al, 1997).  T h e a c t u a l s p e c t r a d r o p o f f m u c h m o r e r a p i d l y at t h e h i g h e r e n e r g y e n d t h a n  predicted b y the m o d e l s . S o e v e n t h o u g h the singly i o n i s e d particles are m o r e penetrating than their higher charged counterparts, they have relatively l o w energies a n d are therefore m o r e e a s i l y attenuated b y s h i e l d i n g . T y l k a e s t i m a t e s that n o particle f r o m t h e A C R w i l l penetrate 50 m i l s o f A l shielding. T h e update to C R E M E  g o e s as f a r as t o e x c l u d e t h e  A C R as h a v i n g a n y e f f e c t o n m i c r o e l e c t r o n i c s . I n d e e d , t h e f l u x p r e s e n t e d b y A C R i n C h a p t e r 4 is a n order o f m a g n i t u d e less t h a n the G C R f l u x a n d thus, e v e n t h o u g h the m o d e l is a k n o w n o v e r e s t i m a t i o n , t h e u n c e r t a i n t y a s s o c i a t e d w i t h it i s n e g l i g i b l e c o m p a r e d to the uncertainty i n the trapped proton a n d electron m o d e l s . S i n c e the p r i m a r y c o m p o n e n t o f radiation i n the M O S T baseline orbit is m a d e u p o f trapped protons a n d electrons i n the S A A ,the updated v e r s i o n o f C R E M E w a s not e m p l o y e d i n this study. A l t h o u g h C R E M E 9 6 is m o r e a c c u r a t e t h a n its p r e d e c e s s o r C R E M E , it is n o t i n t e g r a t e d i n t o S p a c e R a d i a t i o n 4.0. S i n c e the a c c u r a c y d i f f e r e n c e s are n e g l i g i b l e c o m p a r e d to the u n c e r t a i n t i e s i n t h e p r i m a r y p a r t i c l e p o p u l a t i o n , it w a s n o t c o n s i d e r e d necessary to evaluate the G C R o r A C R w i t h greater a c c u r a c y t h a n that afforded b y C R E M E .  45  Chapter 4: Choosing a Baseline Orbit A s o u t l i n e d i n F i g u r e 3.1, the spacecraft e n g i n e e r m u s t s u p p l y a b a s e l i n e orbit to b e e v a l u a t e d i n a r a d i a t i o n a n a l y s i s . C h o o s i n g a b a s e l i n e o r b i t c a n b e c o m p l i c a t e d as a n u m b e r o f factors not related to the r a d i a t i o n e n v i r o n m e n t arise. T h i s C h a p t e r investigates s o m e effects the M O S T  microsatellite w i l l face i n the baseline orbit  established i n the first p h a s e o f m i s s i o n design.  4.1 The M O S T Baseline Orbit M O S T  is b e s t s u i t e d to a l o w - E a r t h o r b i t ( L E O ) w h e r e the r a d i a t i o n e n v i r o n m e n t  is n o t h a r s h , s u c h as a l o w a l t i t u d e g e o - s y n c h r o n o u s o r b i t , o r a p o l a r o r b i t . T h e  M O S T  s c i e n c e t e a m c h o s e a p o l a r s u n - s y n c h r o n o u s o r b i t w i t h a n 8 0 0 k m a l t i t u d e b e c a u s e (a) it a l l o w s stars to r e m a i n i n sight o f the t e l e s c o p e f o r a n e x t e n d e d p e r i o d o f t i m e 4.3),  (b) m i n i m i s e s scattered light c o n t r i b u t i o n s ( S e c t i o n 4.6),  (Section  a n d ( c ) it i s t h e b a s e l i n e  orbit f o r R a d a r s a t 2 (satellites h i t c h i n g a r i d e to s p a c e can't b e too p i c k y a b o u t w h e r e e n d up). T h e L o c a l T i m e o f A s c e n d i n g N o d e ( L T A N ) f o r the b a s e l i n e o r b i t is 6:00  they  p.m.,  so that the satellite a l w a y s r e m a i n s a b o v e the t e r m i n a t o r o f the E a r t h . T h i s t y p e o f d a w n d u s k o r b i t d e s c r i b e d f u r t h e r i n S e c t i o n 4.5  is also f a v o r e d b y E a r t h m o n i t o r i n g m i s s i o n s  as it p r o v i d e s g o o d r e l i e f i n i m a g e s . T h e a d v a n t a g e f o r M O S T r e d u c e s s c a t t e r e d l i g h t as a p h o t o m e t r i c n o i s e One  is t h a t it s i g n i f i c a n t l y  source.  side o f the satellite w i l l r e m a i n p o i n t e d i n the g e n e r a l d i r e c t i o n o f the s u n ,  c o n s t a n t l y s h i e l d i n g t h e o t h e r s i d e o f t h e s a t e l l i t e , w h i l e t h e t e l e s c o p e w i l l s t a y p o i n t e d at a f i x e d spot i n the s k y i n the o p p o s i t e d i r e c t i o n o f the sun. O v e r the c o u r s e o f a year,  the  b e a m the t e l e s c o p e v i e w s o n the s k y s w e e p s out 3 6 0 ° i n R i g h t A s c e n s i o n , so the c o n t i n u o u s v i e w i n g z o n e ( C V Z ) f o r the satellite is a strip a l o n g the celestial equator.  The  g r o u n d t r a c k s s p a n the w h o l e E a r t h , s o there is s u f f i c i e n t c o m m u n i c a t i o n t i m e w i t h p l a n n e d g r o u n d stations i n V a n c o u v e r a n d T o r o n t o ( C a n a d a ) a n d V i e n n a (Austria). H o w e v e r , the satellite w i l l graze the V a n A l l e n r a d i a t i o n belts i n the S A A d u r i n g 1 8 % the orbital passes.  of  46  Figure 4.1 D a t a f r o m the F U S E guide camera comparing regions (a) outside the S A A , and (b) inside the S A A . Over-plotted is the M O S T 'donut' shaped pupil image drawn to scale. E a c h p i x e l that is dark in the second frame is a lit p i x e l , and the lines are tracks where charged particles have hit w i t h a high grazing angle and thus penetrated across many pixels in the direction o f their path through the device.  (Courtesy  o f T i m Hardy, D A O . )  4.2 Cosmic Ray hits A l t h o u g h a l s o c a u s e d b y h i g h - e n e r g y particles i n L E O , c o s m i c ray hits d o not permanently damage any o n b o a r d space components. W h e n an energetic charged particle f r o m a n y o f t h e f o u r p o p u l a t i o n s o f p a r t i c l e s d e s c r i b e d i n S e c t i o n 2.4  h i t s t h e C C D , it c a n  ionise o n e o f the S i atoms, releasing a n electron a n d l e a v i n g a n electron hole pair. T h e e l e c t r o n s are c o l l e c t e d b y t h e e l e c t r i c f i e l d s o f the d e v i c e a n d i n c o r p o r a t e d as s i g n a l . T h u s , pixels hit b y a n i n c o m i n g particle b e c o m e "lit" w i t h a b n o r m a l l y h i g h signal c o m p a r e d to the b a c k g r o u n d signal. T h e F a r U l t r a - V i o l e t Spectroscopic E x p l o r e r ( F U S E ) is o r b i t i n g the E a r t h currently, s e n d i n g data t a k e n f r o m b o t h i n s i d e the S A A a n d f r o m the q u i e s c e n t r e g i o n s o u t s i d e the S A A . F i g u r e 4.1, t w o data f r a m e s f r o m the F U S E  Fine  E r r o r S e n s o r ( F E S ) w h i c h uses a C C D detector, illustrates the n u m b e r o f lit p i x e l s d u e to c r o s s i n g the S A A . T h e s p u r i o u s signals created b y c o s m i c ray hits inside the S A A w i l l b e treated d u r i n g d a t a r e d u c t i o n . T h e m a j o r i t y o f the r a d i a t i o n f l u x is c e n t r a l i s e d i n the S A A , b u t c h a r g e d p a r t i c l e s w i l l s t i l l h i t t h e d e t e c t o r o u t s i d e t h i s r e g i o n at a m u c h l o w e r r a t e . T h u s ,  47  the d a t a r e d u c t i o n s c h e m e e m p l o y e d b y M O S T m u s t c o n s i d e r r e m o v i n g 'lit' p i x e l s d u e to c o s m i c r a y hits, o r t o l e r a t i n g t h e m . T h e r e are t w o o p t i o n s f o r c o s m i c r a y r e m o v a l , b o t h employed by ground based  astronomers:  ( a ) r e m o v e t h e p i x e l c o m p l e t e l y f r o m t h e d a t a set,  or  b ) a s s i g n a v a l u e to the lit p i x e l that is a n a v e r a g e o f the s u r r o u n d i n g p i x e l s . S i n c e c o s m i c ray hits o c c u r o v e r a v e r y short t i m e scale, they c a n b e u n a m b i g u o u s l y d e t e c t e d i f the s i g n a l f r o m a lit p i x e l is h i g h o n e integration, a n d t h e n a r e a s o n a b l e  value  i n the next sequential integration.  4.3 The Continuous Viewing Zone (CVZ) The M O S T  o r b i t is d e s i g n e d s u c h that the o p t i c a l a x i s o f t h e t e l e s c o p e is  s t e e r a b l e w i t h i n a c o n e c e n t e r e d i n t h e a n t i - s o l a r d i r e c t i o n w i t h a d i a m e t e r o f 2 7 . 3 ° (set b y the orbit, F i g u r e 4.2).  A s the E a r t h revolves a r o u n d the sun, this p r o j e c t i o n w i l l s w e e p  out  a p a t h a l o n g the s k y , m u c h like the searchlight o f a lighthouse beam. A s  MOST,  the b e a m s w e e p s a c r o s s the sky, the m i c r o s a t e l l i t e w i l l p o i n t at  stellar  t a r g e t s w h i c h f a l l i n s i d e its a r e a . D e p e n d i n g o n the l o c a t i o n o f the star i n s i d e t h e C V Z , i f it t r a n s v e r s e s t h e C V Z at a h i g h e r p o i n t t h a n at t h e e q u a t o r o f the p r o j e c t i o n , it w i l l  have  a shorter d w e l l t i m e i n the C V Z . F o r e x a m p l e , P r o c y o n is v i s i b l e i n the C V Z f o r 7.9 w e e k s , w h e r e a s G a m  Schematic of the MOST baseline orbit showing Continuous Viewing Zone (CVZ), Orbit Normal Vector (ONV), Inclination angle (/), and Beta Angle (p). F i g u r e 4.2  L e o A is o n l y v i s i b l e f o r 4.2  weeks.  See A p p e n d i x C for the d w e l l t i m e o f the M O S T  p r i m a r y targets as a  function o f orbital parameters. The  d i a m e t e r o f the C V Z is a f u n c t i o n o f orbital i n c l i n a t i o n a n d altitude. I f  M O S T  is p u t i n t o a h i g h e r a l t i t u d e o r b i t , the d i a m e t e r o f the C V Z w i l l i n c r e a s e as t h e l i m i t i n g  boundary of the cone created by the limb of the Earth is at a larger angle. Figure 4.3 shows the MOST targets on the sky with the limits of the MOST CVZ for different inclinations also projected. A higher inclination, higher altitude orbit favours more targets.  49  eVZ  bou  CVZ  bow  IIIIIMII  Illllllll  «fc»y. 986.  Illllllll  XI  i m in II  Illllllll  Illllllll  A  oGem  111 m i l l  Illllllll  Her  • 1 it Peg  a O r 10  Tn  Et  •  F?30ri  o  3B00  1 .mSer CS  E*sV"  BetOel  Itc  a*.  cyon  ^ V l p E l  1  9KtVift  1 1  0  ~  Bft elHer  t  ^mLeoA  Pre c  Illllllll  799.4  B*  I l l l l l l l l  u  Illllllll  8, 603.2 km  M i l l  s a  20  Illllllll  1  40  J_11 • 11 I I I  I I I  60  GamVlr  1 1 1  c -  a <TT3 -'  RT10S q r  „  I l l l l l l l l  I  I l l l l l l l l  -20  1  5  -40  I l l l l l l l l  i II i  0  b)  30  IIIIIIIII  6 0  IIIIIIIII  9 0  IIIIIIIII  1 2 0  1 5 0  Right  IIIIIIIII  l  :  60  IIIIIIIII  IIIIIIIII  III  IIIIIIIII  I  I l l l l l l l l  1 8 0  Ascension  IIIIIIIII  I l l l l l l l l  2 1 0  II II 1 1 I (1I l l l l l l l l  2 4 0  2 7 0  IIIIIIIII  3 0 0  IIIIIIIII  3 3 0  3 6 0  ( d e g r e e s )  IIIIIIIII  IIIIIIIII  IIIIIIIII  TT11111 II I l l l l l l l l  111 II 1 M l  I I I 1 1 1 I I p Z eVZ  boui idoiy, 9 8 6 ,  799.4  Um  boui doiy, 9 7 8.  603.2  Um  -  1 1 1 1  40  2  wk  -  "5 c  f&  Hi 11217  - 2 0  c  onnE  1: '3  113  13  I l l l l l l l l :  O  #  t8  a  HQ 178 2 3 2  1 1 1 1 1 1 1  ~Q c o  I l l l l l l l l  20 U  I l l l l l l l l  1  E24930  z  0  IIIIIIIII  30  i n  6 0  IIIIIIIII  9 0  IIIIIIIII  1 2 0  1 5 0  Right  IIIIIIIII  IIIIIIIII  1 8 0  A s c e n s i o n  2 1 0  11  "  111  2 4 0  IIIIIIIII  2 7 0  3 0 0  1111  3 3 0  :  H i  i  1  1  - 4 0  3 6 0  ( d e g r e e s )  Figure 4.3 MOST targets and limits of CVZ as a function of orbital inclination/altitude projected on the sky for a) the expanded MOST solar type targets, and b) other targets.  50  4.4 The M O S T Duty Cycle The  d u t y c y c l e is d e f i n e d as the a m o u n t o f t i m e that a t e l e s c o p e c a n c o n t i n u a l l y  s t a r e at a s t e l l a r o b j e c t . I n o r d e r t o p e r f o r m a s t e r o s e i s m o l o g y  successfully, a very high  d u t y c y c l e is e s s e n t i a l . I n f a c t , a l i m i t e d d u t y c y c l e is o n e o f the s t r o n g l i m i t a t i o n s o f performing asteroseismology  f r o m the g r o u n d . C o n s o r t i u m s h a v e f o r m e d i n a n attempt to  n e t w o r k t e l e s c o p e s all o v e r the E a r t h into a s i m u l t a n e o u s o b s e r v i n g c a m p a i g n c a l l e d the W h o l e Earth Telescope, W E T . The  C V Z is d e s i g n e d to a l l o w objects to fall into the boresights o f the  microsatellite for an extended,  uninterrupted  period o f time. In asteroseismic  M O S T data  a n a l y s i s , F o u r i e r t i m e s e q u e n c e a n a l y s i s i s p e r f o r m e d . It i s a p o w e r f u l m e t h o d o f d e t e c t i n g r e g u l a r i t i e s i n t h e s i g n a l f r o m t h e star. H o w e v e r , i t i s a l s o a v e r y p o w e r f u l w a y o f d e t e c t i n g a n y t h i n g e l s e i n t h e d a t a set w h i c h h a s a r e p e t i t i v e p a t t e r n , s u c h a s  regular  g a p s o f data. I n a p e r f e c t w o r l d , w h e r e the p h o t o m e t r i c t i m e s e r i e s is c o m p l e t e ,  the  F o u r i e r analysis o f a single f r e q u e n c y w o u l d return the single f r e q u e n c y alone.  However,  f o r a r e d u c t i o n o f the duty c y c l e o f 10%, i l l u s t r a t e d b y F i g u r e 4.4.  s i d e - l o b e s to t h e p r i m a r y s i g n a l are c r e a t e d , as  T h e s e side l o b e s are aliases o r ghost s i g n a l s o f the p r i m a r y  f r e q u e n c y . F o r c o m p a r i s o n , the spectral w i n d o w f r o m a s i n g l e - g r o u n d b a s e d sight is also p r e s e n t e d . It i s c l e a r t h a t t h e s i g n a l d u e t o a n o s c i l l a t i n g s t a r i s a c h a l l e n g e t o e x t r a c t f r o m a g r o u n d b a s e d d a t a set. W h i l e M O S T passes t h r o u g h the S A A , the n u m b e r o f c o s m i c ray hits a n d p r o t o n i n d u c e d S E E s w i l l b e h i g h , a n d p o s s i b l y too h i g h to a l l o w one to utilise data taken d u r i n g the passage. T h u s , there m a y b e s m a l l gaps i n the M O S T when M O S T  d a t a set c o r r e s p o n d i n g t o  times  i s i n t h e S A A . I n a w o r s t - c a s e s c e n a r i o w i t h n o s h i e l d i n g at a l l , M O S T  w o u l d l o o s e data f r o m the i m m e d i a t e b o u n d a r i e s o f the S A A . T h u s , the d u t y c y c l e w o u l d be about 82%  c o m p l e t e (that is, M O S T w o u l d l o s e 1 8 %  o f its d a t a d u e t o the S A A ) .  M o r e realistically, M O S T w i l l still b e able to take g o o d d a t a t h r o u g h o u t the S A A w h e r e p a r t i c l e f l u x i s r e l a t i v e l y l o w . S i n c e the p u p i l i m a g e f o r the s c i e n c e d a t a set o n l y - 6 4 0 0 p i x e l s , the actual n u m b e r o f lit p i x e l s e x p e c t e d i n the s c i e n c e d a t a i n the d e n s e s t part o f the S A A is n o t e x p e c t e d to b e significant. T h e l i m i t i n g f a c t o r to a c c u m u l a t i n g data t h r o u g h the S A A m a y b e the attitude c o n t r o l s y s t e m ( A C S ) p e r f o r m a n c e , a l t h o u g h w i t h appropriate t h r e s h o l d i n g , the a l g o r i t h m f o r g u i d i n g s h o u l d b e s u f f i c i e n t to a l l o w g u i d i n g  51  t h r o u g h the S A A . T h u s , a 9 0 %  d u t y c y c l e is the m i n i m u m expected. S i m u l a t i o n s o f the  photometric performance o f M O S T with an 80%  duty cycle b y K u s c h n i g have s h o w n  t h a t t h e p h o t o m e t r i c p r e c i s i o n r e q u i r e m e n t s c a n s t i l l b e m e t ( K u s c h n i g et a l . 1 9 9 9 ) .  d u t y cycle = 100 %  duty cycle - 90 %  MOST Orbit with shielding I  duty cycle = 80 %  MOST Orbit without shielding I  duty cycle = 3 5 %  frequency [mHzl Figure 4.4 The spectral window for the MOST microsatellite, based on the approximate SAA location shown in the top inset, (a) shows an ideal spectral window for a complete data set, (b) for a duty cycle with a loss of 10% in completeness due to passages through the S A A (c) for a duty cycle 20% incomplete, and (d) for a typical ground based sight that can only observe during the night (Courtesy of Kuschnig, 2000).  53  4.5 Eclipse Season T h e b a s e l i n e o r b i t h a s a L o c a l T i m e o f A s c e n d i n g N o d e ( L T A N , t h e t i m e at w h i c h the satellite c r o s s e s f r o m the S o u t h e r n H e m i s p h e r e o f the E a r t h to the N o r t h e r n )  of  6 p . m . T h u s , the satellite w i l l a l w a y s r e m a i n a b o v e the t e r m i n a t o r o f the E a r t h ( w h e r e the s u n l i t p o r t i o n o f the E a r t h m e e t s the d a r k p o r t i o n ) ( F i g u r e 4.2).  M O S T w i l l peer out over  the d a r k l i m b o f the E a r t h f o r the m a j o r i t y o f the orbit. T h i s is e q u i v a l e n t to a n orbit w i t h L T A N o f 6 a . m . , e x c e p t the satellite crosses f r o m the S o u t h e r n to N o r t h e r n h e m i s p h e r e  at  d a w n as o p p o s e d to d u s k . T h u s , t h e s e o r b i t s are r e f e r r e d to as d a w n - d u s k o r b i t s . D u r i n g w i n t e r a n d s u m m e r e c l i p s e s e a s o n s w h e n t h e S u n i s at its m o s t e x t r e m e i n c l i n a t i o n relative to the Earth's equator, the p l a n e o f M O S T ' s orbit w i l l b e the f r o m t h e t e r m i n a t o r ( w h e t h e r it i s i n a d a w n o r a d u s k o r b i t ) . F o r a m a x i m u m o f minutes (17%  furthest 17  o f t h e o r b i t , i n c l u d i n g u m b r a l a n d p e n u m b r a l p o r t i o n s o f t h e e c l i p s e ) at t h e  s u m m e r solstice M O S T w i l l b e e c l i p s e d b y the E a r t h , cutting o f f direct p o w e r s u p p l y f r o m M O S T ' s s o l a r a r r a y s a n d f o r c i n g t h e satellite t o r e l y o n its b a t t e r i e s . A d d i t i o n a l l y , m a n y satellites are v e r y s e n s i t i v e to t e m p e r a t u r e g r a d i e n t s . A s t h e y are s h a d o w e d f r o m the s u n a n d the temperature drops, they m a y experience m e c h a n i c a l structure.  " t h e r m a l s n a p " , a f l e x u r e o f the  S i n c e M O S T is so s m a l l , t h e r m a l f l e x u r e s h o u l d n o t b e s i g n i f i c a n t .  H o w e v e r , the attitude c o n t r o l s y s t e m w i l l b e m o r e s e n s i t i v e to t h i s p h e n o m e n o n as o p t i c a l focus m a y b e c o m e slightly distorted. F i g u r e 4.5 f r o m O c t o b e r 2001  s h o w s the d u r a t i o n o f i n d i v i d u a l eclipses f o r the M O S T b a s e l i n e orbit t h r o u g h January 2004. S i n c e p o w e r is r e d u c e d , M O S T m a y not b e  a b l e to f u n c t i o n i n n o r m a l o p e r a t i n g m o d e . E c l i p s e season w o u l d b e m o s t spent p e r f o r m i n g testing a n d engineering operations.  appropriately  54  Lunar eclipses, also shown in Figure 4 . 5 have moderate to long duration. However, since there is not very many consecutive eclipses (repeated over many orbits) relative to the number of consecutive eclipses during the eclipse season, power levels should not be significantly affected.  Eclipse Duration, 800km altitude, 6pm LTAN 1200  1000  Umbral eclipses  800  2  600  The eclipse season in the baseline orbit lasts from May 17 to July 27  3  Q  ©  <2  400 200  Penumbral eclipses  21-Oct-02  10-Dec-02  29-Jan-03  20-Mar-03  09-May-03  28-Jun-03  17-Aug-03  06-Oct-03  25-Nov-03  14-Jan-04  Date Figure 4.5 Eclipse duration for the MOST baseline orbit. The central, darkest region of the eclipse is the umbra, while the penumbra is the less shadowed outer portion of the eclipse. Eclipses outside the eclipse season are caused by the moon's shadow.  The L T A N of the orbit determines when the eclipse season will occur. As shown in Figure 4 . 5 , for a L T A N of 6 p.m., the eclipse season is centered on the summer solstice. For a L T A N of 6 am, the eclipse season is offset by 6 months to be centered around the winter solstice. A noon/midnight orbit would have its eclipse season at the vernal or spring equinox.  55  4.6 Stray Light Effects A t t h e o p p o s i t e p o i n t o f the o r b i t w h e r e M O S T is e c l i p s e d f r o m the S u n b y the E a r t h , M O S T w i l l b e f o r c e d to peer out o v e r the sunlit l i m b o f the E a r t h a n d scattered light signal w i l l  increase.  E x p e r i e n c e f r o m the star-sensor a b o a r d the f a i l e d W I R E m i s s i o n , also i n a polar s u n - s y n c h r o n o u s orbit ( i n c l i n a t i o n 9 7 ° , altitude 4 7 0 x 5 4 0 k m ) , s h o w s that the contribution o f signal f r o m p e e r i n g over the bright l i m b o f the E a r t h is substantial. F i g u r e 4.6 shows a b r i e f sequence o f data taken b y W I R E w i t h a g a p d u e to occultation b y the Earth. T h e increase i n integrated signal i m m e d i a t e l y before the occultation  corresponds  directly to the time w h e n the star-sensor observes over the sun-lit l i m b o f the Earth. D a t a p r o c e s s i n g procedures u s e d b y B u s a z i i n a n attempt to detect oscillations i n the A l t a i r  7.0x10-1  WIRE Startracker - Altair field - Star 2 Observation: 18 October 1999 i  6.8x10* H Q  6.6x10'  V  6.4x1 ( M  ^  6.2x10* H  X CO  E  SAA  Target Star occulted by earth  6.0x10'' 5.8x10* H  CO  —  5.6x10* H  .2> CO  5.4x10* H  CO C  5.2x10* H 5.0x10* 1470.20  1470.22  1470.24  1 1470.26  —  i  —  1470.28  1470.30  Time (seconds) Figure 4.6 Light curve taken by the star-sensor onboard the WIRE satellite showing a dramatic increase in signal prior to occultation by the Earth when the satellite peers out over the bright limb of the Earth. (Figure courtesy of Kuschnig, Data courtesy of Busazi.)  56  light c u r v e discards a n y data t a k e n p r i o r to a n o c c u l t a t i o n ( K u s c h n i g , p e r s o n a l communication). Orbits with varying  . RAAN;Oh (dawn Dusk)  L T A N s have been considered, especially since not every  satellite  m a k e s it i n t o t h e i r o p t i m u m o r b i t upon launch. Increasing  Lower edge of CVZ  the  L T A N f r o m d a w n / d u s k to n o o n / m i d n i g h t causes the  satellite  orbit the E a r t h a l o n g a f i x e d p l a n e relative to the E a r t h separated b y a larger angle 9 f r o m the plane o f t h e t e r m i n a t o r ( F i g u r e 4.7).  When 0 is greater than 6 then the MOST boresight will be facing directly over the bright limb of the earth.  orbits w i t h L T A N  In  approaching  n o o n or m i d n i g h t , the  photometric  noise d u e to scattered light f r o m  Figure 4.7 Schematic of MOST orbiting the Earth viewed looking down on the North Pole. Changing the LTAN of the orbit causes MOST to peer out over the sunlit limb of the Earth, increasing photometric noise due to scattered light. Note that Right Ascension of Ascending Node (RAAN) is the same as LTAN measured in degrees of right ascension as opposed to local time.  the E a r t h increases dramatically. A complicating factor in estimating the a m o u n t o f scattered light f r o m the l i m b o f the E a r t h is the a l b e d o o f the E a r t h is not  constant. H e n c e , M O S T s h o u l d be i n an orbit where  9 is less t h a n 9  m a x  (the a n g l e  between  the intersection p o i n t o f the l o w e r m o s t bore-sight o f the C V Z a n d the orbital p l a n e ) i n order to r e d u c e scattered light contributions. G r a n t e d , m a n y o f M O S T ' s targets d o not f a l l d i r e c t l y a l o n g t h i s l o w e r b o r e - s i g h t ( F i g u r e 4.3).  B u t since s u n l i g h t is d i r e c t e d  a n d w i l l reflect o f f the atmosphere or interplanetary particles b a c k t o w a r d s the  outwards telescope,  a c o n s e r v a t i v e o b s e r v a t i o n w o u l d o n l y b e m a d e i n the u p p e r h a l f o f the C V Z f o r s u c h a n orbit. F i g u r e 4.8  s h o w s 9 for three d i f f e r i n g orbits, f r o m the baseline d a w n - d u s k orbit  t h r o u g h to a n orbit w i t h L T A N 7 p . m . . 8 p . m . , a n d L T A N 9 p . m . E v e n f o r a d a w n - d u s k orbit, 9 exceeds 9 ax i n the s u m m e r season (eclipse season). T h e a m o u n t o f t i m e d u r i n g m  57  the y e a r that the satellite w i l l stare out o v e r a sunlit l i m b o f the earth is v e r y s e n s i t i v e to the L T A N .  If the L T A N  is later t h a n 8 p . m . (or c o n v e r s e l y earlier than 4 p . m . ) t h e n the  satellite w i l l a l w a y s stare out o v e r the bright p o r t i o n o f the earth. In a l l n o n d a w n - d u s k o r b i t s , M O S T w i l l n e e d g r e a t e r p o w e r s u p p l i e s t h a n a r e c u r r e n t l y b u d g e t e d . T h u s , it i s i m p e r a t i v e that M O S T g o into a d a w n - d u s k p o l a r orbit ( L T A N either 6 a.m. or 6 p . m . ) .  60  L T A N : 9 p.m.  o l-Oct-02  20-Nov-02  9-Jan-03  28-Feb-03  19-Apr-03  8-Jun-03  28-M-03  16-Sep-03  Date Figure 4.8 0 as a function of LTAN over the course of a year for the MOST baseline orbit (800 km). The amount of time that 0 exceeds 0 ^ increases greatly as the orbit deviates from a dawn dusk orbit.  H e l i o s y n c h r o n o u s orbits are stable o v e r a range o f altitudes w h e r e the i n c l i n a t i o n o f a h e l i o s y n c h r o n o u s o r b i t is a f u n c t i o n o f the altitude ( S e e A p p e n d i x B f o r i n c l i n a t i o n v s . a l t i t u d e o f h e l i o s y n c h r o n o u s o r b i t s ) . 0 is n o t s t r o n g l y d e p e n d e n t o n a l t i t u d e o f t h e o r b i t s i n c e i n c l i n a t i o n o n l y c h a n g e s s l i g h t l y as a f u n c t i o n o f altitude. H o w e v e r , 0 change significantly w i t h altitude ( G a n d 1000  k m respectively). F i g u r e 4.9  ( d e f i n e d as 0 >  0,^)  for a 600  is 2 3 . 9 3 ° , 2 7 . 3 1 ° , a n d 3 0 . 1 8 ° at 6 0 0  m a x  k m , 800  s h o w s the m a x i m u m a l l o w a b l e v a l u e o f the  k m , 800  k m , a n d 1000  does  m a x  km, L T A N  k m sun-synchronous orbit o n  58 January  1,2002. S i n c e 0 c h a n g e s  w i t h the seasons, these m a x i m u m a l l o w a b l e  LTANs  w i l l b e closer to n o o n / m i d n i g h t o v e r s p r i n g a n d f a l l e q u i n o x e s a n d d a w n / d u s k d u r i n g w i n t e r a n d s u m m e r s o l s t i c e s . A s p e r F i g u r e 4.9, season in w h i c h  0 exceeds 0  max  e v e n i n a d a w n - d u s k orbit, there is a  .  10 6  6.5  7  7.5  .  8  8.5  9  Local Time of Ascending Node (hours p.m.)  Figure 4.9 Maximum allowable values of the LTAN for heliosynchronous orbits with 600 (short dash) 800 (solid), and 1000 km (long dash) altitude on January 1, 2002.  59  Chapter 5: The Weather Forecast for the MOST microsatellite Now  that a b a s e l i n e o r b i t is e s t a b l i s h e d a n d it i s c l e a r that t h e o r b i t a l e n v i r o n m e n t  outside o f the r a d i a t i o n e n v i r o n m e n t is tolerable w i t h i n M O S T  m i s s i o n p a r a m e t e r s , it i s  essential to d e t e r m i n e the r a d i a t i o n e n v i r o n m e n t o f the orbit t o ensure the satellite d e s i g n c a n tolerate its i m p a c t . T h i s c h a p t e r o u t l i n e s t h e steps u s e d t o e s t a b l i s h t h e r a d i a t i o n e n v i r o n m e n t f o r t h e M O S T m i c r o s a t e l l i t e b a s e d o n t h e a p p r o a c h d e t a i l e d i n C h a p t e r 3.  5.1 The geomagnetic field A s p e r F i g u r e 3.1, the next step i n e v a l u a t i n g the radiation e n v i r o n m e n t o f the M O S T microsatellite is to calculate the geomagnetic AP8  a n d A E 8 . F i g u r e 5.1 s h o w s g e o m a g n e t i c  baseline orbit based o n the I G R F 2 0 0 0 Due  field  field  field (Section 3.1.2) to b e u s e d i n  strength f o r the M O S T  microsatellite  extrapolated to a flight e p o c h o f 2002.  to the s u g g e s t i o n that o n l y the J e n s e n a n d C a i n m o d e l s s h o u l d b e u s e d w i t h  Longitude  Figure 5.1 Magnetic field strength for the MOST baseline orbit (flight epoch 2002) clearly showing the depression in magnetic field strength associated with the SAA (Section 2.3.1).  60  A P 8 a n d A E 8 ( S e c t i o n 3.3), other g e o m a g n e t i c f i e l d m o d e l s w e r e a p p l i e d to see i f a n y noticeable difference i n proton o r electron f l u x resulted. M o d e l s c o m p a r i n g the proton environments u s i n g G S F C - 1 2 / 6 6 a n d the International G e o m a g n e t i c Reference  Field  ( I G R F , see S e c t i o n 2.3) s h o w e d nearly i d e n t i c a l solar m a x t r a p p e d p r o t o n spectra 5.2).  (Figure  H e n c e , f o r solar m a x i m u m , the u p d a t e d g e o m a g n e t i c f i e l d m o d e l s w e r e e m p l o y e d as  r e c o m m e n d e d f r o m the authors o f S P A C E R A D I A T I O N 4.0. T h e Jensen a n d C a i n solar m i n i m u m m o d e l J C 6 0 w a s not accessible w i t h the Space R a d i a t i o n package a n d hence, variations d u e to the c h o i c e o f m a g n e t i c f i e l d m o d e l d u r i n g solar m i n i m u m w e r e not investigated. Additionally, IGRF2000  w a s imported into Space Radiation. I G R F 2 0 0 0 , the most  recently p u b l i s h e d o f the reference f i e l d m o d e l s , contains t i m e derivates o f the  magnetic  f i e l d s o t h a t s e c u l a r v a r i a t i o n s c a n b e i n c l u d e d a n d t h e field c a n b e e x p t r a p o l a t e d t o t h e M O S T f l i g h t e p o c h o f 2 0 0 2 . T h u s , it is t h e m o s t c u r r e n t r e p r e s e n t a t i o n o f the E a r t h ' s magnetic field a n d the m o s t realistic to use i n m o d e l l i n g . H e n c e , the I G R F s u p p l i e d a n d r e c o m m e n d e d b y the Space R a d i a t i o n software package were used a n d c o m p a r e d to the newly published IGRF2000.  A l t h o u g h the m o d e l coefficients have changed between  e p o c h s , the o v e r a l strength o f the inner m a g n e t i c f i e l d has r e m a i n e d constant.  Thus,  trapped p r o t o n spectra are identical f o r the n e w e r a n d older versions o f I G R F . H o w e v e r , scaling I G R F to the e p o c h o f flight expected f o r the M O S T microsatellite (2002) shows a m i n o r increase i ntrapped p r o t o n spectra, related to the expected increase i n solar protons i n j e c t e d f r o m t h e c u r r e n t p h a s e o f s o l a r m a x i m u m (see F i g u r e 4 . 1 0 ) . D y e r et a l . ( 1 9 9 8 ) note that despite the r e c o m m e n d a t i o n s against u s i n g u p d a t e d f i e l d m o d e l s , u s i n g t h e n e w e r f i e l d m o d e l s d i d predict the correct n u m b e r o f passes o f the S T S - 8 1 m i s s i o n t h r o u g h the S A A . H o w e v e r , the d e f i n i t i v e b o u n d a r i e s o f the are still non-static a n d m a y r e q u i r e r e - e v a l u a t i o n p r i o r to t h e l a u n c h o f the M O S T m i s s i o n .  61  l.OOE+15  Trapped Proton Fluences AP8MAX —IGRF ...GSFC-12/66 . . IGRF2002  1.00E+14  g l.OOE+13 2  1.00E+12  l.OOE+11  1000  10 Energy (MeV)  Figure 5.2 Trapped proton spectra for AP8MAX used with three different magnetic field models: IGRF, GSFC-12/66, and IGRF extrapolated to the MOST flight epoch of 2002. There is no noticeable difference between IGRF and GSFC-12/66. However, the flight epoch spectrum has slightly higher proton fluence due to solar modulation of the trapped proton population.  T h e F a r U l t r a - V i o l e t S p e c t r o s c o p i c E x p l o r e r ( F U S E ) satellite, a n internationally f u n d e d s p a c e satellite e x p l o r e r i n g t h e ultra-violet u n i v e r s e , spent e x t e n s i v e t i m e m a p p i n g o u t t h e b o u n d a r i e s o f t h e S A A d u r i n g t h e first y e a r o f i t s m i s s i o n ( A l e x F u l l e r t o n , personal communication  2000).  H e n c e , M O S T should take advantage o f their  experience  a n d utilise t h e m a p generated b y their t e a m to predict w h e n the satellite w i l l enter the S A A to reject data w h i c h m a y b e c o r r u p t e d d u e to c o s m i c r a y hits o r m a l f u n c t i o n o f the Attitude C o n t r o l System ( A C S ) . D a t a taken during the passage through the S A A c o u l d be a n a l y s e d a n d c o m p a r e d w i t h the predictions f r o m this study to further evaluate the validity o f the models.  62  5.2 Geomagnetic Shielding M O S T w i l l o r b i t at a r e l a t i v e l y l o w a l t i t u d e , w h e r e t h e g e o m a g n e t o s p h e r e s h i e l d the satellite f r o m m u c h interplanetary radiation. T h e g e o m a g n e t i c function  will  transmission  ( S e c t i o n 3.3),  w i t h a n d w i t h o u t the Earth's s h a d o w , f o r the M O S T b a s e l i n e orbit  i s s h o w n i n F i g u r e 5.3.  A l s o s h o w n is the case f o r a s t o r m y m a g n e t o s p h e r e d i s r u p t e d b y a  large C M E .  5  10  15  Cutoff (GV)  Figure 5.3 Geomagnetic transmission function for the MOST baseline orbit, for normal magnetospheric conditions (no storm).  63  5.3 Trapped Protons and Electrons A l o n g w i t h solar energetic particles, the t r a p p e d p r o t o n a n d e l e c t r o n p o p u l a t i o n s are the m o s t s i g n i f i c a n t i n the M O S T b a s e l i n e orbit. A P 8 a n d A E 8 m o d e l s ( S e c t i o n w e r e u s e d to e v a l u a t e p a r t i c l e f l u e n c e o v e r a s p e c i f i e d m i s s i o n l i f e t i m e . F o r the  3.2)  M O S T  m i c r o s a t e l l i t e , it i s p o s s i b l e t o c o m p l e t e a l l p r i m a r y s c i e n c e o b j e c t i v e s w i t h i n m i n i m u m m i s s i o n l i f e t i m e ( o n e year). T h u s , the f l u e n c e levels p r e s e n t e d here are a n o r b i t - i n t e g r a t e d f l u x (particle per unit area per year) o v e r the baseline year l o n g m i s s i o n . F i g u r e 5.4 s h o w s t h e i n t e g r a l f l u e n c e o f t r a p p e d p r o t o n s a n d e l e c t r o n s f o r t h e M O S T baseline orbit for both solar m a x i m u m and solar m i n i m u m ( m i n i m u m m i s s i o n d u r a t i o n o f 1 year). S i n c e the t r a p p e d m a g n e t o s p h e r i c p o p u l a t i o n is i n s i d e the m a g n e t o s p h e r e , n o g e o m a g n e t i c s h i e l d i n g is a p p l i e d . T h e p r o t o n f l u x v a r i e s b y a b o u t a  l.OE+15  —  7  1  10  100  Energy (MeV)  Figure 5.4 Proton and electron fluence over M O S T minimum mission lifetime.  1000  64  factor o f 2 between solar m a x i m u m a n d solar m i n i m u m . H i g h e r fluence during solar m i n i m u m are e x p e c t e d d u e t o a t m o s p h e r i c e x p a n s i o n ( S e c t i o n 2.5).  Since higher energy  particles are also m o r e penetrating to the s h i e l d i n g o f spacecraft, the h i g h - e n e r g y p r o t o n p o p u l a t i o n s are the m o s t i m p o r t a n t to c o n s i d e r . A p p e n d i x D c o n t a i n s m a p s s h o w i n g t h e p a r t i c l e e n v i r o n m e n t s f o r 0.1 M e V , 1 0 M e V , a n d 3 0 0 M e V p r o t o n s as w e l l as 1 M e V , a n d 5 M e V e l e c t r o n s r e s p e c t i v e l y . T h e m o s t d o m i n a n t feature is the S A A , as e x p e c t e d .  L o w e r energy particles, especially the  l o w e r e n e r g y electrons, are a l s o f o u n d to o c c u p y b a n d s i n h i g h latitudes. A t h i g h o r l o w latitudes, t r a p p e d m a g n e t o s p h e r i c particles i n the outer r a d i a t i o n belts c a n r e a c h l o w a l t i t u d e s . T h e s e ' h o r n s ' are m u c h less s t a b l e t h a n t h e S A A as t h e y are a s s o c i a t e d w i t h t h e outer m a g n e t o s p h e r e o f the E a r t h a n d d y n a m i c a l l y interact w i t h the solar w i n d a n d a s s o c i a t e d m a g n e t i c f i e l d s . H e n c e u n c e r t a i n t i e s i n the f l u e n c e o f t h o s e p a r t i c l e s are h i g h e r t h a n estimated here. S i n c e they are l o w - e n e r g y particles a n d w o n ' t b e able to penetrate spacecraft s h i e l d i n g , a m o r e c o m p r e h e n s i v e treatment o f the uncertainty o f the f l u e n c e for this p o p u l a t i o n was not undertaken.  5.4 Galactic Cosmic Radiation The Galactic C o s m i c Radiation component was calculated using ( S e c t i o n 3.4).  C R E M E  Integral a n d differential f l u x spectra f o r the M O S T m i c r o s a t e l l i t e d u e to  G C R during solar m a x i m u m a n d m i n i m u m under n o r m a l geomagnetospheric conditions w i t h a 3 - D s h i e l d i n g representation o f the c a v i t y i n w h i c h the c h a r g e d c o u p l e d d e v i c e (CCD)  r e s i d e s a p p l i e d ( S e c t i o n 1.3.1  respectively.  & 5.7)  are s h o w n i n F i g u r e s 5.5 a n d  5.6  65  Figure 5.5 Integral flux energy spectrum of G C R expected to hit the C C D detector.  66  1i  1  10  100  1000  Energy (MeV/nuc.) Figure 5.6 Differential flux of GCR penetrating behind a 3D shielding geometry. This is the GCR flux expected to hit the CCD detector.  10000  67  The 100G7(m  2  1 M e V integral f l u x d u e to G C R i n s i d e the M O S T s p a c e c r a f t is about  s r s ) , y i e l d i n g a o n e y e a r f l u e n c e o f 2.14  x 10  1 0  / m / s r . C o m p a r e d to the p r o t o n 2  10 f l u e n c e i n s i d e t h e s p a c e c r a f t (~2  x 10  f o r 1 M e V p r o t o n s ; S e c t i o n 5.3) the G C R is  a b o u t 2 orders o f m a g n i t u d e l o w e r t h a n the t r a p p e d p r o t o n f l u x . T h u s , the c u m u l a t i v e i o n i s i n g d o s e d u e to G C R w i l l b e n e g l i g i b l e c o m p a r e d to the p o p u l a t i o n s t r a p p e d i n the V a n A l l e n R a d i a t i o n belts. H o w e v e r , since the G C R i o n s are m u c h heavier, they create m o r e S i n g l e E v e n t U p s e t s ( S E U s , S e c t i o n 6.2)  t h a n the p r o t o n p o p u l a t i o n .  5.5 Anomalous Cosmic Radiation T h e i n t e g r a l a n d d i f f e r e n t i a l flux d u e t o A C R a s p r e d i c t e d b y p r o p a g a t e d t h r o u g h the Earth's g e o m a g n e t o s p h e r e ( S e c t i o n 5.7),  a r e s h o w n i n F i g u r e 5.7.  C R E M E ,  and 5 m m of A l cylindrical shielding  T h e s e spectra scale to larger f l u x e s w i t h less  s h i e l d i n g a n d v i c e v e r s a f o r a d d i t i o n a l s h i e l d i n g . T h e larger first i o n i s a t i o n potential o f N relative to O results i n the f l u x o f N i n the A C R b e i n g s l i g h t l y h i g h e r t h a n that o f O .  68  Figure 5.7 Integral and differential flux energy spectra of ACR.  5.6 Solar Energetic Particles The  e n e r g y spectra associated w i t h the 5 different S E P events d e s c r i b e d i n  S e c t i o n 3.5 are s h o w n i n F i g u r e 5.8, a g a i n w i t h g e o m a g n e t o s p h e r i c a n d 5 m m o f A l cylindrical spacecraft shielding  ( S e c t i o n 5.7) a p p l i e d . T h e J P L 1991  is t h e m o s t r e l i a b l e  a n d s t a t i s t i c a l l y a c c u r a t e m o d e l . H o w e v e r , it r e p r e s e n t s d a t a a v e r a g e d o v e r a 5 - y e a r t i m e f r a m e . S i n c e M O S T i s b e i n g l a u n c h e d d u r i n g t h e d e c l i n e o f s o l a r m a x i m u m , it i s m o r e l i k e l y that a large S E P event w i l l  occur.  69  F i g u r e 5.8  Differential energy spectra of  SEP  events.  70  5.7 Satellite Shielding R e f e r r i n g b a c k t o F i g u r e 3.1 a n d t h e a p p r o a c h t o m o d e l i n g t h e r a d i a t i o n e n v i r o n m e n t , the next step i n the radiation analysis f o r the M O S T microsatellite is apparent. T h econtributions d u e to galactic c o s m i c rays, solar protons a n d magnetospheric particles all contribute to the spacecraft incident  fluence.  However,  s h i e l d i n g stops m a n y o f the i n c i d e n t particles f r o m interacting w i t h spacecraft sensitive c o m p o n e n t s . T h u s , the s h i e l d i n g o f the M O S T d e s i g n m u s t n o wb e c o n s i d e r e d .  Simple Shielding Geometry The  first step i n the r a d i a t i o n analysis is a n assessment o f the r a d i a t i o n  e n v i r o n m e n t b e h i n d standard s i m p l e s h i e l d i n g m o d e l s . T h ethree m o d e l s f o r s i m p l e s h i e l d i n g i n c l u d e d w i t h the S H L E L D O S E - 2 c o d e d e v e l o p e d b y A l V a m p o l a (1996) are (1) a finite A l slab, (2) a s e m i - i n f i n i t e A l slab, a n d (3) a n A l sphere. T h e results f o r the S H I E L D O S E - 2 c a l c u l a t i o n s f o r the M O S T b a s e l i n e orbit are s h o w n i n F i g u r e 5.9. I n c l u d e d is the dose vs. depth profiles f o r trapped protons a n delectrons f r o m A P 8 / A E 8 d u r i n g solar m a x i m u m , as w e l l as solar p r o t o n s f r o m the J P L 9 1 m o d e l c o n t r i b u t i n g to t h e total dose inside a n A l sphere. I n c l u d e d i n the electron dose is b r e h m s t r a h l u n g e m i s s i o n . S i n c e the dose vs. d e p t h curves o f the s i m p l e s h i e l d i n g g e o m e t r y analysis l e v e l o f f ,the a d v a n t a g e s o f t h i c k e r s h i e l d i n g start t o d e c l i n e , e s p e c i a l l y as t h i c k e r s h i e l d i n g q u i c k l y a d d s m a s s t o the satellite. T h u s , i n b a l a n c i n g s h i e l d i n g v s . less m a s s , the o p t i m a l s h i e l d i n g t h i c k n e s s appears to b e a r o u n d 5 m m f r o m F i g u r e 5.9 ( a d d i t i o n a l s h i e l d i n g d o s e not reduce over all dose significantly). A critical dose for M O S T  i s o n t h e o r d e r o f 10  k r a d i n S i ( o n e r a d is e q u i v a l e n t to 100 ergs o f e n e r g y a b s o r b e d b y 1 g r a m o f material), so the m i n i m u m a m o u n t o f s h i e l d i n g n e e d e d is c o n s e r v a t i v e l y estimated as 3 m m . H o w e v e r , s i n c e p r e l i m i n a r y e s t i m a t e s o f m i n i m u m s h i e l d i n g t h i c k n e s s w e r e 10 m m (Matthews, 1997), the revised m i n i m u m estimate o f 5 m m o f A l s h i e l d i n g is sufficient to e n s u r e t h e d u r a t i o n o f the m i s s i o n f o r j u s t less t h a n 10 y e a r s , as w e l l as r e d u c e s o v e r a l l mass o f the instrument.  71 10000000  10-1 0  !  i  1  5  10  —  1  1  15  20  Depth in Al(im) Figure 5.9 Dose vs. Depth curve for simple shielding geometry based on SHIELDOSE-2. The red line is for a 4 Pi Al Spherical shield, the pink for an infinite Al slab, and the blue for a finite Al slab. Also shown are contributions from trapped protons, JPL91 SEP, and trapped electrons for the Al spherical shielding case.  3-D Shielding M o d e l s O f c o u r s e , the M O S T satellite is neither a sphere n o r a p l a n e slab. O n c e the design was significantly mature (incorporating preliminary shielding recommendations), a m o r e detailed s h i e l d i n g m o d e l w a s generated u s i n g S P A C E R A D I A T I O N 4.0. e n g i n e e r i n g s c h e m a t i c o f the M O S T satellite is s h o w n i n F i g u r e 5.10,  A n  along with a  s c h e m a t i c o f the 3 - D c y l i n d r i c a l m o d e l u s e d to generate the c u m u l a t i v e d o s e s o f the M O S T m i s s i o n . T h e telescope structure itself, a p p r o x i m a t e l y a n 8 - m m t h i c k c y l i n d e r o f I N V A R (a steel a l l o y ) a c t s as s h i e l d i n g . T h e m o s t s e n s i t i v e c o m p o n e n t w i t h i n the s t r u c t u r e is the C h a r g e d C o u p l e d D e v i c e ( C C D ) w h i c h sits a b o u t 3/4 o f t h e w a y  satellite  towards  the b a c k o f the telescope. I m m e d i a t e l y s u r r o u n d i n g the C C D is the c a m e r a h o u s i n g , another cylinder about 2 m m thick a n d m a d e o f T i .  72  449 mm  Figure 5.10 Engineering schematic of the preliminary MOST satellite design (top) and schematic of 3D shielding geometry used to represent satellite shielding. The outer casing is representative of the telescope tube, and the inner casing is representative of the camera housing.  73  A l u m i n u m (or A l equivalent) shielding removes low-energy particles m o r e effectively t h a n h i g h - e n e r g y particles. F i g u r e 5.11 illustrates this b y c o m p a r i n g the t r a p p e d p r o t o n integral fluence spectra f o r a non-shielded, a n d a shielded case (inside tube 5 - m m thick w i t h the M O S T t e l e s c o p e d i m e n s i o n s ) . R e c a l l that integral f l u e n c e is t h e f l u x integrated o v e r a baseline m i s s i o n lifetime o f one year.  1.00E+15 -j  1.00E+09 H 0.1  1  1  1  1  10  100  1  1 1000  Energy (MeV) Figure 5.11 Comparison of the integral trapped proton fluence outside the spacecraft, and behind 5 mm of Al cylindrical shielding with the MOST satellite dimensions as shown in  T h e c h a r g e d particle must penetrate the s h i e l d i n g w i t h sufficient energy to further penetrate into the sensitive v o l u m e o f the spacecraft c o m p o n e n t to d o a n y d a m a g e .  Thus,  the i n c i d e n t spectra are d e s c r i b e d i n terms o f the linear e n e r g y transfer o r L E T s p e c t r u m ( F i g u r e 5.12). L E T is a m e a s u r e o f the rate o f e n e r g y d e p o s i t i o n i n a sensitive v o l u m e o f t h e d e v i c e p e r u n i t p a t h l e n g t h . It i s e s s e n t i a l l y a d e s c r i p t i o n o f t h e a b i l i t y o f t h e p a r t i c l e  74  t o t r a n s f e r its e n e r g y i n t o c r y s t a l l a t t i c e o f the d e v i c e , o r i n t o i o n i s i n g o n e o f t h e a t o m s . O f c o u r s e , t h e L E T is d e p e n d e n t o n the d e v i c e structure i t s e l f , w h a t it is m a d e o f a n d h o w e a s i l y the b o n d s b e t w e e n m o l e c u l e s are b r o k e n . S i n c e the m a j o r i t y o f e l e c t r o n i c s b a s e d o n a S i l i c o n crystal lattice, the L E T p r e s e n t e d here are f o r S i detectors.  are  Knowing  the L E T thresholds o f the d e v i c e s o n b o a r d the spacecraft a l l o w s a direct c o m p a r i s o n b e t w e e n the r a d i a t i o n e n v i r o n m e n t a n d the sensitive c o m p o n e n t s . F i g u r e s 5.12  and  5.13  s h o w the L E T spectra f o r t r a p p e d protons a n d S E P ( J P L 9 1 ) respectively d u r i n g solar m a x i m u m f o r 3 different s h i e l d i n g m o d e l s . F i g u r e 5.14  s h o w s the L E T spectra f o r G C R .  S i n c e the m a j o r i t y o f the i o n s i n G C R are h e a v y i o n s , the L E T is not s e n s i t i v e to the shielding model.  1.00E+13T  I.OOETOS-1 1  j  .  10  ,  I  100  LCT(IVfev/(g'cm2))  Figure 5.12 LET Spectra of trapped Protons for (a) 2 mm Ti shielding with MOST camera housing dimensions, (b) 5 mm Al shielding with MOST telescope tube dimensions, and (c) 8 mm Al shielding with MOST telescope tube dimensions.  ; 1000  75  l.OOE+06 -I  1  1  1  10  100  L E T (MeV/Cgfan )  «  1 1000  2  Figure 5.13 LET spectra of solar energetic protons as modeled by JPL91.  1.00E+11  T  1.00E-05 -1 1.00E+00  1 1.00E+01  1 1.00E+02  1 1.00E+03  LET (MeV/(g/cm )) 2  Figure 5.14 LET spectra due to GCR.  1 1.00E+04  1 1.00E+05  76  5.8 Cumulative doses C h a r g e d particles interact w i t h S i l i c o n b a s e d electronics o n b o a r d i n the satellite i n t h r e e w a y s : (a) i o n i s a t i o n , (b) d i s p l a c e m e n t , o r (c) S E U ( S e c t i o n 6.2). W h e t h e r a p r o t o n w i l l cause i o n i s a t i o n d a m a g e o r d i s p l a c e m e n t d a m a g e d e p e n d s o n the incident particle's energy. T h e L E T spectra contain information o n whether the particle w i l l damage the electronic d e v i c e through ionisation o r displacement. H e n c e , there are t w o c u m u l a t i v e doses to b e calculated: ionisation dose a n d displacement dose. C u m u l a t i v e a n d d i s p l a c e m e n t doses are tabulated i n A p p e n d i x E . M O S T w i l l e x p e r i e n c e about 1 k r a d o f i o n i s i n g dose p e r year. S i n c e o n l y the telescope itself has b e e n c o n s i d e r e d as s h i e l d i n g , this is a v e r y conservative estimate. T h e satellite structure itself w i l l contribute a d d i t i o n a l s h i e l d i n g . I f there is a large flare a n d a large geomagnetic disturbance, then M O S T c o u l d experience a n additional krad ionising dose. A g a i n , a c c o r d i n g to recent experience, this is a grossly conservative n u m b e r . T h e g o o d n e w s is that M O S T s h o u l d b e a b l e to w i t h s t a n d e v e n the harshest r a d i a t i o n e n v i r o n m e n t i t i s l i k e l y t o e n c o u n t e r i n t h e r a n g e o f o r b i t s c o n s i d e r e d f o r it. T o t a l d i s p l a c e m e n t d a m a g e w i l l b e o n the order o f 0.7 rads i n S i , o r 7 x 1 0 M e V proton equivalents/cm ). 2  10  7  8  1  O n er a d (Si) is approximately equivalent to a f l u x o f 4 x  (energy - l M e V ) ionising particles/cm  2  ( B a i l e y 1996). T h e effects o f the  d i s p l a c e m e n t d a m a g e a s w e l l as t h e i o n i s i n g d o s e a r e f u r t h e r i n v e s t i g a t e d i n C h a p t e r 6. T h e uncertainties i n the radiation e n v i r o n m e n t m o d e l s are d i s c u s s e d i n S e c t i o n 3.6. T a b l e 5.1 s u m m a r i z e s t h e e s t i m a t e d u n c e r t a i n t i e s i n t h e m o d e l s . D o s e s c a l c u l a t e d u s i n g these m o d e l s are o n l y as g o o d as the m o d e l s themselves.  Particle Population  Model  Uncertainty  Under/Over-prediction  Trapped Protons  AP8  1.7-2.0  Under-prediction  Trapped Electrons  AE8  2  Over-prediction  Solar Protons  JPL1991  97%  G C R  C R E M E  2-5  A C R  C R E M E  Table 5.1 Uncertainty in radiation environment models.  accuracy  Depends o n Flare N u m b e r Depends o n Solar M a x / M i n Gross Over-prediction  77  5.9 Dose versus Altitude A l t h o u g h there are m a n y a r g u m e n t s f a v o r i n g the b a s e l i n e orbit, there is n o g u a r a n t e e that the M O S T m i c r o s a t e l l i t e w i l l e n d u p i n its i d e a l o r b i t . T h u s , the d o s e as it v a r i e s w i t h o r b i t a l a l t i t u d e o v e r a set o f s u n - s y n c h r o n o u s o r b i t s h a s b e e n e v a l u a t e d quiet magnetospheric  for  c o n d i t i o n s at s o l a r m a x i m u m i n t h e c e n t e r o f a n a l u m i n u m s p h e r e  (as p e r s i m p l e s h i e l d i n g g e o m e t r y d e s c r i b e d i n S e c t i o n 5.7).  A s expected, the  dose  i n c r e a s e s v e r y s l i g h t l y at h i g h e r a l t i t u d e s a s t h e o r b i t c r e e p s u p w a r d i n t o t h e h e a r t o f t h e V a n A l l e n R a d i a t i o n belts. A s s h o w n i n F i g u r e 5.15  the d o s e increases b y a b o u t 50  krad  p e r y e a r p e r 100 k m i n c r e a s e i n o r b i t a l altitude ( w i t h 5 m m o f A l s h i e l d i n g ) .  0  2  4  6  8  10  12  14  16  18  20  Alumiiumum Shielding Thickness (mm)  Figure 5.15 Dose at the center of an Aluminum Sphere for heliosynchronous orbits with different altitudes rangingfrom600 to 1000 km.  T h e S A A c o v e r s a m u c h l a r g e r a r e a at h i g h e r a l t i t u d e , a s t h e o r b i t a l p l a n e c r e e p s u p w a r d into the heart o f the V a n A l l e n R a d i a t i o n Belts. A t 1000 f r o m b o t h h o r n s w i l l b o m b a r d the satellite. F i g u r e 5.16  k m , energetic particles  s h o w s t h e o u t l i n e o f t h e S A A at  78  600  k m , 800  k m , a n d 1000  k m . S i n c e data c o l l e c t i o n m a y be r e d u c e d for m u c h o f the  h i g h energy regions o f the S A A , M O S T s h o u l d not c o n s i d e r increasing orbital altitude significantly.  Figure 5.16 Approximate boundary of the SAA at 600, 800, and 1000 km altitude.  79  Chapter 6: Rain or Shine? Implications of Space Weather on MOST T h e p r i m a r y effects o f the radiation e n v i r o n m e n t o n the M O S T microsatellite are the following: (a) i o n i s i n g p a r t i c l e s w i l l d a m a g e t h e C C D a n d e l e c t r o n i c s ( i n c l u d i n g d e g r a d a t i o n o f c h a r g e t r a n s f e r e f f i c i e n c y ( C T E ) a n d l o c a l i s e d 'hot' r e g i o n s o f p e r m a n e n t l y d a m a g e d pixels); (b) h i g h e n e r g y particles w i l l alter structure o f s i l i c a lattice i n electronics c a u s i n g displacement damage; (c) single event effects  (SEEs);  (d) a n d a r e d u c e d d u t y c y c l e f r o m a p o s s i b l e loss o f o b s e r v a t i o n s t h r o u g h t h e S A A ( S e c t i o n 4.4). T o u n d e r s t a n d h o w s e r i o u s effects (a) - (c) a r e , it is n e c e s s a r y t o e x a m i n e the C C D detectors to b e u s e d o n M O S T . A p p e n d i x F contains the Specification Sheet f o r the o f f t h e - s h e l f v e r s i o n o f the M O S T C C D . T h e r a d i a t i o n s p e c i f i c a t i o n o f the C C D ( S e c t i o n 1.4) i s s u e d b y M a r c o n i i s t h e following: " D e v i c e parameters m a y b e g i n to change i f subject to greater than 1 0 corresponds to 1 0  1 3  o f 15 M e V n e u t r o n s / c m , 2 x 1 0 2  1 3  4  rads. T h i s  o f 1 M e V gamma/cm , or 4 x 10 2  1 1  o f ionising particles/cm ." ( M a r c o n i 2000) M a r c o n i c o n d u c t s m a n y r a d i a t i o n tests o f t h e i r o w n C C D s i n o r d e r t o p r o v i d e their c u s t o m e r s w i t h estimates o f the c h a n g e i n s p e c i f i c a t i o n w i t h r a d i a t i o n d e g r a d a t i o n . T h u s , the document prepared b y R o b b i n s (2000) o n the performance o f M a r c o n i  CCDs  u n d e r r a d i a t i o n d a m a g e is u t i l i s e d to e v a l u a t e t h e p e r f o r m a n c e o f the M O S T C C D i n t h e e n v i r o n m e n t d e s c r i b e d i n C h a p t e r 5.  80  6.1 CCD Damage C C D t e c h n o l o g y has a d v a n c e d greatly i n the past d e c a d e i n response to d e m a n d s f o r b e t t e r s c i e n t i f i c i m a g i n g . H o w e v e r , t h e i n c r e a s e i n s e n s i t i v i t y o f C C D s h a s b e e n at t h e p r i c e o f a n i n c r e a s e d s e n s i t i v i t y t o r a d i a t i o n d a m a g e i n s p a c e t h r o u g h (1) i o n i s a t i o n d a m a g e , a n d (2)  displacement  damage.  I o n i s a t i o n d a m a g e , as s h o w n i n F i g u r e 6.1, c a n h a v e t w o e f f e c t s o n C C D s . I f a n i n c o m i n g e n e r g e t i c p a r t i c l e h i t s t h e s e m i c o n d u c t o r l a t t i c e , it a c t s m u c h l i k e a n i n c o m i n g signal p h o t o n w o u l d , f r e e i n g an electron a n d l e a v i n g a n d electron-hole pair. T h e electron is f r e e d a n d a n e l e c t r o n - h o l e p a i r r e m a i n s . H o w e v e r , t h e e l e c t r o n - h o l e p a i r s t e n d t o c o n g r e g a t e at t h e o x i d e - c o n d u c t o r i n t e r f a c e , c r e a t i n g a p o s i t i v e c h a r g e b u i l d u p at t h e i n t e r f a c e . T h i s d i r e c t l y s h i f t s the f l a t - b a n d p o t e n t i a l o f the C C D . M i d - g a p t r a p p i n g states are a l s o generated, c r e a t i n g a n increase i n d a r k current t h r o u g h t h e r m a l ' h o p p i n g ' o f electrons. D e e p t r a p p i n g states a l s o are c r e a t e d , r e d u c i n g C T E ( c h a r g e t r a n s f e r e f f i c i e n c y , S e c t i o n 6.1.5).  Photon or charged particle  Electron is collected as signal  Figure 6.1 Charge generation or ionisation damage occur in the same manner in CCDs  81 2 I o n i s a t i o n is s t r o n g l y d e p e n d e n t o n the c h a r g e o f the i n c o m i n g i o n s q u a r e d , Z . T h u s , t h e m o r e a b u n d a n t , l o w e r Z i o n s c a n d o as m u c h d a m a g e as the less a b u n d a n t , h i g h e r Z p a r t i c l e s ( T r i b b l e et a l . 1 9 9 9 ) . D i s p l a c e m e n t d a m a g e (or b u l k damage) has m o r e lasting effects o n C C D s . A n i n c o m i n g particle ( p r o t o n o r h i g h - e n e r g y neutron) strikes the lattice o f the s e m i c o n d u c t o r a n d d i s p l a c e s o n e o f the ( S i ) a t o m s i n the lattice, l e a v i n g a v a c a n c y as s h o w n i n F i g u r e 6.2.  T h e v a c a n c i e s t e n d to c o n g r e g a t e t o g e t h e r a n d a r o u n d i m p u r i t i e s i n the  lattice, c r e a t i n g p e r m a n e n t t r a p p i n g states w i t h i n t h e s e m i - c o n d u c t o r itself. S h a l l o w t r a p p i n g states i n c r e a s e d a r k c u r r e n t a n d d e e p t r a p p i n g states b o t h d e c r e a s e C T E a n d increase r e a d noise s i m i l a r l y to i o n i s a t i o n d a m a g e . " H o t " p i x e l s , or r e g i o n s w i t h e x t r e m e intensities u n r e l a t e d to the s i g n a l , d e v e l o p w h e r e the v a c a n c i e s c o n g r e g a t e , d u e to the e x t r e m e d a r k c u r r e n t a s s o c i a t e d w i t h t h e m i d - g a p t r a p p i n g states.  Figure 6.2 An energetic particle strikes a Si atom of the semiconductor lattice and kinks the structure, leaving a vacancy. Vacancies congregate together and about impurities in the crystal lattice. (After Hardy 1997)  82  Either ionisation damage or bulk damage w i l l induce the following: (a) D a r k current increase d u e to i o n i s i n g r a d i a t i o n ; (b) D a r k current increase d u e to b u l k d a m a g e ; (c) D a m a g e d p i x e l s ; (d) R a n d o m t e l e g r a p h signals; (e) F l a t b a n d v o l t a g e shifts; (f)  C T E degradation.  6.1.1 Dark Current In e v e r y C C D , s o m e current is generated e v e n w h e n photons are n o t incident o n the detector. T h i s b a c k g r o u n d signal is c a l l e d the  dark current.  Since dark current  g e n e r a t i o n c r e a t e s a r a n d o m n u m b e r o f e l e c t r o n s p e r p i x e l , it i s a n o i s e s o u r c e . D a r k current is p r i m a r i l y a t h e r m a l effect. I f electrons i n the v a l e n c e b a n d (or i n the v a l e n c e shells o f s i l i c o n i n the crystal lattice) possess e n o u g h energy, t h e n they m o v e to the c o n d u c t i o n b a n d , i.e., to the i n v e r s i o n layer w h e r e they are t r a p p e d a n d t h e n c o l l e c t e d as s i g n a l . T h u s , i f a n e l e c t r o n i n the v a l e n c e b a n d possesses s u f f i c i e n t t h e r m a l e n e r g y , it c a n b e attracted t o t h e c o n d u c t i o n b a n d w i t h o u t a d d e d e n e r g y g e n e r a t e d b y t h e photoelectric effect or b y interactions w i t h a charged particle.  T h e probability o f a n  electron possessing sufficient thermal energy to d o this is g i v e n b y the f o l l o w i n g f o r m u l a :  p=  I l  where E  c  ^ c-E VkT E  +  (6.1)  e  f  is the energy o f electrons i n the c o n d u c t i o n b a n d , a n d E f is the F e r m i energy o f  the electron, k is the B o l t z m a n constant a n d T i s the o p e r a t i n g temperature o f the C C D ( H a r d y 1 9 9 7 ) . T h u s , at l o w e r o p e r a t i n g t e m p e r a t u r e s , d a r k c u r r e n t i s s u p p r e s s e d . T h i s i s o n e o f t h e m a i n r e a s o n s t o o p e r a t e t h e M O S T C C D at - 4 0 ° C . C o o l e r t e m p e r a t u r e s  would  reduce dark current e v e n further, but require a m o r e expensive c r y o g e n i c c o o l i n g system as o p p o s e d to a p a s s i v e c o o l i n g m e c h a n i s m . D a r k current due to radiation d a m a g e c a n b e created b y either i o n i s i n g or b u l k damage. Ionising radiation induces e n h a n c e d dark current b y increasing the interface state d e n s i t y o f t h e d e p l e t e d s u r f a c e a r e a s o f t h e d e v i c e ( R o b b i n s 2 0 0 0 ) . I f a n i n c o m i n g  83  c h a r g e d p a r t i c l e i n t e r a c t s w i t h a s i l i c o n a t o m at t h e s i l i c o n - s i l i c o n d i o x i d e i n t e r f a c e ( i n a n o n - b u r i e d c h a n n e l d e v i c e ) , it c a n c r e a t e a m i d - g a p t r a p p i n g state, e s s e n t i a l l y a l o w e r e n e r g y p a t h w a y f o r electrons to m o v e a b o u t i n the s i l i c o n lattice. T h i s e f f e c t i v e l y i n c r e a s e s the e n e r g y o f the v a l e n c e b a n d e l e c t r o n s , o r d e c r e a s e s the e f f e c t i v e e n e r g y o f t h e c o n d u c t i o n b a n d e l e c t r o n s as s e e n b y the v a l e n c e b a n d e l e c t r o n s . T h e r m a l h o p p i n g o f electrons increases a n d the d a r k current o f the detector increases. S i n c e the M O S T  device  is o p e r a t e d u n d e r i n v e r s i o n , the m a j o r i t y o f this surface g e n e r a t e d s i g n a l is s u p p r e s s e d . H e n c e , t h e r e are n o t m a n y r a d i a t i o n tests to c o m p a r e to. T h e o n l y a b s o l u t e m e a s u r e m e n t has b e e n m a d e b y B r u n e i University, but not o n a M a r c o n i device. T h e y f o u n d an i n c r e a s e o f 1.5 p A / c m / k r a d ( S i ) , o r 15 e 7 p i x / s / k r a d ( S i ) a t 3 0 ° C . T h e e f f e c t o f i o n i s i n g 2  r a d i a t i o n d a m a g e o n d a r k c u r r e n t at - 4 0 ° C s h o u l d b e n e g l i g i b l e ( - 0 . 0 3 e 7 p i x / m i n / k r a d ) . D a r k signal increases can also be caused b y displacement damage. In a similar f a s h i o n to the c r e a t i o n o f m i d - g a p t r a p p i n g states i n the c a s e o f i o n i s a t i o n d a m a g e , b u l k d a m a g e also results i n the f o r m a t i o n o f a l o w e r energy p a t h w a y f o r electrons. T h e biggest d i f f e r e n c e is that the l o w e r e n e r g y p a t h w a y is n o w i n the b u l k structure o f the p - t y p e s i l i c o n , as o p p o s e d to b e i n g c o n f i n e d t o the i n t e r f a c e r e g i o n . S i n c e the t r a p p i n g states c r e a t e d b y d i s p l a c e m e n t d a m a g e a r e v e r y g o o d at t r a n s f e r r i n g e l e c t r o n s t h r o u g h t h e r m a l h o p p i n g , the d i s r u p t e d lattice c a n essentially b e c o n s i d e r e d a d a r k current g e n e r a t i o n c e n t e r . B u l k d a m a g e is i n d e p e n d e n t o f b i a s state. S i n c e the s i g n a l g e n e r a t i o n c e n t e r s a r e i n the d e p l e t i o n r e g i o n a n d not i n the surface r e g i o n , i n v e r s i o n w i l l not suppress this signal. T e s t s o f a T e k t r o n i k s b a c k s i d e i l l u m i n a t e d , b u r i e d c h a n n e l d e v i c e s i m i l a r to the M O S T C C D w e r e m a d e b y H a r d y (1997). F o r the d e v i c e r u n n i n g i n M P P m o d e , the b a s e l i n e d a r k c u r r e n t w a s l e s s t h a n 1 e 7 p i x e l / s (at - 4 0 ° C ) . A f t e r i r r a d i a t i o n w i t h 6.0 x 3 M e V p r o t o n s / c m , t h e d a r k c u r r e n t i n c r e a s e d t o 2 e 7 p i x e l / s , a n d a f t e r 1.5 x 1 0  10  9  3 M e V  p r o t o n s / c m , t h e s i g n a l w a s u p t o 9 e 7 p i x e l / s , a g a i n at - 4 0 ° C . I n a w o r s t c a s e s c e n a r i o 2  w i t h a n e x t r e m e l y l a r g e f l a r e e v e n t , M O S T w i l l s e e 1.93  x 10  1 M e V protons/cm . Thus,  w e c a n e x p e c t a s m a l l i n c r e a s e i n d a r k c u r r e n t d u e t o b u l k r a d i a t i o n , o n t h e o r d e r o f 1 e" /pixel/s. B o t h effects are temperature d e p e n d e n t . T h e i o n i s a t i o n d a m a g e increase i n d a r k c u r r e n t v a r i e s as  j ^' 3  ^,  7000  w h e r e a s the d a r k current increase d u e to b u l k d a m a g e is  84  j2 (-7ooon) j e  j  e  n  c  e  ;  o p e r a t i n g at T ~ - 4 0 ° C i s v e r y i m p o r t a n t . T e m p e r a t u r e s t a b i l i t y i s a l s o  v e r y i m p o r t a n t to suppress drifts i n the d a r k current. In s u m m a r y , s i n c e M O S T is u s i n g a d e v i c e o p e r a t i n g i n i n v e r s i o n , m o s t o f the d a r k current is s u p p r e s s e d . T h e total increase i n d a r k current d u e to i o n i s a t i o n a n d b u l k d a m a g e is o n the o r d e r o f 1 e"/pixel/s.  6.1.2 Damaged Pixels F r e q u e n t l y , d i s p l a c e m e n t d a m a g e o c c u r s i n m o r e than o n e p l a c e i n the S i lattice i n the s a m e p i x e l . A h i g h e n e r g y particle c a n b o m b a r d the first S i a t o m i n the lattice, b e d e f l e c t e d , b u t c o n t i n u e o n its d e s t r u c t i v e p a t h t h r o u g h t h e d e v i c e . A l s o , i f d i s p l a c e m e n t d a m a g e o c c u r s i n the lattice i n a r e g i o n w h e r e there is a v e r y s t r o n g electric f i e l d a p p l i e d , t h e n the p i x e l w i l l s h o w a v e r y h i g h g e n e r a t i o n rate d u e to a s i g n i f i c a n t l o w e r i n g o f the p o t e n t i a l b a r r i e r . E l e c t r o n s w i l l f l o o d t h e p i x e l , m a k i n g it a p p e a r lit. T h e v o l u m e o f a n y p i x e l i n a h i g h f i e l d r e g i o n is e x t r e m e l y s m a l l , so this is a n u n l i k e l y effect ( R o b b i n s , private communication). M a r c o n i estimates that ~ 0 . 1 %  o f p i x e l s w i l l d i s p l a y a b o u t 3 1 , 5 0 0 e7pixel/s d u e to  d a m a g e d p i x e l s a f t e r i r r a d i a t i o n w i t h 2 k r a d o f 10 M e V e q u i v a l e n t p r o t o n s . A l t h o u g h t h i s is a v e r y s m a l l n u m b e r o f p i x e l s , the M O S T data r e d u c t i o n a l g o r i t h m n e e d s to i n c l u d e a process for identifying pixels w h i c h consistently give a signal above a certain threshold, e v e n w h e n there is n o l i g h t f a l l i n g o n that p i x e l . T h e lit p i x e l s c a n e f f e c t i v e l y b e r e m o v e d f r o m t h e d a t a set, t h u s r e m o v i n g a n y n o i s e d u e t o t h e l i t p i x e l s .  85  6.1.3 R T S R a n d o m T e l e g r a p h S i g n a l i n g ( R T S ) is a m o r e significant type o f p i x e l d a m a g e that the M O S T  data r e d u c t i o n a l g o r i t h m s h o u l d b e set to m o n i t o r . A l t h o u g h it is u n c l e a r  w h a t c a u s e s R T S (it d o e s n o t a p p e a r t o b e c r e a t e d b y n u c l e a r i n t e r a c t i o n s ) , it i s d e f i n i t e l y an effect seen i n experiments p e r f o r m e d o n M a r c o n i C C D s a n devaluated b y R o b b i n s (2000). A f t e r significant proton irradiation, single pixels b e g i n to s h o w fluctuations i n their signals, shifting f r o m a l o w signal r e g i m e to a h i g h signal regime. T h e a m o u n t o f t i m e spent i n o n e r e g i m e is n o t we}l characterized. H o w e v e r , the a v e r a g e t i m e s b e t w e e n t h e d i s c r e t e g e n e r a t i o n states i s w e l l d e f i n e d b y t h e f o l l o w i n g e q u a t i o n :  (6.2)  T w h e r e t is t h e a v e r a g e t i m e i n e a c h state, R is a c o n s t a n t ( ~ 1 0  1 3  -10  1 4  /s), a n d E is a  c o n s t a n t ( 0 . 9 ± 0.1 e V ) . T h e a v e r a g e t i m e p e r state is s t r o n g l y t e m p e r a t u r e d e p e n d e n t . A t the M O S T o p e r a t i n g temperature ( - 4 0 ° C ) , the t i m e constant is o n the o r d e r o f 6 days. A b o u t 6 % o f the p i x e l s w i l l d i s p l a y R T S after o n e year (i.e. 6 0 , 0 0 0 p i x e l s w i l l b e d a m a g e d p e r 1 krad) a n dthe effect w i l l b e e n h a n c e d i n lit pixels. S i n c e the t i m e constant f o r R T S is m u c h l o n g e r t h a n the o s c i l l a t i o n p e r i o d s that M O S T  is s e a r c h i n g f o r i n the stellar s i g n a l , this effect w i l l n o t create aliases i n the  F o u r i e r f r e q u e n c y r e g i o n s o f interest. H o w e v e r , s i n c e t h e actual t i m e d u r a t i o n i n e a c h g e n e r a t i o n state i s n o t w e l l k n o w n , t h e r e c o u l d b e p h o t o m e t r i c n o i s e i n t r o d u c e d i n t h e r e g i o n o f interest w h e n a p i x e l d i s p l a y i n g R T S stays i n o n e d i s c r e t e state f o r a p e r i o d o f t i m e s i g n i f i c a n t l y shorter t h a n its t i m e constant. T h u s , the M O S T data r e d u c t i o n a l g o r i t h m needs to i n c l u d e a m o n i t o r f o r this effect, utilising o n b o a r d temperature  sensor  d a t a t o k e e p a n a c c u r a t e v a l u e f o r x. P i x e l s d i s p l a y i n g a n R T S e f f e c t s h o u l d b e d i s c a r d e d c o m p l e t e l y o r g i v e n values taken f r o m a n average o f the s u r r o u n d i n g pixels. A l t h o u g h 6 % is a large n u m b e r o f total p i x e l s , the data that M O S T uses o n l y s p a n s 6 4 0 0 p i x e l s ( 0 . 6 % o f the total c h i p ) . T h u s , it is n o t l i k e l y that R T S w i l l result i n a m a j o r loss o f data f o r M O S T .  86  6.1.4 Flat Band Voltage Shifts F l a t b a n d v o l t a g e shifts o c c u r u n d e r i o n i s i n g r a d i a t i o n d a m a g e . T h e lattice o f the s i l i c o n d i o x i d e i n s u l a t i n g layer is not i m m u n e to interactions w i t h c h a r g e d particles. W h e n a n i o n i s i n g particle hits the lattice i n the i n s u l t i n g r e g i o n , the h i g h electric  field  tends to i m m e d i a t e l y s w e e p a w a y a n y electrons, l e a v i n g e l e c t r o n - h o l e pairs (positive charge). A f t e r t i m e , the electron h o l e pairs a c c u m u l a t e a n d c o n g r e g a t e together. T h i s effectively changes the operating potential o f the M I S capacitor, increasing the potential b y a n a m o u n t e q u a l to a flat b a n d v o l t a g e shift. M a n y e x p e r i m e n t s h a v e b e e n c o n d u c t e d o n M a r c o n i C C D s to investigate flat b a n d v o l t a g e shifts. H o w e v e r , the m a j o r i t y o f t h e m are d o n e o n front i l l u m i n a t e d d e v i c e s . S i n c e the i n s u l a t i n g r e g i o n is b u r i e d i n a b a c k s i d e i l l u m i n a t e d d e v i c e , c o m p a r i s o n s  are  not valid. H o w e v e r , C C D 2 6 , a M a r c o n i b a c k s i d e i l l u m i n a t e d d e v i c e was tested for  flat  b a n d v o l t a g e shifts a n d a n i n c r e a s e o f 100 ± 2 0 m V / k r a d ( S i ) w e r e f o u n d ( c f . R o b b i n s 2000). T h u s , M O S T will likely experience - 1 0 0  m V voltage shift o v e r the course o f one  y e a r o f o p e r a t i o n . T h i s m a y d e c r e a s e the r e s p o n s i v i t y o f the C C D , but is s u c h a s m a l l shift that effects w i l l be n e g l i g i b l e o v e r the first y e a r o f the m i s s i o n ( J o h n s o n ,  private  c o m m u n i c a t i o n ) . I f the m i s s i o n l i f e t i m e is e x t e n d e d , the issue o f a flat b a n d v o l t a g e shift should be revisited.  87  6.1.5 C T E Degradation C T E degradation occurs i n response to b u l k radiation damage. In response to a d a m a g e d S i lattice, d e e p t r a p p i n g states a r e c r e a t e d , c a p a b l e o f t r a p p i n g e l e c t r o n s .  With  t i m e they c o n g r e g a t e together. C T E is d e f i n e d as the f r a c t i o n o f the s i g n a l w h e n transferred f r o m o n e p i x e l to the next. C h a r g e T r a n s f e r Inefficiency  ( C T I ) is the quantity  (1-CTE). It i s k n o w n t h a t C T I i n c r e a s e s w i t h s m a l l e r s i g n a l s , a n e f f e c t t h a t i s e n h a n c e d b y r a d i a t i o n d a m a g e ( H a r d y 1997). S i n c e M O S T w i l l o b s e r v e s o m e o f the b r i g h t e s t stars i n the s k y , it is n o t e x p e c t e d that C T I w i l l b e a m a j o r  obstacle.  S i n c e C T I is a result o f d i s p l a c e m e n t d a m a g e , it scales w i t h n o n - i o n i s i n g e n e r g y loss ( N I E L ) , or the a m o u n t o f energy deposited b y a charged particle through a n y process other t h a n i o n i s a t i o n ( u s u a l l y n u c l e a r interactions). R o b b i n s ( 2 0 0 0 ) c o n f i r m s that M E L is a g o o d first a p p r o x i m a t i o n . M E L c a n b e c a l c u l a t e d u s i n g a m o d e l d e v e l o p e d b y the E u r o p e a n Space Research a n dT e c h n o l o g y Center a n davailable online through the Space Environment Information System ( S P E N V I S , http://www.spenvis.oma.be/spenvis/). F o r the M O S T baseline orbit, M E L i n S i l i c o n as a f u n c t i o n o f spherical A l s h i e l d i n g is s h o w n i n F i g u r e 6.3.  88  1.00E+09  1.00E+06 -1  1  0  1  5  10  Al shielding thickness (mm)  Figure 6.3 Non-ionising energy loss as a function of spherical Aluminum shielding thickness.  The  s c a l i n g factor u s e d to estimate C T I f r o m N I E L is u s u a l l y d e t e r m i n e d  experimentally. S i n c e radiation testing o f the C C D s w a s not d o n e as part o f this analysis, an arbitrary s c a l i n g constant o f 1 x 10"  11  g ( S i ) / M e V w a s c h o s e n b a s e d o n D a l e ( 1 9 9 3 ) . It  is p r o b a b l y a n o v e r e s t i m a t e o f t h e a c t u a l f a c t o r a n d h e n c e , t h e r e s u l t s p r e s e n t e d h e r e a r e e x p e c t e d to b e a n o v e r e s t i m a t i o n o f the actual C T I . T h e s c a l i n g factor c a n b e r e f i n e d i f radiation testing is ever p e r f o r m e d o n the M O S T For  CCDs.  5 m m o f s p h e r i c a l A l s h i e l d i n g , t h e N I E L i s 1.97 x 10  M e V / g ( S i ) , a n dthe  relative degradation i n C T E over the course o f a 1 year m i s s i o n i n the M O S T  baseline  o r b i t is 1.97 x 10" . T h u s , after o n e y e a r , t h e l o w e s t m e a s u r e d C T E o f t h e M O S T 4  grade C C D w i l l b e degraded to 0.999799%.  science  89  6.1.6 Implications for the Photometric Error Budget In s u m m a r y , the f o l l o w i n g 'worst case' scenario m a y o c c u r to the M O S T C C D d u r i n g the first year o f operations: a)  D a r k c u r r e n t i n c r e a s e s t o ~ 16 e 7 p i x e l / s ;  b)  R T S / D a m a g e d pixels remove - 6 % o f pixels f r o m functioning;  c)  C T E is d e g r a d e d to 9 9 . 9 9 9 7 9 9 % ;  d)  a n d R e d u c t i o n o f duty c y c l e to 8 0 % ( S e c t i o n 4.4). K u s c h n i g (2000) has incorporated these values into n u m e r i c a l simulations o f the  M O S T microsatellite. E v e n after r a d i a t i o n d a m a g e c o m b i n e d w i t h a l l other n o i s e sources, the s i m u l a t i o n s s h o w that M O S T w i l l b e able to o b s e r v e oscillations o n the o r d e r o f a f e w p p m ( K u s c h n i g , private c o m m u n i c a t i o n 2000).  6.2 Single Event Effects (SEEs) S E E s differ f r o m ionising a n d b u l k d a m a g e effects because they are n o n cumulative. T h ebroadest definition o f a single event covers all energetic  particle  interactions w i t h a d e v i c e w h i c h cause a n o b s e r v a b l e effect. In general, S E E s are c a u s e d b y a n energy transfer f r o m the c h a r g e d particle to S i l i c o n or S i l i c o n d i o x i d e i n microelectronics (or G a l l i u m A r s e n i d e i n solar panels). T h e s e effects c a n o c c u r i n a n y electronics system or c o m p u t e r device a n d are not limited to the C C D .  Multi-Oxide-  S e m i c o n d u c t o r s ( M O S ) devices are particularly sensitive to s u c h effects, especially f i e l d effect transistors ( M O S F E T s ) w h i c h are a c o m m o n c i r c u i t element. O t h e r parts w h i c h m a y experience upsets i n c l u d e the digital signal processor, solar cells, m e m o r y devices, l o g i c circuits, a n d other sensitive circuit n o d e s . T h e p r i m a r y effects that a c h a r g e d particle c a n cause i n c l u d e ( L a b e l 1997): •  A Single E v e n t U p s e t ( S E U ) occurs w h e n the charged particle causes a bit flip i n a m e m o r y d e v i c e (a binary transition f r o m 0 to 1 o r v i c e versa).  •  A Single H a r d E r r o r ( S H E ) occurs w h e n a S E E causes permanent damage i n one bit o f memory.  90  •  A S i n g l e E v e n t F u n c t i o n a l Interrupt ( S E F I ) o c c u r s w h e n a S E U causes a string o f c o d e to b e read incorrectly, c a u s i n g temporary interruption o f n o r m a l system  •  performance.  A Single E v e n t L a t c h u p ( S E L )occurs i n circuits w h e n the energy deposition c a u s e s a b u r s t i n c u r r e n t c a u s i n g a b u r n - o u t i n t h a t c i r c u i t . It i s p o t e n t i a l l y catastrophic.  •  A S i n g l e E v e n t B u r n o u t ( S E B ) is a h i g h l y l o c a l i z e d S E L w h i c h causes a burnout i n the drain source o f M O S F E T S associated w i t h p o w e r generation.  •  T h e M O S F E T s are also sensitive to S i n g l e E v e n t G a t e R u p t u r e ( S E G R ) w h e n the c h a r g e d particle interacts w i t h a n o x i d e gate layer, c a u s i n g destruction a n d possible failure.  A l t h o u g h it i s p o s s i b l e t o c a l c u l a t e t h e n u m b e r o f i n t e r a c t i o n s a d e v i c e w i l l h a v e p e r d a y , o r p e r o r b i t , it i s n o t p o s s i b l e t o p r e d i c t w h i c h o f t h e a b o v e e f f e c t s it w i l l  cause.  T h e catastrophic effects are less c o m m o n , o n l y b e c a u s e the v o l u m e s o f m a t e r i a l sensitive to that t y p e o f effect are s m a l l . Integrated circuits are also susceptible to S E E s , d e p e n d i n g o n h o w the circuit b o a r d is m a n u f a c t u r e d . D e v i c e s m a n u f a c t u r e d o n b u l k substrate are h i g h l y susceptible to SEUs  b e c a u s e circuit j u n c t i o n s are c o n n e c t e d to the substrate (Johnston 1996). I f the  c i r c u i t i s m a n u f a c t u r e d s u c h that j u n c t i o n i s i s o l a t e d f r o m t h e s u b s t r a t e s o it c a n n o t b u i l d u p e l e c t r o n s g a t h e r e d w i t h i n t h e substrate itself, t h e n it is less s e n s i t i v e to t h e r a d i a t i o n e n v i r o n m e n t . T h i s is a c c o m p l i s h e d u s i n g a n epitaxial l a y e r i n g p r o c e s s to insulate the junctions f r o m the surrounding material. Junctions c a n also b e isolated i n special oxides to p r e v e n t c h a r g e b u i l d u p . T h e s e latter p r o c e s s e s are m o r e e x p e n s i v e t h a n the first, b u t w o r t h the cost. W h e t h e r a n S E E h a p p e n s d e p e n d s b o t h o n the i n c o m i n g particle's e n e r g y a n d o n t h e s u s c e p t i b i l i t y o f t h e d e v i c e it i n t e r a c t s w i t h . T h e n u m b e r o f p r o t o n - i n d u c e d u p s e t s i n a device c a n be calculated using a semi-empirical B e n d e l & Petersen 2-parameter m o d e l ( P e t e r s e n 1996). T h i s m o d e l is appropriate o n l y f o r S i d e v i c e s . I n a s i m i l a r f a s h i o n to t h e radiation e n v i r o n m e n t m o d e l s , the basis o f the relationships w e r e f o u n d a p p l y i n g particle interaction theories, b u t the coefficients a n d overall f o r m u l a e were d e v e l o p e d b y c o m p a r i n g t h e o r y to a c t u a l data.  91  T h e 2 parameters i n the B e n d e l & Petersen m o d e l relate to the cross s e c t i o n f o r proton upset i n the f o l l o w i n g w a y :  4 (6.3)  V  J  w h e r e a is the cross section f o r p r o t o n upsets i n c m / b i t , B a n dA are the B e n d e l 2  parameters i n M e V , a n d E is the particle's energy i n M e V (Petersen 1996). T h e cross section f o r upset, integrated w i t h the energy spectra o f the proton e n v i r o n m e n t yields the n u m b e r o f u p s e t s , o r S E E rate. T h e t e r m ( B / A )  1 4  effectively describe the "limiting cross  s e c t i o n " o f t h e d e v i c e : It i s t h e v a l u e w h i c h b e s t f i t s t h e d e v i c e ' s c r o s s s e c t i o n f o r susceptibility to a particle w i t h infinite energy, o r the m a x i m u m cross section f o r the d e v i c e . T h e s e parameters are e x t r e m e l y d e v i c e - d e p e n d e n t . O n a series o f parts tested b y S t a p o r et a l . ( 1 9 9 0 ) , b o t h A a n d B r a n g e d f r o m a b o u t 5 - 5 0 M e V . H o w e v e r , f o r a g i v e n d e v i c e , A a n d B are w i t h i n a f e w M e V o f e a c h other. I f A w a s l o w , t h e n B w a s a l s o l o w . T h e s t a n d a r d m e t h o d o f testing a part f o r its d u r a b i l i t y i n the c o c k t a i l o f particles a s s o c i a t e d w i t h t h e o r b i t a l e n v i r o n m e n t i s t o p e r f o r m g r o u n d b a s e d tests o n e n g i n e e r i n g grade devices. In order to f i n d the B e n d e l Parameters f o r a g i v e n device, the cross s e c t i o n s a r e f o u n d e x p e r i m e n t a l l y , u s u a l l y b y i r r a d i a t i n g t h e d e v i c e w i t h p r o t o n s at o n l y 1 o r 2 energies (as a c c e l e r a t o r t i m e is e x p e n s i v e ! ) . F r o m t h e c r o s s s e c t i o n , the data are least square fit to the m o d e l a n dA a n dB are f o u n d . A l t e r n a t i v e l y , o n e c a n b u y parts that are specifically designed f o r space a n d have a l o w susceptibility to the environment. N o g r o u n d - b a s e d t e s t i n g h a s y e t b e e n p e r f o r m e d o n M O S T parts. H e n c e , F i g u r e 6.4 s h o w s t h e p r o t o n i n d u c e d S E U rate p e r d a y as a f u n c t i o n o f b o t h A a n d B . S i n c e t h e m a j o r i t y o f tests o n v a r i o u s e l e c t r o n i c s s h o w that A ~ B , t h e S E U rate w i l l l i k e l y b e o n the o r d e r o f 1 x 10" S E U / d a y . O n l y r a d i a t i o n testing c a n c o n f i r m this. 6  92  l.OOE-30 H 0  :  i  1  H-  10  20  30  •  i  =4  40  50  A parameter (MeV)  Figure 6.4 Proton induced single event effect rate as a function of Bendel and Petersen model parameters A and B for the MOST baseline orbit radiation environment.  S i n g l e event effects c a n also b e c a u s e d b y h e a v y ions. S i n c e the nuclear interactions are different, a d i f f e r e n t m o d e l is u s e d to d e s c r i b e the n u m b e r o f S E E s t h e y m a y c a u s e . T h o u g h the f l u x o f h e a v y i o n s is m u c h less t h a n the p r o t o n f l u x , the  particles  are m u c h m o r e penetrating t h r o u g h the m a g n e t o s p h e r e a n d s p a c e c r a f t s h i e l d i n g b e c a u s e o f their h e a v i e r m a s s e s a n d h e n c e , m a y p l a y as large, i f n o t a larger r o l e i n the n u m b e r o f upsets a spacecraft  experiences.  T h i s analysis e m p l o y s the P i c k e l a n d B l a n f o r d m o d e l for h e a v y i o n upset ( P i c k e l 1 9 9 6 ) , s i n c e it is a l r e a d y i n t e g r a t e d i n t o S P A C E R A D I A T I O N 4.0.  T h e m o d e l requires  k n o w l e d g e o f the sensitive r e g i o n o f the s e m i c o n d u c t o r d e v i c e , a n d the f l u x o f the ions that m a y hit the sensitive area. T h e d e v i c e is d e s c r i b e d i n t e r m s o f the sensitive v o l u m e  93 (dxxdyx  dz) and the critical charge (Q). The critical charge is the minimum charge that  must be built up in device in order to cause an effect, such as an SEU. The lower the critical charge, the easier it is to cause an upset. Q is given by 6.4:  Q = {f}LET(dz)  (6.4)  where {f} is a function of the material properties of the device, L E T is the Linear Energy Transfer (LET) threshold (i.e. the minimum energy which the incident particle must have in order to confer any charge to the device through ionisation), and dz is the depth to which the device is sensitive. {/} for Si is 0.0103, for SiC>2 it is 0.00196, and for GaAs it is 0.0177. Again, radiation testing of the device is needed to determine the L E T threshold as well as the sensitive depth. Sensitive volume can be estimated by knowing the structure of the device and making assumptions about its workings (Johnston, 1996). Since radiation testing has not been done on the majority of the M O S T electronics, Figure 6.5 shows the upsets/bit/day for the MOST baseline orbit for a range of sensitive volume as a function of critical charge, and two different volume dimensions. From figure 6.5 it is apparent that there is a plateau in the number of upsets a device experiences at low critical charges. This occurs because the device essentially saturates if exposed to more charge. At the high limit of critical charge, the curves show a very steep drop off. This makes sense, as very few particles will have sufficient energy to actually induce sufficient charge in the device to cause an effect, if the critical charge exceeds about .01 pC.  94  100&08  1.0OBO5  1.00BQ2  l.OOBOl  1.00BO4  Upset OBnj;(pQ  Figure 6.5 Heavy ion  induced upsets for the MOST baseline orbit for a range of sensitive volume as a function of critical charge.  A s the s e n s i t i v e v o l u m e o f the d e v i c e i n c r e a s e s , s o d o e s the u p s e t rate. S e n s i t i v e v o l u m e s a r e l i k e l y n o t c u b i c a l as p r e s e n t e d i n F i g u r e 6 . 5 , b u t w i l l h a v e s o m e s h a p e . T h e d e p t h o f p e n e t r a t i o n o f p a r t i c l e s w i l l d e f i n e t h e d i r e c t i o n dz,  rectangular  so realistically,  dz w i l l b e s m a l l e r t h a n t h e s u r f a c e a r e a o f t h e d e v i c e . T h e h e a v y b l u e l i n e i n d i c a t e s t h i s scenario. T h e results d o not d i f f e r s i g n i f i c a n t l y f r o m a s y m m e t r i c c u b e w i t h the s a m e sensitive v o l u m e .  95  E a c h d e v i c e w h i c h is i n t e g r a t e d i n t o the M O S T m i c r o s a t e l l i t e s h o u l d b e c o m p a r e d to the a b o v e d i a g r a m . T h e f o l l o w i n g q u e s t i o n s a b o u t the d e v i c e s h o u l d b e answered: •  W h a t is the s e n s i t i v e v o l u m e o f the d e v i c e ?  •  W i l l the d e v i c e experience excessive  •  H o w d o e s the d e v i c e affect other  upsets?  systems?  T h e latter q u e s t i o n is p e r h a p s the m o s t i m p o r t a n t ( p r o v i d e d the d e v i c e d o e s e x p e r i e n c e upsets). In c o n s i d e r i n g a n y r i s k factor f o r a space m i s s i o n , there is a  constant  battle b e t w e e n cost, e f f i c i e n c y ( m a s s b u d g e t , d e l i v e r y s c h e d u l e , e t c . . . ) a n d r i s k . W h e n is the r i s k great e n o u g h to warrant s p e n d i n g m o r e m o n e y o n a s p e c i f i c part? T h u s , the d e v i c e s t h e m s e l v e s are not the o n l y i m p o r t a n t p i e c e o f the p u z z l e . H o w t h e y interact w i t h the other systems, a n d w h a t i m p l i c a t i o n s their failure c o u l d h a v e o n other systems  will  mitigate whether or not they should be f l o w n . S i n c e c o m p r e h e n s i v e radiation testing has not b e e n done, a n d device s p e c i f i c a t i o n s are n o t c u r r e n t l y a v a i l a b l e , it is n o t p o s s i b l e to p r o v i d e a f u r t h e r e s t i m a t e the s i n g l e e v e n t rate. O n c e these v a l u e s are k n o w n , a c o m p l e t e a s s e s s m e n t o f the s i n g l e effect effects c a n be m a d e . A l l devices utilised s h o u l d be radiation hardened.  of  96  Chapter 7 : Mitigation of environmental damage T h e radiation e n v i r o n m e n t o f the M O S T microsatellite has b e e n evaluated u s i n g S P A C E R A D I A T I O N 4.0, to f i n d yearly i o n i s i n g doses a n d d i s p l a c e m e n t damage. T h e radiation e n v i r o n m e n t w i l l s l o w l y degrade the C C D detector i n the f o l l o w i n g w a y : a)  dark current w i l l increase b y about 1 e7pixel/s over the course o f o n e year,  b)  C T E w i l l be degraded to 9 9 . 9 9 7 9 9 % f r o m 9 9 . 9 9 9 9 9 % over the course o f one year,  c)  6% o f pixels will be damages or display R T S , and  d)  the detector w i l l experience a flat-band voltage shift o n the order o f a f e w m V p e r year.  O t h e r o n - b o a r d microelectronics w i l l b e susceptible to S i n g l e E v e n t Effects. R a d i a t i o n testing o f sensitive devices a n d circuits is n e e d e d to further quantify the S E E rate. T h e m a j o r i t y o f t h e s e e f f e c t s o c c u r as t h e satellite passes t h r o u g h t h e c h a r g e d particle r i c h r e g i o n o f the S A A . T h u s , the c o m b i n e d effect m a y cause M O S T to e x p e r i e n c e a t e m p o r a r y loss o f data w h i l e i n the densest parts o f the S A A . T h i s r e d u c t i o n i n d u t y c y c l e w i l l c r e a t e a l i a s e s i n t h e d a t a set.  7.1 Recommendations for the M O S T Microsatellite T h e f o l l o w i n g r e c o m m e n d a t i o n s h a v e b e e n m a d e to mitigate the effects o f the radiation environment o n the M O S T a)  microsatellite:  T h e satellite needs a m i n i m u m o f 5 m m o f A l s h i e l d i n g . C u r r e n t d e s i g n h a s about 8 m m o f Invar s h i e l d i n g f r o m the telescope structure itself. E x c e s s i v e s h i e l d i n g m a y i n d u c e s e c o n d a r y r e a c t i o n s w h i c h m a y c a u s e as m u c h d a m a g e as p r i m a r y i n t e r a c t i o n s ( D y e r et a l . 1 9 9 6 ) . T h u s , s h i e l d i n g s h o u l d n o t b e i n c r e a s e d .  b)  C o s m i c r a y h i t s s h o u l d b e r e m o v e d f r o m t h e M O S T d a t a set a n d p i x e l v a l u e s r e p l a c e d b y the m e a n o f the s u r r o u n d i n g p i x e l values, or discarded.  c)  T h e M O S T d a t a r e d u c t i o n a l g o r i t h m s h o u l d i n c l u d e a filter f o r p i x e l s d i s p l a y i n g R a n d o m T e l e g r a p h Signaling ( R T S ) u s i n g the average lifetime i n the high signal  97  g e n e r a t i o n state v s . l o w state a s t h e d i s t i n g u i s h i n g s i g n a l o f R T S a n d d i s c a r d a n y pixels s h o w i n g this effect. d)  T h e b o u n d a r y o f the S A A m e a s u r e d b y the F U S E satellite t e a m s h o u l d b e utilized to filter data taken d u r i n g passage t h r o u g h the S A A . T h e data taken d u r i n g this t i m e s h o u l d b e a n a l y z e d a n d c o m p a r e d to the predictions presented i n this study to further validate the m o d e l s .  e)  T h e satellite s h o u l d n o t b e l a u n c h e d into a h i g h e r altitude orbit d u e to the i n c r e a s e d e x p a n s e o f t h e S A A at h i g h e r a l t i t u d e s .  7.2 Other Asteroseismology Missions M O S T w i l l b e f o l l o w e d b y t w o other space satellite m i s s i o n s also a i m i n g t o study stars t h r o u g h a s t e r o s e i s m o l o g y : C O R O T a n d M O N S . C O R O T h a s a b a s e l i n e o r b i t s i m i l a r to that o f M O S T w i t h a 9 0 0 k m altitude a n d 9 9 . 5 ° i n c l i n a t i o n . T h e p r i m a r y d i f f e r e n c e is t h a t t h e satellite w i l l b e i n a n i n e r t i a l o r b i t s o it c o n s t a n t l y f a c e s o n e h a l f o f t h e s k y . T h e M O N S baseline orbit is a M o l n i y a type orbit, a h i g h l y eccentric (E=0.741) orbit w i t h a s e m i m a j o r access o f 2 6 5 6 0 a n d a n inclination o f 6 3 . 4 ° . M O N S w i l l s p e n d a great deal o f its t i m e f a r f r o m t h e E a r t h a n d s o , w i l l h a v e a c c e s s t o a g r e a t p o r t i o n o f t h e s k y . H o w e v e r , it w i l l a l s o b e o u t s i d e o f g e o m a g n e t i c s h i e l d i n g a n d b e v e r y e x p o s e d t o S o l a r E n e r g e t i c Particles ( S E P s ) . A c o m p a r i s o n o f the dose vs. depth curves f o r the three m i s s i o n s s h o w s that w i t h 5 m m o f s p h e r i c a l A l s h i e l d i n g , M O S T a n d C O R O T w i l l e x p e r i e n c e  about  e q u i v a l e n t d o s e s ( F i g u r e 7.1). M O N S w i l l n e e d t o s h i e l d sensitive c o m p o n e n t s w i t h u p to 10 m m o f A l i n o r d e r t o b r i n g d o w n t h e i r c u m u l a t i v e i o n i s i n g d o s e s t o a l e v e l s a f e f o r most devices.  C O R O T has a curve slightly lower than the M O S T dose vs. depth curve  b e c a u s e the i n c l i n a t i o n o f the o r b i t is s l i g h t l y greater, a n d h e n c e it e x p e r i e n c e s geomagnetic  greater  shielding.  S i n c e M O S T i s t h e f i r s t o f t h e t h r e e m i s s i o n s s c h e d u l e d f o r l a u n c h , it w i l l r e a l l y b e t h e test o f c o n c e p t f o r t h e o t h e r t w o m i s s i o n s . E x p e r i e n c e s f a c e d b y t h e M O S T m i c r o s a t e l l i t e s h o u l d b e u s e d b y the other satellite t e a m s i n p e r f e c t i n g their d e s i g n to withstand the orbital e n v i r o n m e n t i n order to p e r f o r m asteroseismology.  98  Figure 7.1 Ionising doses for the MOST (red), MONS (blue), and COROT (green) satellite missions as a function of spherical Al shielding thickness. Models are identical, for flight epochs in 2002.  7.3 Future Work Although it has been determined that the radiation environment will not impede the sensitivity of the M O S T detector over the course of the baseline mission lifetime, there are other effects due to the radiation environment which should be considered prior to launch. Spacecraft charging is a common phenomenon. Particles hit the satellite, and buildup in metallic reservoirs, a process known as di-electric charging. If the charge becomes sufficiently high, arcing will occur, and parts of the satellite may be permanently damaged. Radiation testing of sensitive components to experimentally determine the proton upset cross sections and thus, Bendel parameters should be undertaken. If accelerator time is deemed too expensive, then an alternate fixed source (such as Cobalt ) can be 60  99 used to simulate the radiation environment. Sensitive volumes of all microelectronics should be considered in order to quantify the Single Event Upset rate. In learning from a previous space satellite with Canadian involvement, FUSE, there were many more SEUs than anticipated. In fact, FUSE must uplink on board command sequences after every pass through the SAA. Since MOST will not have access to as complete a network of ground stations as FUSE, a significant attempt to reduce the SEU rate should be made.  100  References: A l p e r t , M . , 2 0 0 0 , F i r e i n the S k y : S p a c e weather turns gusty as solar activity a p p r o a c h e s its p e a k , S c i e n t i f i c A m e r i c a n , J u l y 2 0 0 0  B a d h w a r , G . D . , O ' N e i l l , M . , 1996, G a l a c t i c C o s m i c R a d i a t i o n M o d e l a n d its A p p l i c a t i o n s , A d v . S p a c e R e s . , V . 17, N o . 2, p p . 7 - 1 7  B a i l e y , P., 1993, R a d i a t i o n D a m a g e Effects i nE E V C C D s , C C D T e c h n i c a l N o t e E E V  12,  limited  Barth, J., 1997, N S R E C Short C o u r s e : A p p l y i n g C o m p u t e r S i m u l a t i o n T o o l s to R a d i a i o n Effects Problems,  1997 I E E E N u c l e a r a n d Space Radiation Effects  Conference  Beaujean, R . , B a r z , S., Jonathal, D . , E n g e , W . , 1996, O n the O r i g i n o f T r a p p e d H e a v y I o n s at L = 1 . 4 - 1 . 6 , A d v . S p a c e R e s . , V . 17, N o . 2, p p . 1 6 7 - 1 7 0  B u z a s i , D . , Catanzarite,  J., Laher, R . , C o n r o w , T . , Shupe, D . , Gautier, T . N . , Ill, K r e i d l ,  T . , Everett, D . , 2000, T h eDetection o f M u l t i m o d a l Oscillations o n a Ursae Majoris, T h e A s t r o p h y s i c a l J o u r n a l , V . 5 3 2 , N o . 2, p p . L 1 3 3 - L 1 3 6  C h a p m a n , S . , B a r t e l s , J . , 1 9 4 0 , G e o m a g n e t i s m , v o l s 1 a n d 2. 1 0 4 9 p p . O x f o r d : O x f o r d University Press, C l a r e n d o n Press  C r o n i n , J . W . , G a i s s e r , T . K . , S w o r d y , S . P . , 1 9 9 7 , C o s m i c R a y s at t h e E n e r g y  Fronteir,  Scientific A m e r i c a n , January 1997  D e m a r q u e , P . , G u e n t h e r , D . B . , 1 9 9 9 , H e l i o s e i s m o l o g y : P r o b i n g t h e i n t e r i o r o f a star, Proc. Natl. A c a d . Sci., V . 96, pp. 5356-5359  D y e r , C . S . , Truscott, P . R . ,Peerless, C . L . , Watson, C . J . , E v a n s , H E . , Knight, P . , C o s b y , M . , U n d e r w o o d , C , C o u s i n s , T . , N o u l t y , 1998, U p d a t e d M e a s u r e m e n t s  from  and C R E D O a n dImplications for Environment a n d Shielding M o d e l s , I E E E  C R E A M Transactions  o n N u c l e a r S c i e n c e , V . 4 5 , N o . 3, p p . 1 5 8 4 - 1 5 8 9  D y e r , C . S . , Truscott, P . R . ,Peerless, C . L . , Watson, C . J . , E v a n s , H . E . , K n i g h t , P., C o s b y , M . , U n d e r w o o d , C , C o u s i n s , T . ,N o u l t y , R . , M a a g , C , 1999, Implications f o r space radiation environment models f r o m C R E M E & C R E D O measurements over h a l f a solar cycle, Radiation Measurements,  V . 30, pp. 569-578  F e y n m a n n , J., A r m s t r o n g , T . P . , D a o - G i b n e r , L . , S i l v e r m a n , S., 1990, N e w Interplanetary P r o t o n F l u e n c e M o d e l , J o u r n a l o f S p a c e c r a f t s , V . 2 7 , N o . 16, p p . 4 0 3 - 4 1 0  F e y n m a n n , J., Spitale, W a n g , 1993, Journal o f G e o p h y s i c a l Research, 13294  V . 98, pp. 13281-  101  Gasser, T . K . , 1990, C o s m i c R a y s a n dParticle Physics, C a m b r i d g e University Press, N e w York  G u s s e n h o v e n , M . S . , M u l l e n , E . G . , 1993, Space R a d i a t i o n Effects Program: A n O v e r v i e w , I E E E T r a n s a c t i o n s o n N u c l e a r S c i e n c e , V . 4 0 , N o . 2, p p . 2 2 1 - 2 2 7  H e d i n , A . E . , 1991, E x t e n s i o n o f the M S I S T h e r m o s p h e r i c M o d e l into the M i d d l e a n d Lower Atmosphere,  J. Geophys. Res., V . 96, p .  1159  H u s t o n , S . L . , K u c k , G . A . , Pfitzer, K . A . , 1998, S o l a r C y c l e V a r i a t i o n o f the l o w - A l t i t u d e T r a p p e d P r o t o n F l u x , A d v . S p a c e R e s . , V . 2 1 , N o . 12, p p . 1 6 2 5 - 1 6 3 4  Jenkins, J . M . , B o r u c k y , W . J . , D u n h a m , E . W . , M c D o n a l d , J.S., 1996, H i g h P r e c i s i o n Photometry w i t h B a c k - I l l u m i n a t e d C C D s , Planets B e y o n d O u r Solar S y s t e m a n d N e x t G e n e r a t i o n S p a c e M i s s i o n s , A S P C o n f e r e n c e Series, conference  proceedings  Johnston, A . , 1996, E f f e c t s i n E l e c t r o n i c D e v i c e s a n d S E E Rates, In S i n g l e E v e n t  Effect  Criticacality Analysis, N A S A , C o d e Q W  K i n g , J . H . , 1974, Solar P r o t o n Fluences f o r 1977-1983 S p a c e M i s s i o n s , Journal o f S p a c e c r a f t a n d R o c k e t s , V . 11, n o . 6, p p . 4 0 1 - 4 0 8  K l e c k e r , B . , 1996, Energetic Particle E n v i r o n m e n t i n N e a r - E a r t h Orbit, A d v a n c e s i n S p a c e R e s e a r c h , V o l 17, N o . 2, p p . 3 7 - 4 5  Krischiunas, K , Sinton, W . , Tholen, D . , Tokunaga, A . , Golisch, W . , Griep, D . , K a m i n s k i , C , Impey, C , Christian, C , 1987, A t m o s p h e r i c Extinction a n d N i g h t - S k y B r i g h t n e s s at M a u n a K e a , P u b l i c a t i o n s o f t h e A s t r o n o m i c a l S o c i e t y o f t h e P a c i f i c , V . 9 9 , pp.  887-894  K u c h n e r M . J . , B r o w n , M . E . , 2000, A Search for Exozodiacal Dust a n dFaint C o m p a n i o n s N e a r Sirius, P r o c y o n , a n d A l t a i r w i t h the N I C M O S C o r o n o g r a p h , astro-ph/0002043  K u s c h n i g , R . , 2000, i n preparation  L a b e l , K , 1996, Single E v e n t Effects Criticality A n a l y s i s , N A S A C o d e Q W  Lauriente, M . , V a m p o l a , A . L . , G o s i e r , K . , 1996, E x p e r i m e n t a l V a l i d a t i o n o f S o u t h A t l a n t i c A n o m a l y M o t i o n U s i n g a T w o - D i m e n s i o n a l C r o s s - C o r r e l a t i o n T e c h n i q u e , in L e m a i r e , J . F . , H e y n d e r i c k x , D . , B a k e r , D . N . (eds), R a d i a t i o n Belts: M o d e l s a n d Standards, G e o p h y s i c a l M o n o g r a p h Series 1997, A m e r i c a n G e o p h y s i c a l U n i o n ,  1996  M a n d e a , M . , M a c m i l l a n , S., B o n d a r , R . , G o l o v k o v , V . , Langlais, B . , L o w e s , F . , O l s e n , N . , Q u i n n , J., Sabaka, T . , 2000, International G e o m a g n e t i c Reference F i e l d - 2 0 0 0 :  102  International A s s o c i a t i o n o f G e o m a g n e t i s m a n dA e r o n o m y ( I A G A ) , D i v i s i o n V , W o r k i n g G r o u p 8, P h y s i c s o f t h e E a r t h a n d P l a n e t a r y I n t e r i o r s , V . 1 2 0 , p p . 3 9 - 4 2  M a t t h e w s , J . M . , 1990, G o o d V i b r a t i o n s f r o m the Stars, N e wScientist, V . 2 7 , p p . 4 6 - 5 0  M a t t h e w s , J . M . , 1997, M O S T P h a s e A report, D y n a c o n D o c u m e n t 9 7  Matthews, J . M . , K u s c h n i g , R . , 2000a, N u m e r i c a l Simulation o f Photometry, Document,  Most  MOST-UBC-0004/002  Matthews, J . M . , Kuschnig, R., 2000b, Science Operations Guide, M o s t Document, MOST-TJBC-0005/7  M c l l w a i n , C a r l E . , 1961, Coordinates f o r M a p p i n g the D i s t r i b u t i o n o f M a g n e t i c a l l y T r a p p e d P a r t i c l e s , J o u r n a l o f G e o p h y s i c a l R e s e a r c h , V . 6 6 , N o . 11  Panasyuk, M . I . , 1996, E m p i r i c a l M o d e l s o f Terrestrial T r a p p e d Radiation, A d v . Space R e s , V . 17, N o . 2, p p . 3 7 - 4 5  R o b b i n s , M . , 2000, T h eRadiation D a m a g e Performance  ofMarconi CCDs,  Technical  Note, M a r c o n i D o c u m e n t , S & C 906/424  Rigault, F., D a l m o n , D . , Arsenault, R . , Thomas, J., L a i , O . , Rouan, D . , Veran, J.P., G i g a n , P . , C r a m p t o n , D . , Flethcher, J . M . , Stilburn, J., 1998, Performance  o f the C a n a d a -  F r a n c e - H a w a i i T e l e s c o p e A d a p t i v e O p t i c s Bonnette, P u b l i c a t i o n s o f the A s t r o n o m i c a l S o c i e t y o f the P a c i f i c , 110: 152-164,  February  T a s s o u l , M . , 1990, A s t r o p h y s i c a l Journal, V . 358, p. 313  T y l k a , A . J . , A d a m s , J.H.Jr., Boberg, P.R., Brownstein, B . , Deitreich, W . , Flueckiger, E . O . , Petersen, E X . , Shea, M . , Smart, D . F . , S m i t h , E . C . , 1997, C R E M E 9 6 : A R e v i s i o n o f the C o s m i c R a y E f f e c t s o n M i c r o - E l e c t r o n i c s C o d e , I E E E T a n s . N u c l . S c i , V . 18, p . 9 4 9 9  V a n A l l e n , J a m e s A . , 1983, O r i g i n s o f M a g n e t o s p h e r i c P h y s i c s , S m i t h s o n i a n Institution Press, W a s h i n g t o n D . C .  V e t t e , J.I., 1966, M o d e l s o f the T r a p p e d R a d i a t i o n E n v i r o n m e n t , V l - 7 , N A S A , Washington  W a l t , M . , 1994, Introduction to G e o m a g n e t i c a l l y T r a p p e d R a d i a t i o n , C a m b r i d g e University Press, C a m b r i d e  Walt, M . , 1996, Source a n dL o s s Processes f o r R a d i a t i o n Belt Processes, i n L e m a i r e , J . F . , H e y n d e r i c k x , D . , B a k e r , D . N . (eds), R a d i a t i o n Belts: M o d e l s a n d Standards, G e o p h y s i c a l M o n o g r a p h 97, A m e r i c a n G e o p h y s i c a l U n i o n , Washington D C  103  W a t s o n , C . J . , D y e r , C . S . , Truscott, P.R., Peerless, C . L . , S i m s , A . J., Barth, J . L . , The L o w Earth Orbit Environment Observed using C R E A M and C R E D O , S p a c e R e s e a r c h , V . 2 1 , N o . , 12, p p 1 6 2 1 - 1 6 2 4  1998,  Advances in  104  Appendix A: Selected MOST Target Stars RA  DEC  hh: m m : ss  deg  mm:  SS  RA  DEC  deg  deg  Proper Motion RA  Mag. DEC  V  B  SolarType Stars Procyon  7  Beta Gem  7  39 20.44 45 21.259  5  14  21.22  114.83517  8.5884167  -0.712  -1.029  0.34  0.74  28  1  36.61  116.33858  28.402542  -0.628  -0.071  1.15  2.15  19  57.2  19  50  37.34  154.98833  31.655583  0.307  -0.152  2.61  3.76  Gam Leo  10  Eta B o o  13  54 41.217  18  24  9.72  208.67174  24.0405  -0.064  -0.363  2.68  3.26  Gam V i r  12  41 41.407  -1  -26  -58.08  190.42253  -7.742  -0.567  0.004  3.65  4.01  A  Bet H e r  16  30 13.465  21  29  23.27  247.5561  28.346958  -0.099  -0.017  2.77  3.71  Bet O p h  17  43 28.398  4  33  54.25  265.86833  12.476042  -0.042  0.159  2.77  3.93  Zet H e r  16  41 18.996  31  35  50.87  250.32915  39.961958  -0.552  0.386  2.81  Eps V i r  13  2 11.454  10  57  32.1  195.54773  24.38375  -0.275  0.017  2.83  3.46 3.77  Subdwarf HD2249 30  0  2  7  27  5  44.9  0.5291667  28.437083  0.841  -0.985  5.75  6.42  HD 76932  8  58  43.01  -16  -8  -8.1  134.67921  -18.03375  0.234  0.214  5.86  6.39  3  55  Ro-Aps 16.4  -12  -5  -55.4  58.818333  -13.480833  -0.062  -0.039  6  6.32  gam E q u  21  10 20.251  10  8  0.99  317.58438  12.004125  0.061  -0.14  4.69  4.95  HD1762 32  18  58  46.82  13  54  26.2  284.69508  26.609167  -0.008  -0.044  5.9  6.14  -0.022  0.015  HR1217  WolfRayets W R 113  18  19  7.22  -11  -37  -58.8  274.78008  -20.495  9.43  9.86  WR128  19  48  8  297.13375  11.025  10.5  10.51  19  3  12 -49  6  W R 123  32.1 59  0  285.99583  -16.25  11.27  11.74  -4  105  Appendix B: Orbital Parameters M O S T Baseline Orbit: •  Altitutde: 800 k m  •  Inclination: 9 8 . 6 °  •  O r b i t a l P e r i o d : 1.86 h o u r s  •  Duration: 1 orbit  •  Date:  12/01/2000  •  Start time: Oh O m i n 0.00s  •  M a g n e t i c F i e l d M o d e l : I G R F 1995  •  Orbital Epoch: 2002  •  C o l a t i t u d e o f the d i p o l e p o l e :  •  L o n g i t u d e o f the dipole pole: - 7 1 . 8 °  •  D i p o l e tilt a n g l e :  10.47°  -25.2°  T I M E (h)  L O N . (°)  LAT.(°)  0  269.6  0  0.02  268.8  0.03  A L T I T U D E  B  L  800  0.217036  1.2049  3.5  800.1  0.227873  1.2336  268  7.1  800.3  0.240166  1.2726  0.05  267.2  10.6  800.6  0.253634  1.3236  0.07  266.4  14.2  801  0.267975  1.3883  0.08  265.6  17.7  801.5  0.282875  1.4694  0.1  264.8  21.3  802.2  0.298027  1.5698  0.12  263.9  24.8  802.9  0.313136  1.6939  0.13  263  28.3  803.7  0.327923  1.8472  0.15  262  31.9  804.6  0.342131  2.0376  0.17  261  35.4  805.5  0.355521  2.275  0.18  259.9  38.9  806.5  0.36787  2.5739  0.2  258.7  42.4  807.4  0.378973  2.9539  0.22  257.4  45.9  808.4  0.388643  3.4432  0.23  256  49.4  809.4  0.396727  4.0824  0.25  254.4  52.8  810.3  0.403119  4.9313  0.27  252.6  56.3  811.2  0.40778  6.0772  0.28  250.4  59.7  812  0.410761  7.6498  0.3  247.9  63.1  812.7  0.412213  9.834  0.32  244.7  66.4  813.4  0.412392  >10  0.33  240.6  69.7  813.9  0.411646  >10  0.35  235.2  72.8  814.3  0.410388  >10  (km)  0.37  227.7  75.8  814.7  0.40905  >10  0.38  216.7  78.4  814.8  0.408027  >10  0.4  200.5  80.4  814.9  0.407625  >10  0.42  178.5  81.4  814.8  0.408015  >10  0.43  155  81  814.5  0.409203  >10  0.45  136.1  79.4  814.2  0.411034  >10  0.47  123.1  76.9  813.6  0.41321  10.0863  0.48  114.3  74.1  813  0.415339  7.845  0.5  108.1  71  812.3  0.416985  6.2161  0.52  103.5  67.8  811.4  0.417721  5.0191  0.53  100  64.5  810.4  0.417171  4.1259  0.55  97.2  61.1  809.4  0.415043  3.4492  0.57  94.9  57.7  808.2  0.411148  2.9286  0.58  93  54.3  807  0.405402  2.523  0.6  91.3  50.8  805.8  0.397825  2.2032  0.62  89.8  47.4  804.6  0.388526  1.9488  0.63  88.4  43.9  803.3  0.377693  1.7452  0.65  87.2  40.4  802.1  0.365581  1.5811  0.67  86.1  36.9  800.9  0.352503  1.449  0.68  85  33.3  799.7  0.338827  1.3427  0.7  84.1  29.8  798.6  0.324976  1.2576  0.72  83.1  26.3  797.5  0.311431  1.1905  0.73  82.2  22.7  796.6  0.298722  1.1387  0.75  81.4  19.2  795.8  0.287411  1.1003  0.77  80.5  15.6  795.1  0.278047  1.0739  0.78  79.7  12.1  794.5  0.271103  1.0585  0.8  78.9  8.5  794.1  0.266888  1.0535  0.82  78.1  5  793.8  0.265479  1.0586  0.83  77.3  1.4  793.7  0.26669  1.0739  0.85  76.5  -2.1  793.7  0.270099  1.1002  0.87  75.8  -5.7  793.9  0.275124  1.1382  0.88  75  -9.2  794.2  0.281127  1.1893  0.9  74.2  -12.8  794.6  0.287504  1.2553  0.92  73.3  -16.3  795.2  0.293757  1.3383  0.93  72.5  -19.9  796  0.299521  1.4412  0.95  71.6  -23.4  796.8  0.304578  1.5674  0.97  70.7  -27  797.8  0.308835  1.7214  0.98  69.8  -30.5  798.8  0.312301  1.9082  1  68.8  -34  800  0.315058  2.1347  1.02  67.7  -37.6  801.1  0.317225  2.4086  107  1.03  66.6  -41.1  802.4  0.31894  2.7403  1.05  65.3  -44.6  803.6  0.320339  3.1415  1.07  64  -48.1  804.9  0.321552  3.6258  1.08  62.4  -51.5  806.1  0.322699  4.2084  1.1  60.7  -55  807.3  0.323889  4.9028  1.12  58.7  -58.4  808.5  0.325223  5.7166  1.13  56.3  -61.8  809.6  0.326784  6.6417  1.15  53.4  -65.2  810.7  0.328629  7.6385  1.17  49.7  -68.5  811.6  0.330772  8.618  1.18  44.9  -71.6  812.5  0.333169  9.4342  1.2  38.3  -74.7  813.2  0.335707  9.9106  1.22  28.7  -77.5  813.8  0.338201  9.9137  1.23  14.7  -79.8  814.3  0.340407  9.4315  1.25  354.6  -81.2  814.6  0.342044  8.5855  1.27  330.9  -81.3  814.8  0.34283  7.5619  1.28  309.9  -80.1  814.9  0.342513  6.5251  1.3  294.8  -77.9  814.8  0.340913  5.5772  1.32  284.6  -75.2  814.6  0.337942  4.7614  1.33  277.6  -72.2  814.3  0.333611  4.0834  1.35  272.5  -69  813.9  0.328033  3.5302  1.37  268.6  -65.8  813.3  0.321395  3.0824  1.38  265.6  -62.4  812.6  0.313929  2.7203  1.4  263.1  -59  811.9  0.305877  2.4266  1.42  261.1  -55.6  811  0.297459  2.1875  1.43  259.3  -52.2  810.2  0.28885  1.991  1.45  257.7  -48.7  809.2  0.280172  1.8285  1.47  256.3  -45.2  808.2  0.271503  1.6934  1.48  255.1  -41.7  807.3  0.262893  1.5804  1.5  253.9  -38.2  806.3  0.254396  1.4856  1.52  252.8  -34.7  805.3  0.246092  1.4061  1.53  251.8  -31.2  804.4  0.238109  1.3396  1.55  250.9  -27.6  803.5  0.230633  1.2845  1.57  250  -24.1  802.7  0.2239  1.2394  1.58  249.1  -20.6  802  0.218193  1.2035  1.6  248.3  -17  801.4  0.21381  1.1758  1.62  247.4  -13.5  800.9  0.211041  1.1558  1.63  246.6  -9.9  800.5  0.210135  1.1436  1.65  245.8  -6.4  800.2  0.211271  1.1387  1.67  245  -2.9  800.1  0.21452  1.1413  1.68  244.3  0.7  800  0.219871  1.1518  .  1.7  243.5  4.2  800  0.227162  1.171  1.72  242.7  7.8  800.3  0.236166  1.1995  1.73  241.9  11.3  800.6  0.246593  1.2383  1.75  241.1  14.9  801  0.258124  1.2888  1.77  240.2  18.4  801.6  0.270451  1.3526  1.78  239.4  22  802.2  0.283289  1.4319  1.8  238.5  25.5  803  0.296393  1.5294  1.82  237.6  29  803.8  0.309547  1.6489  1.83  236.6  32.6  804.7  0.322551  1.7953  1.85  235.6  36.1  805.6  0.335209  1.9746  109  Appendix G: Selected Target Star Dwell Time in the CVZ (Time in weeks) Solar Type Stars:  Inclination Altitude Radius Procyon of C V Z 8.6 10.1 96.0 101.4 2.8 96.2 162.1 12.8 3.6 96.4 221.5 4.3 14.9 16.7 96.6 279.5 4.8 96.8 336.3 18.2 5.3 391.9 19.6 5.7 97.0 446.3 20.8 6.0 97.2 97.4 499.6 22.0 6.4 97.6 551.9 23.0 6.7 97.8 603.2 24.0 7.0 98.0 653.6 24.9 7.2 703.1 98.2 25.7 7.5 26.5 98.4 751.6 7.7 98.6 799.4 . 27.3 7.9 846.3 28.0 98.8 8.1 99.0 892.5 28.7 8.3 99.2 937.9 29.3 8.5 99.4 982.6 29.9 8.7 30.5 99.6 1026.6 8.8 99.8 31.1 1069.9 9.0 100.0 1112.6 31.6 9.2 100.2 1154.7 32.1 9.3 100.4 1196.1 32.6 9.4 33.1 100.6 1237.0 9.6 100.8 1277.3 33.6 9.7 1317.0 34.0 9.8 101.0 101.2 1356.2 34.4 10.0 101.4 101.6 101.8 102.0  1394.9 1433.1 1470.9  34.9 35.3 35.6  1508.1  36.0  Beta Gem 28.4 0.0 0.0 0.0 0.0 0.0  Gam Leo A 31.7 0.0 0.0 0.0 0.0 0.0  Eta Boo 24.0 0.0 0.0 0.0 0.0 1.7  Gam Vir -7.7 0.0 0.0 1.3 2.5 3.2  Bet Her 28.3 0.0 0.0 0.0 0.0 0.0  Bet Oph 12.5 2.3 3.2 3.9 4.5 5.0  Zet Her 40.0 0.0 0.0 0.0 0.0 0.0  Eps Vir 24.4 0.0 0.0 0.0 0.0 1.4  0.0 0.0 1.9 2.9 3.6 4.1 4.6 5.1 5.4 5.8 6.1 6.4 6.7 7.0 7.2 7.5 7.7  0.0 0.0 0.0 0.0 0.7 2.3 3.1 3.7 4.2 4.7 5.1 5.5 5.8 6.1 6.4 6.7  2.8 3.6 4.2 4.7 5.1 5.5 5.9 6.2 6.5 6.8 7.1 7.3  3.7 4.2 4.6 5.0 5.3 5.6 5.9 6.1  0.0 0.0 1.9 2.9 3.6 4.2 4.6 5.1  0.0  7.6 7.8 8.0 8.2 8.4  6.3 6.6 6.8 6.9 7.1 7.3 7.4 7.6 7.7  5.5 5.8 6.1 6.4 6.7 7.0 7.2 7.5  5.5 5.8 6.2 6.5 6.8 7.1 7.4 7.6 7.8 8.1 8.3 8.4 8.6 8.8  2.6 3.4 4.0 4.6 5.0 5.4 5.8 6.1 6.5 6.7 7.0 7.3 7.5 7.7 8.0 8.2  8.6 8.8 8.9  7.9 8.0 8.1  9.1 9.3 9.4  8.2 8.3  7.9 8.1 8.3 8.5 8.7  10.1  8.8  10.2 10.3 10.4  9.0 9.1 9.3  Metal Poor Subdwarfs: Inclination  Altitude  HD224930  HD76932  96.0  101.4  28.4  -18.0  0.0  96.2  0.0  162.1  0.0  0.0  96.4  221.5  0.0  0.0  96.6  279.5  0.0  0.0  96.8  336.3  0.0  0.0  97.0  391.9  0.0  0.0  6.9 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8  9.6 9.7 9.8  8.4 8.5 8.6 8.7  9.0 9.1  . o.o 0.0 0.0 0.0 0.0 0.0 0.0  0.0 0.0 0.0 0.0 0.0 0.9 2.2 2.9  7.7 7.9 8.1  9.3 9.4 9.6  3.5 4.0 4.4  8.3 8.5 8.7  9.7 9.9 10.0  4.8 5.2 5.5  8.8 9.0 9.2  10.1 10.2 10.3 10.4  5.8 6.1 6.3 6.6  9.3  8.4 8.5 8.7 8.9 9.1 9.2 9.4 9.5 9.7 9.8  97.2  446.3  0.0  97.4  499.6  1.8  0.0  97.6  551.9  2.8  0.0  97.8  603.2  3.5  0.0  98.0  653.6  4.1  0.0  98.2  703.1  4.6  98.4  751.6  5.0  98.6  799.4  5.4  1.7  98.8  846.3  5.8  2.3  99.0  892.5  6.1  2.8  99.2  937.9  6.4  3.2  99.4  982.6  6.7  3.5  99.6  1026.6  7.0  3.8  99.8  1069.9  7.2  4.0  100.0  1112.6  7.4  4.2  100.2  1154.7  7.7  4.5  100.4  1196.1  7.9  4.6  100.6  1237.0  8.1  4.8  100.8  1277.3  8.3  5.0  101.0  1317.0  8.5  5.1  101.2  1356.2  8.6  5.3  101.4  1394.9  8.8  5.4  101.6  1433.1  9.0  5.5  101.8  1470.9  9.1  5.7  102.0  1508.1  9.3  5.8  WR113  W R 128  W R 123 -16.3  0.0  0.0 .  0.7  Wolf Rayet Stars: Inclination  Altitude  -20.5  11.0  96.0  101.4  0.0  2.6  0.0  96.2  162.1  0.0  3.4  0.0  96.4  221.5  0.0  4.1  0.0  96.6  279.5  0.0  4.7  0.0  96.8  336.3  0.0  5.1  0.0  97.0  391.9  0.0  5.6  0.0  97.2  446.3  0.0  5.9  0.0  97.4  499.6  0.0  6.3  0.0  97.6  551.9  0.0  6.6  0.0  97.8  603.2  0.0  6.9  0.0  98.0  653.6  0.0  7.2  1.6  98.2  703.1  0.0  7.4  2.3  98.4  751.6  0.0  7.7  2.9  98.6  799.4  0.0  7.9  3.3  98.8  846.3  0.0  8.1  3.6  99.0 99.2 99.4 99.6 99.8 100.0 100.2 100.4 100.6 100.8 101.0 101.2 101.4 101.6 101.8 102.0  892.5 937.9 982.6 1026.6 1069.9 1112.6 1154.7 1196.1 1237.0 1277.3 1317.0 1356.2 1394.9 1433.1 1470.9 1508.1  Ro-Ap stars: Inclination Altitude  -  0.0 0.0 0.5 1.5 2.0 2.4 2.8 3.1 3.3 3.5 3.7 3.9 4.1 4.2 4.4 4.5  HR 1217 -13.5  8.3  3.9  8.5 8.7 8.8 9.0 9.2 9.3 9.5 9.6 9.7 9.9 10.0 10.1 10.2 10.3 10.4  4.2 4.5 4.7 4.9 5.1 5.3 5.5 5.6 5.8 5.9 6.0 6.2 6.3 6.4 6.5  Gam Equ HD176232 12.0 26.6  96.0 96.2 96.4 96.6 96.8 97.0 97.2 97.4  101.4 162.1 221.5 279.5 336.3 391.9 446.3 499.6  0.0 0.0 0.0 0.0 0.0 0.0 0.7 2.0  2.4 3.3 4.0 4.6 5.1  0.0 0.0 0.0 0.0 0.0  5.5 5.9 6.2  0.0 2.2 3.1  97.6 97.8 98.0 98.2 98.4 98.6 98.8 99.0 99.2 99.4 99.6 99.8 100.0 100.2 100.4 100.6  551.9 603.2 653.6 703.1 751.6 799.4 846.3 892.5  2.7 3.2 3.7 4.0  6.6 6.8 7.1 7.4 7.6 7.9 8.1 8.3 8.5 8:6 8.8 9.0 9.2 9.3 9.5 9.6  3.8 4.3 4.8 5.2 5.6 5.9  937.9 982.6 1026.6 1069.9 1112.6 1154.7 1196.1 1237.0  4.4 4.7 4.9 5.2 5.4 5.6 5.8 6.0 6.1 6.3 6.4 6.6  6.3 6.6 6.8 7.1 7.3 7.6 7.8 8.0 8.2 8.4  100.8  1277.3  6.7  101.0  1317.0  101.2  1356.2  101.4  9.7  8.6  6.8  9.9  8.8  7.0  10.0  8.9  1394.9  7.1  10.1  9.1  101.6  1433.1  7.2  10.2  9.2  101.8  1470.9  7.3  10.3  9.4  102.0  1508.1  7.4  10.4  9.5  113  A p p e n d i x D: M a p s o f T r a p p e d P r o t o n s a n d E l e c t r o n s i n the M O S T B a s e l i n e O r b i t  00  Figure A - l Positional plot of trapped proton flux >1.00 MeV. The SAA is clearly the only feature of this environment.  ^  114  Longitude  Figure A-2 Positional plot of trapped proton flux > 10 MeV.  Longitude  Figure A-3 Positional plot of trapped proton flux > 300 MeV. Higher energy protons remain confined to a smaller portion of the radiation belt.  115  Longitude  Figure A-4 Positional plot of trapped electron flux > 1.0 MeV. Bands in high and low latitudes are a result of the outer radiation belt penetrating to lower altitude.  -180  -120  -60  0 Longitude  60  120  160  Figure A-5 Positional plot of trapped electron flux > 5.0 MeV. Highenergy electrons are not found in the SAA.  116  Appendix E : Cumulative Doses for the MOST Microsatellite Description of Radiation Models M i s s i o n Duration  A  B  F  J  P  W  3.15E+07  3.15E+07  3.15E+07  3.15E+07  3.15E+07  3.15E+07  normal  normal  normal  normal  normal AI Cylinder 8  Geomagnetospheric normal Conditions Spacecraft Shielding A l C y l i n d e r  A l Cylinder  A l Cylinder  A l Cylinder  5  5  Ti Cylinder  5  5  Inner Radius (mm)  75  2  75  75  75  Height (mm)  449  47.5  449  75  449  449  33  449  Thickness (mm)  R a d i a l Distance (mm)  0  0  0  A x i a l Distance (mm)  0  123  0  123  0  123  123  0  Solar C y c l e  Solar M a x  Solar M a x  Solar M a x  Solar M i n  Geomagnetic Reference Field  Solar M a x  IGRF2000  123 Solar M a x  IGRF2000  IGRF  IGRF  IGRF  IGRF  Epoch  2000  2002  0  0  Solar Proton M o d e l  0  JPL91  0  JPL91  JPL91  JPL91  Confidence L e v e l  97%  JPL91  JPL91  97%  97%  97%  97%  97%  Heavy Ion/Proton Ionising Dose Trapped Protons (rad/yr Si)  369  422  369  503  Solar Protons (rad/yr Si)  410  516  410  306  410  410  964  Galactic C o s m i c R a d i a t i o n (rad/yr Si)  1.54  231  1.54  1.54  3.65  1.58  1.51  7.14  Electron Ionising Dose Primary Electrons  177  170  177  103  Bremmstrahlung  2620  8.79  11.6  8.79  5.31  Total Electron D o s e  24.2  185.79  8  181.6  185.79  108  2644.2  15.1  T O T A L IONISING D O S E  966.33  1015.14  966.33  1024.65  4125.78  553.61  Displacement Damage Trapped Protons (rad/yr Si)  3.30E-01  1.60E-01  3.30E-01  1.94E-01  Solar Protons (rad/yr Si)  1.85E-01  5.37E-01  1.23E-01  5.37E-01  5.37E-01  1.20E-01  2.68E-01  Galactic C o s m i c Radiation (rad/yr Si)  5.13E-04  7.01E-02  5.13E-04  5.12E-04  1.20E-03  5.12E-04  5.12E-04  TOTAL DISPLACEMENT D O S E (rad/yr Si)  8.68E-01  6.98E-01  8.68E-01  3.15E-01  4.54E-01  1.94E-01  TOTAL DISPLACEMENT D O S E (1 M e V proton equivalents (protons/cm )  8.76E+08  7.05E+08  8.76E+08  3.18E+08  4.58E+08  1.96E+08  2  117  Doses due to SEP as modelled by the C R E M E code Solar Energetic Particle Model Spacecraft Shielding Thickness (mm) Geomagnetospheric Conditions  Ordinary Scenario  1972 Scenario  Composite Worst Case Scenario  A l Cylinder  90% Worst Case Scenario A l Cylinder  A l Cylinder  A l Cylinder  5  5  5  5  normal  normal  normal  normal  Ionising Dose (rad/day Si)  9.03E-01  5.95  318  348  Displacement Dose (rad/day Si)  2.71E-04  1.75E-03  9.69E-02  1.22E-01  Ordinary Scenario A l Cylinder  90% Worst Case Scenario A l Cylinder  1972 Scenario  Composite Worst Case Scenario  A l Cylinder  5  5  5  A l Cylinder 5  stormy  stormy  stormy  stormy  1.14  7.55  402  436  3.42E-04  2.22E-03  1.22E-01  1.50E-01  Ordinary Scenario A l Cylinder  90% Worst Case Scenario A l Cylinder  1972 Scenario  Composite Worst Case Scenario A l Cylinder  5  5  5  5  worst  worst  worst  worst  Solar Energetic Particle Model Spacecraft Shielding Thickness (mm) Geomagnetospheric Conditions Ionising Dose (rad/day Si) Displacement Dose (rad/day Si) Solar Energetic Particle Model Spacecraft Shielding Thickness (mm) Geomagnetospheric Conditions Ionising Dose (rad/day Si) Displacement Dose (rad/day  A l Cylinder  4.18  27.7  1460  1550  1.23E-03  8.03E-03  4.38E-01  5.10E-01  Ordinary Scenario  1972 Scenario  Composite Worst Case Scenario  Ti Cylinder  90% Worst Case Scenario Ti Cylinder  Ti Cylinder  Ti Cylinder  2  2  2  2  normal  normal  normal  normal  Si) Solar Energetic Particle Model Spacecraft Shielding Thickness (mm) Geomagnetospheric Conditions Ionising Dose (rad/day Si) Displacement Dose (rad/day  2.5  13.7  635  665  7.07E-04  3.86E-03  1.83E-01  2.08E-01  Ordinary Scenario A l Cylinder  90% Worst Case Scenario  1972 Scenario  Composite Worst Case Scenario  A l Cylinder  A l Cylinder  A l Cylinder  Si) Solar Energetic Particle Model Spacecraft Shielding Thickness (mm) Geomagnetospheric  8  8  8  8  normal  normal  normal  normal  118  Conditions Ionising Dose (rad/day Si) Displacement Dose (rad/day Si)  5.68E-01 1.76E-04  3.65 1.10E-03  199 6.35E-02  230 8.85E-02  119  Appendix F: Specifications Sheet for Marconi CCD47-20 N o t e : T h i s is the s p e c i f i c a t i o n s sheet f o r the c o m m e r c i a l o f f - t h e - s h e l f m o d e l . T h e  M O S T  C C D is c u s t o m d e s i g n e d as w e e l as c u s t o m p a c k a g e d s o the s p e c i f i c a t i o n s f o u n d h e r e m a y not be representative o f the M O S T  C C D . F o r details o n the M O S T  C C D see  Section  1.3.1.  1 1Q  120  EEV  CCD47-20 High Performance C C D Sensor  FEATURES •  1024 by 1024 1:1 Image Format  •  Image Area 13.3 x 13.3 mm  •  Frame Transfer Operation  •  13 |im Square Pixels  •  Symmetrical Anti-static Gate Protection  •  Very Low Noise Output Amplifiers  •  Gated Dump Drain on Output Register  •  100% Active Area  APPLICATIONS •  Spectroscopy  •  Scientific Imaging  •  Star Tracking  •  Medical Imaging  TYPICAL P E R F O R M A N C E  INTRODUCTION This version of the CCD47-20 is a front-face illuminated, frame transfer C C D sensor with high performance low noise output amplifiers, suitable for use in slow-scan imaging systems. The image area contains a full 1024 by 1024 pixels which are 13 nm square. The output register is split, allowing either or both of the two output amplifiers to be employed, and is provided with a drain and control gate for charge dump purposes. In common with all EEV C C D Sensors, the CCD47-20 is available with a fibre-optic window or taper, a UV coating or a phosphor coating for X-ray detection. Other variants of the CCD47-20 include IMO, back-thinned and full-frame devices. Designers are advised to consult EEV should they be considering using C C D sensors in abnormal environments or if they require customised packaging.  Maximum readout frequency Output responsivity Peak signal Dynamic range (at 20 kHz) . Spectral range Readout noise (at 20 kHz) Q E a t 700nm  .  .  5 MHz 4.5 nV/e~ 120 ke~/pixel ~ 6 0 000:1 4 0 0 - 1100 nm 2.0 e~ rms 45 %  GENERAL D A T A Format Image area Active pixels (H) (V) Pixel size Storage area Pixels (H) (V)  13.3x13.3 1024 1024 13 x 13 13.3 x 13.3 1024 1024  mm  nm mm  Additional pixels are provided in both the image and storage areas for dark reference and over-scanning purposes. Number of output amplifiers Weight (approx, no window)  7.5  2 g  Package Package size Number of pins Inter-pin spacing Window material Type  EEV Limited, Waterhouse Lane, Chelmsford, Essex C M 1 2QU Internet: w w w e e v . c o m  England  Telephone: + 44 (011245 493493  Facsimile: +44 (011245 492492  e-mail: info@eev.com  Holding Company: The General Electric Company, p.I.e. A member of the Marconi Electro-Optics Group.  EEV, Inc. 4 Westchester Plaza, PO Box 1482, Elmsford, NY10523-1482 U S A  ©1998 EEV Limited  22.7 x 42.0 mm 32 2.54 mm quartz or removable glass ceramic DIL array  Telephone: (914) 592-6050  Facsimile: (914) 682-8922  e-mail: info@eevinc.com  A 1 A - C C D 4 7 - 2 0 Issue 4, December 1998 411Z4664  I  2-1  PERFORMANCE Min  80k  Peak charge storage (see note 1) Peak output voltage (no binning) Dark signal at 293 K (see notes 2 and 3) Dynamic range (see note 4) Charge transfer efficiency (see note 5): parallel serial Output amplifier responsivity (see note 3) Readout noise at 243 K (see notes 3 and 6): grade 0 and 1 grade 2 Maximum readout frequency (see note 7) Response non-uniformity (std. deviation) Dark signal non-uniformity (std. deviation) (see notes 3 and 8)  -  Typical  120k 540 10k 60000  -  99.9999 99.9993 4.5  3.0  -  2.0 3.0 5.0 3  _  1000  Max  -  e /pixel mV  20k  e~ /pixel Is  -  % %  6.0  nV/e~  10  rms e~/pixel rms e~/pixel MHz % of mean  2000  e~/pixel/s  4.0 6.0  -  ELECTRICAL INTERFACE CHARACTERISTICS Electrode capacitances (measured at mid-clock level)  S0/S0 interphase 10/10  interphase  10/SS and S0/SS  R0/R0 interphase  R0/(SS + DG + OD1  0R/SS  Output impedance (at typ. operating condition)  Min  Typical  Max  -  3.5 3.5 4.5 40 60 10 300  -  White spots  NOTES 1. Signal level at which resolution begins to degrade. 2. Measured between 233 and 253 K and V +9.0 V. Dark signal at any temperature T (kelvin) may be estimated from: Q /Q = l22T ewhere Q is the dark signal at T = 293 K (20 °C). 3. Test carried out at EEV on all sensors. 4. Dynamic range is the ratio of readout noise to full well capacity measured at 243 K and 20 kHz readout speed. 5. CCD characterisation measurements made using charge generated by X-ray photons of known energy. 6. Measured using a dual-slope integrator technique (i.e. correlated double sampling) with a 20 us integration period. 7. Readout at speeds in excess of 5 MHz into a 15 pF load can be achieved but performance to the parameters given cannot be guaranteed. 8. Measured between 233 and 253 K, excluding white defects. s s  3  d  6400/T  d0  BLEMISH SPECIFICATION  Slipped columns  Pixels where charge is temporarily held. Traps are counted if they have a capacity greater than 200 e " at 243 K. Are counted if they have an amplitude greater than 200 e . Are counted when they have a signal level of less than 90% of the local mean at a signal level of approximately half fullwell. _  Black spots  CCD47-20, page 2  n  Are counted when they have a generationrate 25 times the specified maximum dark signal generation rate (measured between 233 and 253 K). The amplitude of white spots will vary in the same manner as dark current, i.e.: Qc/Qdo = i 2 2 T e A column which contains at least 21 white defects. A column which contains at least 21 black defects. 3  d 0  Traps  nF nF nF pF pF pF  White column Black column  6400/T  GRADE  0  1  2  Column defects: black or slipped white Black spots Traps >200e~ White spots  0 0 15 1 20  2 0 25 2 30  6 0 100 5 50  Grade 5  Devices which are fully functioning, with image quality below that of grade 2, and which may not meet all other performance parameters.  Minimum separation between adjacent black columns  50 pixels  Note The effect of temperature on defects is that traps will be observed less at higher temperatures but more may appear below 233 K. The amplitude of white spots and columns will decrease rapidly with temperature.  ©1998 EEV Limited  12Z TYPICAL O U T P U T CIRCUIT NOISE (Measured using clamp and sample) V  S  = 9 V  S  V  VRQ = 18  Voo =  29  V  TYPICAL S P E C T R A L RESPONSE (No window)  50h  500  600  W A V E L E N G T H (nm)  TYPICAL VARIATION O F D A R K SIGNAL WITH S U B S T R A T E V O L T A G E (Two 1 0 phases held low) 60  7509  50  40  \  V  " 20  0  1  2  SUBSTRATE VOLTAGE V  © 1 9 9 8 EEV Limited  3 s  s  4  6  6  | TYPICAL R A N G E |  7  .  8  9  10  11  (V)  CCD47-20, page 3  TYPICAL VARIATION OF DARK CURRENT WITH TEMPERATURE  P A C K A G E T E M P E R A T U R E ("Cl  DEVICE SCHEMATIC 3 DARK REFERENCE R O W S SS ABD  R01L  1  O  2  0  16  O  017 8 BLANK ELEMENTS  CCD47-20, page 4  R01R  8 BLANK ELEMENTS  ©1998 EEV Limited  CONNECTIONS, TYPICAL V O L T A G E S A N D A B S O L U T E M A X I M U M RATINGS PULSE AMPLITUDE OR PIN  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 •26 27 28 29 30 31 32  REF  DESCRIPTION  SS ABD  Substrate Anti-blooming drain (see note 10) Image area clock Image area clock Image area clock Substrate Output gate Reset transistor drain (left amplifier) No connection Output transistor source (left amplifier) Output transistor drain (left amplifier) Substrate Output reset pulse (left amplifier) Output register clock (left section) Output register clock (left section) Output register clock (left section) Output register clock (right section) Output register clock (right section) Output register clock (right section) Output reset pulse (right amplifier) Substrate Output transistor drain (right amplifier) Output transistor source (right amplifier) No connection Reset transistor drain (right amplifier) Dump gate (see note 12) Substrate Storage area clock Storage area clock Storage area clock Anti-blooming gate  I03  I02 101 SS OG RDL  OSL ODL SS 0RL R03L R02L  R01L R01R R02R R03R  0RR SS ODR OSR  RDR DG SS S01 S02 S03  ABG SS  Substrate  Maximum voltages between pairs of pins: pin 10 (OSL) to pin 11 (ODL) pin 22 (ODR) to pin 23 (OSR) Maximum output transistor current  DC LEVEL (V) (See note 9) Min Typical Max  M A X I M U M RATINGS with respect to V  0  9  -  8 8 8 0 1 15  12 12 12 9 3 17  10  VoD  27 0 8 8 8 8 8 8 8 8 0 27  15 15 15 10 5 19  0 8 8 8 0 0  - 0 . 3 to +25 V ±20V ±20 V ±20 V  ±20 V - 0 . 3 to +25 V  -  -  see note 11 29 9 12 10 10 10 10 10 10 12 9 29 see note 11  - 0 . 3 to +25 V - 0 . 3 to +35 V  31 10 15 15 15 15 15 15 15 15 10 31  15  s s  17 0 9 12 12 12 0 9  ±20 V ±20 V ±20 V ±20 V ±20 V ±20 V ±20 V ±20 V  -0.3 to +35 V -0.3 to +25 V  19 10 15 15 15 5 10  -0.3 to +25 V ±20 V  ±20 V ±20 V ±20 V±20 V  -  ±15 V ±15 V 10 mA  NOTES 9. 10. 11. 12. 13.  Readout register clock pulse low levels +1 V; other clock low levels 0±0.5 V. Drain not incorporated, but bias is still necessary. 3 to 5 V below OD. Connect to ground using a 2 to 5 mA current source or appropriate load resistor (typically 5 to 10 kQ). Non-charge dumping level shown. For operation in charge dumping mode, DG should be pulsed to 12 ± 2 V. All devices will operate at the typical values given. However, some adjustment within the minimum to maximum range may be required for to optimise performance for critical applications. It should be noted that conditions for optimum performance may differ from device to device. 14. With the R 0 connections shown, the device will operate through the left hand output only. In order to operate from both outputs R 0 K R ) and R02(R) should be reversed.  © 1 9 9 8 EEV Limited  CCD47-20, page 5  25 FRAME TRANSFER TIMING DIAGRAM CHARGE COLLECTION  PERIOD  -1 ~77 111 ^ UU  77  77  -77-  -WJUL^UI  77  74.  .a  .a77"  1033  101  J U T 77'  102  J U L 77.  103  77-  77  CYCLES  -IU juir 77 umj ^Lru|irj— m i ^ 7  S02  > 1028  Hi .77 JUL "77  CYCLES  7  J U L 77 J L M W J U I p L L - 7 7 J l # U L # J U L 1 7 7 J L U  # Jill  S03  S E E DETAIL O F F R A M E T R A N S F E R PERIOD  LINE T R A N S F E R  •>1 R01  R02  R03  LINE TIME  Hr  TTL3 ULUL[JLp ^ ' 7 7  3^nnu^Lii|jiijiji^" 3  77 ^ ^ W T 7 D  Vr T j u ^ n n n r j ^ H  UL  fr  .ii.  -77  77-  l  77  nn^nnnn^. u u •77 u u u u 7 7  S E E DETAIL O F O U T P U T CLOCKING  LL 7 7  R E A D O U T PERIOD  DETAIL OF LINE TRANSFER  (For output from a single amplifier) V T,—J 3  \  S01  -toi S02  -tdrl  • tdir -  S03  • U L / U U - A A A /  1 A A J  J U L A  A A A _  1 ± U  L U U L  R02  R03  0R  U l A A  CCD47-20, page 6  (©1998 EEV Limited  DETAIL OF VERTICAL LINE T R A N S F E R (Single line dump) S01  -•  X-J  S02  S03  \  R01  \J\J\ J  R02  r\j\.  R03  _ n _ . DG  - END OF PREVIOUS LINE READOUT  LINE TRANSFER INTO REGISTER  DUMP SINGLE LINE FROM REGISTER TO DUMP DRAIN  LINE TRANSFER INTO REGISTER  START OF LINE READOUT  DETAIL OF VERTICAL LINE TRANSFER (Multiple line dump) S01  S02  S03  R01  R02  A I A /  -  . J U  \J\J\  R03  0R  .JL END OF PREVIOUS LINE READOUT  © 1 9 9 8 EEV Limited  1ST LINE I 2ND LINE  3RD LINE  DUMP MULTIPLE LINE FROM REGISTER TO DUMP DRAIN  CLEAR READOUT REGISTER  LINE TRANSFER INTO REGISTER  START OF LINE READOUT  CCD47-20, page 7  I2  T  DETAIL OF O U T P U T C L O C K I N G  R01  V  R02  R03  A  0R  SIGNAL OUTPUT  OUTPUT VALID OS  RESET FEEDTHROUGH  LINE O U T P U T F O R M A T 15 DARK REFERENCE  1024 ACTIVE O U T P U T S  15 DARK REFERENCE  7S12 8 BLANK  I IIIIII I RECOMMENDED D.C. C L A M P TIME I  = Partially shielded transition elements  C L O C K TIMING REQUIREMENTS Symbol Ti ••wt  tri tfi  *oi tdir 'dri  T  r  trr tfr tor  W trx. tfx 'dx  Description  Image clock Image clock Image clock Image clock Image clock  period pulse width pulse rise time (10 to 90%) pulse fall time (10 to 90%) pulse overlap  Delay time, S0 stop to R0 start Delay time, R0 stop to S0 start Output register clock cycle period Clock pulse rise time (10 to 90%) Clock pulse fall time (10 to 90%) Clock pulse overlap Reset pulse width Reset pulse rise and fall times Delay time, 0R low to R 0 3 low  Min  Typical  Max  2 1 0.1  5 2.5 0.5 0.5 0.5 2 1 1000 0.1T 0.1T, 0.5t  see note 15 see note 15 0.2T 0.2T 0.2T see note 15 see note 15 see note 15 0.3T 0.3T 0.1T 0.3T 0.1T 0.8T  t„  (t,i+t |)/2 f  1 1 200 50  r  t„  20 30 0.2t 30  rr  wx  0.1 TR 0.5t 0.5T rr  r  r  r  r  r  r  r  US US  us us US  us us ns ns ns ns ns ns ns  NOTES 15. No maximum other than that necessary to achieve an acceptable dark signal at the longer readout times. 16. To minimise dark current, two of the 10 clocks should be held low during integration. 10 timing requirements are identical to S 0 (as shown above).  CCD47-20, page 8  £51998 EEV Limited  12$ O U T P U T CIRCUIT RD  Q  R03  0R  0  S02 (SEE NOTE 17)  0  OD Q  I  1  I OG  OS  •A ss  OUTPUT  EXTERNAL LOAD (SEE NOTE 18)  ss  T  0 V  NOTES 17. The amplifier has a DC restoration circuit which is internally activated whenever S 0 2 is high. 18. Not critical; can be a 2 to 5 mA constant current supply or an appropriate load resistor.  ©1998 EEV Limited  CCD47-20, page 9  \2S\ OUTLINE (All dimensions without limits are nominal) A -M-  -H IMAGE AREA  PIN 1 INDICATOR  RECESSED TEMPORARY COVERGLASS IMAGE PLANE  J PITCH -  Ref  Millimetres  A B C D E  42.00 ± 0.42 22.73 ± 0.26 16.60 ± 0.25 3.64 ± 0.37 22.86 ± 0.25 „ + 0.051 0254 „ „ „ - 0.025 5.0 + 0.5 0.457 ± 0.051 2.54 + 0.13 38.1 1.65 + 0.50 13.3 13.3  F G H J K L M N  CCD47-20. page 10  ©1998 EEV Limited  130 ORDERING INFORMATION  HANDLING C C D S E N S O R S  Options include: •  Permanent Quartz Window  •  Temporary Glass Window  CCD sensors, in common with most high performance MOS IC devices, are static sensitive. In certain cases a discharge of static electricity may destroy or irreversibly degrade the device. Accordingly, full antistatic handling precautions should be taken whenever using a CCD sensor or module. These include:-  •  Permanent Glass Window  •  •  Temporary Quartz Window  Working at a fully grounded workbench  •  Fibre-optic Coupling  •  Operator wearing a grounded wrist strap  •  UV Coating  •  All receiving socket pins to be positively grounded  •  X-ray Phosphor Coating  •  Unattended CCDs should not be left out of their conducting foam or socket.  For further information on the performance of these and other options, please contact EEV.  Evidence of incorrect handling will invalidate the warranty. All devices are provided with internal protection circuits to the gate electrodes (pins 3, 4, 5, 7, 13, 14, 15, 16, 17, 18, 19, 20, 26, 28, 29 , 30, 31) but not to the other pins.  HIGH ENERGY RADIATION Device parameters may begin to change if subject to greater than 10 rads. This corresponds to: 10 of 15 MeV neutrons/cm 2 x 10 of 1 MeV gamma/cm 4 x 10 of ionising particles/cm Certain characterisation data are held at EEV. Users planning to use CCDs in a high radiation environment are advised to contact EEV. 4  13  2  ,3  2  11  2  T E M P E R A T U R E LIMITS Min  Typical  Max  Storage . . . . . . . 73 373 K Operating . . . . . . . 73 243 323 K Operation or storage in humid conditions may give rise to ice on the sensor surface on cooling, causing irreversible damage. Maximum device heating/cooling  . . . .  5 K/min  Whilst EEV has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any use thereof and also reserves the right to change the specification of goods without notice. EEV accepts no liability beyond that set out in its standard conditions of sale in respect of infringement of third party patents arising from the use of tubes or other devices in accordance with information contained herein.  © 1 9 9 8 EEV Limited  Printed in England  CCD47-20, page 11  

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