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An investigation of polar substorms observed at Halley Bay, Antarctica Boteler, David H. 1980

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AN INVESTIGATION OF POLAE SUBSTOEMS OBSERVED AT HALLEY BAY, ANTARCTICA by DAVID H..BOTELEE B.Sc, The University of Wales, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE EEQUIEEMENTS FOE THE DEGEEE OF MASTER OF SCIENCE i n THE FACULTY OF GEADUATE STUDIES (Department of Geophysics and Astronomy) We accept this thesis as conforming To the required standard THE UNIVEESIIY OF BEITISH COLUMBIA March 1980 © David H..Boteler 1980 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l 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 an advanced degree at the 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 agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that 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 f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date 31st M*A IW i i Abstract Ground observations of magnetic and ionospheric substorms are reviewed, and the processes involved are explained^ Then magnetic and absorption r e s u l t s from an auroral zone station, Halley Bay, are examined*.Positive H bays occur i n the evening sector between 16.00 and 22.00 l o c a l time while negative H bays occur on the nightside between 22.00 and 06.00 l o c a l time. Clockwise and counterclockwise rotating Pi2 pulsations respectively have s i m i l a r times cf occurrence, as do westward and eastward moving radio aurora. Short-period absorption occurs coincident with magnetic bays while lcnger-period absorption occuxs on the dayside (04.00 to 20.00 l o c a l time), This information i s used to determine the disturbance pattern i n l o c a l time seen at auroral zone st a t i o n s . A simple model i s presented to i l l u s t r a t e the processes responsible for the e f f e c t s observed i n the 3 d i f f e r e n t l o c a l -time zones,. On the nightside there i s d i r e c t i n j e c t i o n of p a r t i c l e s from the t a i l , into the ionosphere, accompanied by s h o r t - c i r c u i t i n g of the c r o s s - t a i l current through the ionosphere to produce the westward e l e c t r o j e t . Protons are injected into the radiation b e l t on the evening side, and their westward d r i f t constitutes a p a r t i a l ring current. The plasmasphere bulge modifies the d i s t r i b u t i o n of proton p r e c i p i t a t i o n and influences the location of the ionospheric return current, the eastward e l e c t r o j e t . .Trapped electrons d r i f t into the morning and dayside sectors and prec i p i t a t e steadily into the ionosphere producing periods of slowly-varying absorption, The diurnal and seasonal variations of t h i s absorption at Halley Bay suggests that photodetachment of negative ions i n the D region i s a s i g n i f i c a n t factor i n the production of auroral absorption. I t i s shown that variations i n f x l s i g n i f i c a n t l y a f f e c t the occurrence of blackout on the ionoscnde, making t h i s an unreliable indicator of absorption. Anomalies i n the magnetic bays observed at Halley Bay are shown to be due to the presence of induced currents i n the sea flowing p a r a l l e l to the continental shelf. An analysis of e l e c t r i c a l power system disturbances i d e n t i f i e s magnetic substorms as the primary cause, and discusses how knowledge about substorms can aid prediction of the power system disturbances. . i v CONTENTS 1. .. I NTRODUCTION .............. ............. i . . . . . . . . . . 1 1.1. What i s a substorm? ........ ... 3 1.2- Halley bay . ..„....- .. 4 1,3. Outline of the thesis ............................. 7 2. MAGNETIC SUBST0R.1S ...„.......-..--» 10 2. U _ Ground obs. ..of magnetic bays and pulsations .... . 10 2.2. Equivalent current systems ...... .,.„,....... 14 2.3. Latitude p r o f i l e of substorms ., » „ , , , . 1 9 C i r c u i t diagrams of the substorm current system ........ 24 3. IONOSPHERIC SUBSTORMS ., »,,,,...,i ... 28 3.1..Morphology of auroral absorption . . . . . ; . . . . . . . i i . . . 30 3.2. Response of the ionosphere to prec i p i t a t i o n 34 3.4. P r e c i p i t a t i o n mechanisms 36 4. .MAGNETIC OBSERVATIONS AT HALLEY BAY- ......... ........... 39 4.1. Magnetic bay r e s u l t s .............................. 39 4.2. Pi2 pulsation results 49 4.3. Correlation with other phenomena .................. 54 5. ABSORPTION RESULTS FROM HALLEY BAY 57 5.1..Rapidly varying absorption ........................ 59 5.2, Slowly varying absorption .........................61 5.3. Comparison with IGY r e s u l t s .. 64 5.4..Correlation with VLF emissions .....................67 V 6. DISCUSSION 68 6.1. Local time disturbance pattern i 69 6.1.1. Magnetic disturbances ........................ 69 6.1.2- Ionospheric disturbances ...................... 70 6.1;3A Overall picture of substorm effects .......... 72 6.2. The cause of the eastward e l e c t r o j e t .............. 76 6.2.1. Association with p a r t i a l ring current ,. .,.1...^ 76 6.2. 2..Evening bulge i n the plasmapause . . . . . i . . . . . . . 78 Proposed mechanism for the eastward e l e c t r o j e t ...... 79 6.3,. Role of photodetachment i n auroral a b s o r p t i o n . ^ . . 80 D region chemistry .................................. 80 6.3.2. Halley bay re s u l t s ........................... 81 6.3.3i .Production of auroral absorption .......... 82 7. CONCLUSIONS ..... 83 REFERENCES ........... 85 APPENDIX A: THE EFFECT OF FXI ON BLACKOUT OCCURRENCE (submitted To J. Atmos. Terr,; Phys; , Oct., 1979) ....... 95 APPENDIX B: THE EFFECT OF INDUCED CUBRENTS (pub. i n J . Atmos. Terr; Phys., May, 1978) ............. 106 APPENDIX C: THE PROBLEM OF SOLAR INDUCED CURRENTS (pres. „ at I. S.T. P. _ Workshop, Boulder, A p r i l , 1979) 117 v i Acknowledgements The author would l i k e to thank Prof T. Watanabe for his advice and encouragement. Valuable c r i t i c i s m s of the manuscript wer made by Prof G. K. C. . Clarke and Prof R,. Mi. . E l l i s . Many useful discussions were also had with Dr J.R. Dudeney and Mr A.S, Rodger. The author i s also gra t e f u l to the B r i t i s h Antarctic Survey f o r the opportunity to c o l l e c t the Halley Bay data, and for permission to publish the results;. vxa PREFACE Geomagnetic storms have been the cause of disturbances on man-made systems since the l a s t century (see review by Lanzerotti, 1978), but technological improvements over the years have reduced the impact of geomagnetic storms on most of the systems.. The e l e c t r i c power d i s t r i b u t i o n network, however, with the tendency toward higher voltage lines and more i n t e r -connected systems, i s becoming more, not l e s s , susceptable to disturbance during geomagnetic storms (see Appendix C on 'The Problem of Solar Induced Currents'). Thus there i s an urgent p r a c t i c a l need for a greater understanding of the geomagnetic disturbances observed on the ground. This thesis presents an analysis of ground observations of polar substorms, one of the fundamental components of a geomagnetic storm. 1 Is. - INTRODUCTION This thesis has i t s o r i g i n s when the author was running the ionospheric programme at Halley Bay, a geophysics observatory i n the Antarctic..Curiosity about the disturbances seen on the records prompted comparison between the magnetometer, ionosonde and riometer records; which revealed that disturbances often occurred simultaneously on a l l the records. The disturbances were accompanied by bright auroral displays and usually also meant a loss of radio contact with the outside world. A l l these apparently unrelated phenomena ( i e . magnetic disturbances, radio wave absorption, and aurora) are different aspects of polar substorms, and their cause can be traced back to an explosive process (calle d a magnetospheric sutstorm) in the t a i l region of the earth's magnetic f i e l d . . The opportunity to study i n Canada, with Dr. T. Watanabe at U.B»C., allowed the author to further investigate polar substorms. Substorms occupy a c e n t r a l position i n space physics as they are the fundamental process by which energy from the solar wind i s transfered i n t o the earth's atmosphere. Also investigations of the substorm processes i n the earth's magnetosphere are at the f r o n t i e r s of research i n plasma physics,; Substorms also have considerable e f f e c t s on the ground; an i n v e s t i g a t i o n of e l e c t r i c a l power system problems i d e n t i f i e d magnetic substorms as the source of the disturbances; also ionospheric substorms are responsible for disruptions i n H.F. radio communications. Many theories have been proposed to account f o r magnetic and ionospheric substorms, but over the years a generally 2 accepted picture of substorm processes has evolved. The purpose of t h i s t h e s i s i s to test t h i s substorm picture against the data collected by the author at Halley Bay. Shortcomings i n the present substorm picture, revealed by t h i s study, are then investigated and new theories suggested. 3 ls.1 What Is A Substorm? Geomagnetic storms have c e r t a i n recognised features, such as a depression of the f i e l d strength at low l a t i t u d e s l a s t i n g several hours, and shorter (approx 1 hour) changes i n the f i e l d strength at high latitudes.„ The l a t t e r feature was termed a "polar magnetic substorm" by Chapman (1935) and i s now recognised as the fundamental component of a magnetic storm. Ionospheric disturbances accompanying these magnetic substorms have been s i m i l a r l y l a b e l l e d ionospheric substorms, and are characterised by anomalous ionization i n the lower ionosphere which i s responsible for increased absorption of high frequency radio waves. I t i s believed that when the interplanetary magnetic f i e l d (IMF) turns southward there i s a greater interaction between the solar wind and the magnetosphere. The energy accumulated i n the magnetosphere as a r e s u l t seems to be released i n a succession of explosive processes (termed magnetospheric substorms) involving acceleration of p a r t i c l e s in the magnetospheric t a i l towards the earth. These p a r t i c l e s injected from the t a i l are guided by the f i e l d l i n e s and precipitate, d i r e c t l y or i n d i r e c t l y , into the ionosphere i n the north and south auroral zones. This p r e c i p i t a t i o n gives r i s e to the magnetic and ionospheric e f f e c t s that constitute polar substorms. 4 1.2. Halley Bay Halley Bay i s situated on the Brunt Ice Shelf on the coast of the Weddell Sea (f i g 1) . The sta t i o n was established by the Royal Society as a geophysics observatory for the International Geophysical Year (IGY, 1957/8) and i s now run by the B r i t i s h Antarctic Survey,. H a l l e y / Bay p 1 / 270 / / 0 ^ \ / \ — - X \ / \ ) \ H \ \ \ \ \ 180 + G M \ \ 9 0 . Pole / \ 1 ^ J> / J Dip / J -J P o l e ^ / / / < z \ y / / Figw 1. Map of Antarctica showing the location of Halley Bay, together with the mean position of the austral auroral zone as reported by Bond and Jacka (1960, 1962). Co-ordinates are geographic longitude. 5 The c h a r a c t e r i s t i c s of ground observations of polar substorms are determined by the d i s t r i b u t i o n of the p r e c i p i t a t i o n from the magnetosphere and by the c h a r a c t e r i s t i c s of the ionosphere and i t s response to that p r e c i p i t a t i o n . The p a r t i c l e p r e c i p i t a t i o n occurs along the auroral zone (which i s roughly centred around the invariant pole) and so has a dependence on invariant latitude* Ionospheric c h a r a c t e r i s t i c s , however, are cont r o l l e d by the i n t e n s i t y of incident solar radiation and so exhibit a dependence on geographic l a t i t u d e . At guiet times the auroral oval i s south (poleward) of Halley Bay, however during disturbed periods the auroral oval moves eguatorward and passes over the station. This makes the station an i d e a l place for studying storm e f f e c t s along the auroral oval and for examining phenomena with an auroral zone d i s t r i b u t i o n . Also Halley Bay i s at a uniquely high geographic lati t u d e f o r an auroral zone sta t i o n (ie. compared to i t s invariant l a t i t u d e - see table 1). Thus i t i s p a r t i c u l a r l y suited to d i f f e r e n t i a t e between phenomena with a geographic and an in v a r i a n t latitude dependence and hence determine whether they are controlled by solar radiation or by magnetospheric effects* . 6 Geographic Latitude 75.5° South Longitude 333.4° East Geomagnetic Latitude 65.8° South Longitude 24.6° East L - s h e l l value at 400km a l t i t u d e 4.6 L-s h e l l value at surface 4.2 Invariant Latitude (Surface) 60.9° Geocentric Dip -64.3° Local zone or W30 mean time, LZT = UT.- 2hours Local magnetic time (July 15, 1972), MLT = UT - 3hours ± 20min, Table 1.. Co-ordinates of Halley Bay. 7 1.3. Outline Of The Thesis This thesis deals with two aspects of polar substorms: magnetic substorms, and the ionospheric substorms characterised by auroral absorption. F i r s t , to i l l u s t r a t e the presently accepted substorm picture, a survey i s made of key results presented by other authors* ^Chapter 2 i s concerned with magnetic substorms and presents a summary of ground based observations and explains how these can be used to determine the d i s t r i b u t i o n of e l e c t r i c currents responsible. This provides a f u l l description of the s p a t i a l c h a r a c t e r i s t i c s of the magnetic disturbances. The e l e c t r i c - c i r c u i t approach of Bostrom (1972) i s used to explain the temporal variations of the disturbances. Chapter 3 describes the morphology of the auroral absorption and considers reports of a day/night asymmetry i n the r a t i o of absorption to the flux of precipitated electrons responsible. Theories of the p r e c i p i t a t i o n mechanism involved are also considered* The next two chapters feature an analysis made by the author of substorm features observed at Halley Bay, p r i n c i p a l l y using data personally c o l l e c t e d ty the author. Chapter 4 presents a detailed analysis of magnetic bays and Pi2 pulsations observed at Halley Bay, and comparison i s also made with observations of radio aurora..These phenomena occur during night hours and a l l shew di f f e r e n t behaviour before and af t e r 22.00 l o c a l time. This i s shown to be i n agreement with the picture of eastward and westward e l e c t r o j e t s seperated by the Harang discontinuity. Chapter 5 d e t a i l s the absorption results from Halley Bay* including both the r a p i d l y varying absorption that 8 i s coincident with magnetic bays, and the slowly varying absorption that occurs on the dayside* The r e s u l t s support the view of Hartz and Brice (1967) that seperate classes of 'splash' and ' d r i z z l e ' p r e c i p i t a t i o n are responsible for the absorption observed. The Discussion i n chapter 6 examines the substorm picture developed by other authors by using the Halley Bay r e s u l t s obtained by the author.. As a re s u l t of t h i s a new unified picture of substorm features i s developed. Comparison between the Halley Bay results and the 'prevailing' substorm picture presented i n chapters 2 and 3 reveal two areas of confusion: the cause of the eastward e l e c t r o j e t and the role of photodetachment with regard to auroral absorption. Both these points are examined and possible explanations of these features are proposed^ In the course of t h i s study several important r e s u l t s have been established, which have been submitted or accepted for publication, and are contained i n appendices. A comparison between the riometer and ionosonde techniques of measuring absorption (Appendix A) shows that variations in f x l seriously affect the occurrence of blackout on the ionosonde, making t h i s an unreliable indicator of enhanced absorption. An e a r l i e r study (Appendix B) of the azimuth of disturbing vectors responsible for the magnetic bays at Halley Bay revealed the presence of induced currents i n the sea, and f o r the f i r s t time demonstrated t h e i r e f f e c t on magnetic bays i n the D component*. A t o p i c a l review of substorm effects on e l e c t r i c power 9 systems i s presented i n Appendix C, and the factors a f f e c t i n g the severity of power system disturbances are discussed* Knowledge about substorm c h a r a c t e r i s t i c s i s used to suggest ways of predicting power system disturbances. 10 2-. MAGNETIC SUBSTORMS J i J i Ground Observations Of Magnetic Bays And Pulsations A magnetic bay i s simply a departure of the magnetic f i e l d strength from i t s quiet time l e v e l and i s generally accepted as the signature of a magnetic substorm. Magnetic bays are usually characterised by an increase (or decrease) of the f i e l d strength to a peak (or trough) i n 15 to 30 minutes, followed by a more gradual decrease (or increase). Magnetic bays are observed at night (22.00 to 06.00 l o c a l time) and i n the H component have a positive sign at mid l a t i t u d e s and a negative sign at high l a t i t u d e s . In addition positive H bays are sometimes observed at high l a t i t u d e s in the evening sector (16.00 to 22.00 l o c a l time). The signs of bays in the Z component i n the midnight-morning sector are generally p o s i t i v e (downward) north of the auroral oval and negative to the south, with the signs reversed in the evening sector. These features are well documented (Akasofu, 1968) and are attributed to a l i n e current i n the ionosphere flowing along the auroral oval, westward i n the midnight-morning sector, eastward i n the evening sector. However bays i n the D component are not so easy to explain and a consistent picture of t h e i r behaviour i s yet to emerge. In a s t a t i s t i c a l study of bays observed at Scandinavian s t a t i o n s , Harang(1946) showed that negative bays i n H were usually accompanied by an eastward d e f l e c t i o n i n D, while l a t e r workers (eg Akasofu and Meng, 1969) found cases where the D deflection was to the east i n the l a t e evening sector but tc the west i n the morning sector* 11 MIDLATITUDE H-COMPONENT S U B S T O R M S I G N A T U R E S 0 0 0 1 . 1 . — — ^ D o w n ^ •—> 1 1 1 • s — k N o o n \ V. t3?9 Dusk \ / ^ - ^ 6 3 4 ^ i i i 1 1 1 I 1 I I I  I I I - 4 - 3 - 2 ) 0 1 2 3 4 O N S E T TIME (Hours) AURORAL ZONE H-COMPONENT I i — i — i 1 1 1 — i — i 30 GAMMA I I I I 1 I I I I - 4 - 3 - 2 - 1 0 1 2 3 4 O N S E T T I M E (HOURS) F i g . 2. Substorm signatures i n the H component obtained by Caan et a l (1978) : a) at midlatitudes, b) i n the auroral zone 12 I d e n t i f i c a t i o n of a substorm from magnetic records by using the c r i t e r i a above i s s t i l l rather subjective. However recently Caan et a l (1978) , using a computer pattern-recognition program, analysed 1800 substorm events and determined the s t a t i s t i c a l magnetic signature for d i f f e r e n t l a t i t u d e s and l o c a l times. They i d e n t i f i e d the time of a substorm onset by the occurrence of a positive H bay at a mid-latitude s t a t i o n , within t 5 hours of station l o c a l midnight. For the onset times so determined, the magnetic records from many locations were analysed and, with a superposed-epoch technique, used to determine c h a r a c t e r i s t i c substorm signatures f o r d i f f e r e n t l o c a l times- The substorm signature i n the mid-latitude H component is- shown i n f i g 2a while that i n the auroral zone H component i s shown i n f i g 2b, Shorter period variations than magnetic bays are termed pulsations* Pulsations are c l a s s i f i e d as continuous (Pc) or i r r e g u l a r (Pi) and each group i s further subdivided according to frequency as shown in table 2 (see Jacobs, 1970, f o r further d e t a i l s ) . . Pi2 pulsations occur p r i n c i p a l l y i n the night-time auroral zone and frequently occur coincident with magnetic bays (see f i g 8) sc w i l l be included i n t h i s i n v e s t i g a t i o n . .The observed polarization c h a r a c t e r i s t i c s indicate that " Pi2 pulsations cannot be described by a simple equivalent current system such as that used to describe a bay, Such a current system provides an in-phase r e l a t i o n s h i p between the H and D components which i s not observed to be the case (Jacobs 1970). The dominant rotation, counterclockwise i n the north and clockwise i n the south, suggests that Pi2 are subject to propagation effects and they are commonly attributed to an odd-13 mode o s c i l l a t i o n of dipole f i e l d l i n e s . Such an o s c i l l a t i o n would give a conjugate r e l a t i o n s h i p in which the H variations were i n phase and the 0 variat i o n s were out of phase. However such a relationship was also found by Campbell (1971) for the magnetic bays observed at the conjugate stations Great Whale River and Byrd. Period (sec) Magnitudes (#) Pel 0.2 - 5 0,05 - 0.1 Pc 2 5 10 0* 1 - 1 Pc3 10 - 45 0.1 - 1 Pc 4 45 - 150 0. 1 - 1 Pc5 150 - 600 1 - 10 Pi1 1 45 0.01 . - 0.1 Pi 2 45 - 150 1 - 5 Table 2. C l a s s i f i c a t i o n of continuous pulsations (Pc) and i r r e g u l a r pulsations ( P i ) , (after Jacobs, 1970). 14 2. 2 . , Equivalent Current Systems Extensive studies have been made of the pattern of magnetic bays in order to determine the complete current system responsible.(see review paper by Andersen and Vondrak, 1975), To examine the d i s t r i b u t i o n of a magnetic disturbance i t i s more convenient to represent magnetic bays by an equivalent disturbing vector which represents the amplitude and orientation of the t o t a l disturbance of the component bays. The observed d i s t r i b u t i o n of disturbing vectors i s then often >expressed i n terms of an equivalent current system i n the ionosphere ( f i g 3) which, as Akasofu (1968) pointed out, i s a mathematically correct method of representation. This should not be confused, however, with models of the actual current system f o r , as Chapman (1935) stated, i t i s not possible to determine uniquely the current system from ground observations alone-The equivalent current system shows an intense westward current along the auroral oval, c a l l e d the auroral e l e c t r o j e t , together with return currents spread eguatorward of t h i s and over the polar cap.„ (Some authors have suggested that the current system should include an eastward e l e c t r o j e t , to the west of the main e l e c t r o j e t , to account f o r bays i n the evening sector.) Such a configuration was postulated for the actual current system by Chapman (1935) and Vestine and Chapman (1938). An al t e r n a t i v e current system featuring f i e l d - a l i g n e d currents was suggested by Birkeland (1908) and Alfven (1939), (see f i g 4) . Fig* . 3. The d i s t r i b u t i o n of the magnetic disturbance vectors and the equivalent current l i n e s which corresponds to the averaqe f o r many bays (Silsbee and Vestine, 1942). 16 Fig. 4. Current systems proposed to account f o r polar magnetic substorms..Note that the currents occur simultaneously i n both hemispheres,, a) shows the B i r k e l a n d — Alfven system which features f i e l d - a l i g n e d currents, b) shows the purely ionospheric Chapman - Vestine current system. The two current systems were compared by Fukushima (1969) i The Birkeland-Alfven system can be considered as the superposition of the Chapman-Vestine system and two v e r t i c a l currents each combined with horizontal spreading. .Fukushima has shown that a v e r t i c a l current in the magnetosphere combined with horizontal spreading produces no magnetic effect below the ionosphere. In consequence the Birkeland-Alfven and Chapman-Vestine systems ars. equivalent i n terms of the magnetic bays they produce on the ground. 17 More. recently, s a t e l l i t e observations (Heikkila and Winnington, 1971; McPherron et a l , 1973; Yasuhara et a l , 1975) have provided evidence that the actual current system involves f i e l d - a l i g n e d currents. I t has also been shown (Zmuda and Armstrong, 1974) that downward and upward f i e l d - a l i g n e d currents always appear as a pair on opposite sides of the auroral oval. If the emf i s of magnetospheric o r i g i n the westward e l e c t r o j e t can be explained as a H a l l current, along the auroral oval, set up as the r e s u l t of a strong eguatorward e l e c t r i c f i e l d {and Pederson current) across the auroral oval (fig 5) • A l t e r n a t i v e l y an ionospheric emf can be used to account f o r the f i e l d aligned currents* Fukushima (1971) demonstrated that a westward e l e c t r i c f i e l d wculd produce a, primary Hall current with a pattern s i m i l a r to that of the Sg system. When th i s i s combined with enhanced conductivity along the auroral oval there i s a space charge accumulation along the edges of the auroral oval. The r e s u l t i n g e l e c t r i c f i e l d would produce a secondary Hall current that would flow westward along the auroral oval and return via the ionosphere. . Pederson currents across the oval and f i e l d aligned currents would be produced as a by-product and would represent leakage of the space charge* Such a current system i s represented i n f i g 6 (after Fukushima, 1971) and Fukushima emphasised that the systems involving magnetospheric or ionospheric emfs produce the same magnetic disturbance pattern on the ground. However they produce d i f f e r e n t magnetic e f f e c t s above the ionosphere so i t w i l l be possible to i d e n t i f y the correct current system using s a t e l l i t e observations. 18 al Q, CO O 6 E jB-a. (A o c o Conductivity "a—0) E M F of magnetospheric o r i g i n 00^.01—0 0|>02 R Pedersen current V V V V E M F of ionospheric or ig in cro, a\t (72^=0 Meridional cross-section of the three-dimensional current flow and t o r o i d a l magnetic f i e l d when the electromotive force i s of magnetospheric o r i g i n (left) and of ionospheric o r i g i n (right), after Fukushima (1971) Toroidal magnetic f i e l d i n and above the auroral zone ionosphere associated with the meridianal f i e l d -aligned current (after Fukushima, 1971). 19 2.3,.Latitude P r o f i l e Of Substorms Much useful information about the auroral e l e c t r o j e t can be obtained from simultaneous magnetic observations at a chain of stations, on a magnetic meridian, extending across the auroral zone. Such a chain in Canada has been extensively used for e l e c t r o j e t studies (eg Kisabeth and Eostoker, 1977) and this section w i l l examine some of the r e s u l t s obtained. However i n i t i a l l y i t i s useful to consider the t h e o r e t i c a l l a t i t u d e p r o f i l e ( f i g 7) computed by Kisabeth (1972) for a current system featuring a westward e l e c t r o j e t connected to f i e l d - a l i g n e d currents as shown in f i g 4a. , 500 Y LATITUDE Fig..7. Theoretical l a t i t u d e p r o f i l e of the magnetic disturbance i n the H, D, and Z components, produced by a three-dimensional current system featuring a westward e l e c t r o j e t connected to f i e l d - a l i g n e d currents. (Kisabeth, 1972). 20 Fig 7 shows the c h a r a c t e r i s t i c substorm features: a negative H disturbance at auroral l a t i t u d e s with a positive H disturbance at lower l a t i t u d e s and i n the polar cap; and a reversal of the Z disturbance immediately below the e l e c t r o j e t . Kisabeth (1972) also reported that the D bay configuration i s determined by whether the meridian of the l a t i t u d e p r o f i l e i s east or west of the central meridian of the current system. Magnetic records, from the Canadian chain, for a substorm on 14th July 1970 are presented i n f i g 8a which shows the magnetic bays and f i g 8b which shows the simultaneous occurrence of Pi2 pulsations. Latitude p r o f i l e s of the three components for d i f f e r e n t stages of the substorm are shown i n f i g 9. Fig 9c shows l a t i t u d e p r o f i l e s s i m i l a r to the t h e o r e t i c a l p r o f i l e of f i g 7, however f i g 9d shows a noticeably d i f f e r e n t p r o f i l e and examination of other r e s u l t s showed that there are considerable dynamic changes to the e l e c t r o j e t system within the course of a t y p i c a l substorm. 21 UJ z Q_ O (_) Q 0715 07*4 RSLT CflMB FTRL FTSM FTCH FTMU \ * CRLK MEAN LEDU CRLG NWPT CRLK MERN LEDU CRLG NWPT LU o Q_ 21 O CJ LU o Q_ O (_) RSLT CRMB FTRL FTSM FTCH FTMU J . CRLK MERN LEDU CRLG NWPT RSLT CRMB FTRL FTSM FTCH FTMU J » . CRLK MERN LEDU CRLG NWPT RSLT CRMB FTRL FTSM FTCH FTMU J « i CRLK MEAN LEDU CRLG NWPT Fig..8. Magnetic bays and Pi2 pulsations recorded by the Canadian chain of magnetometers during a substorm on July 2, 1970 (Kisabeth and Rostoker, 1977).. 22 F i g . .9. Latitude p r o f i l e s for 4 times during the substorm on July 2, 1970, shown i n f i g 8. 23 The application of inversion theory to the e l e c t r o j e t problem by Oldenburg (1976,1977) has provided a more sophisticated technique for i n t e r p r e t i n g magnetic records from a chain of stations* Solution of the inverse problem involves the manipulation of a suite of ground-lased data to i n f e r ionospheric and magnetospheric current densities within the framework.of a s p e c i f i c curren geometry* Inversion of magnetic variations during a substorm on 15th July 1970 by Oldenburg (1976), assuming the current system shown i n f i g 6a, indicated that the magnitude of the westward current density was at l e a s t 0.90±0.15 A m_1 and that the current density could not be approximated by a constant value over widths much greater than 1 degree of l a t i t u d e . Inversion techniques were also used to analyse the magnetic f i e l d values at the Canadian chain of stations during quiet times. A study by Hughes et a l (1978) showed that eastward and westward e l e c t r o j e t s ex i s t i n the evening and morning sectors respectively even when no substorm a c t i v i t y could be detected. This i s of considerable s i g n i f i c a n c e f o r , as Hughes et a l pointed out, i t implies that s i m i l a r current systems e x i s t both as a steady state feature and a disturbed feature which wculd indicate that the substorm i s a perturbation of a pre-existing current pattern.. 24 2.4. C i r c u i t Diagrams Of The Substoim C crrent S ystem (a) 7 M o g n e l o p o u s e 1 Bn ->~ Bn -> F i e l d l i n e = > S o l a r w i n d A U R O R A ( c ) A U R O R A PLASMA MAGNIETOTAIL CIRCUIT A U R O R A L O V A L C I R C U I T 0 <-Positive particle Negative particle Plasma motion Electric current Solar wind - Magnetosphere Dynamo Magnetotail current Polar ionosphere Fig. 10*. Schematic diagram showing the magnetospheric processes and current systems that give rise to the e l e c t r o j e t i n the auroral oval, and consequently magnetic substorms. 25 The primary energy source for magnetospheric processes, including the substorm current system, i s the inte r a c t i o n between the solar wind and the flanks of the magnetosphere- This i n t e r a c t i o n can be considered as a dynamo which drives a current across the central region of the magnetotail (see f i g 10). During substorms an increase i n the resistance of the magnetotail c i r c u i t causes the current to flow down f i e l d l i n e s , around the auroral oval, and back up f i e l d l i n e s to the t a i l . Falthammar and Bcstrom (Bostrom,1972) developed a c i r c u i t f o r the substorm current system defined e n t i r e l y by lumped c i r c u i t components, as shown i n f i g 11a. Energy i s stored i n the magnetospheric t a i l i n the form of magnetic energy and as k i n e t i c energy of plasma motion;. The magnetic energy can be d i r e c t l y represented by an inductance while the k i n e t i c energy is analogous tc the energy stored in a capacitor. The magnetotail current i s estimated to be about 106A so to account for the energies released during substorms ( t y p i c a l l y 1 0 1 3 to 10* * Joules) would require an L value of 100 Henrys which would give a stored energy value, 0.5LI 2, of 5 x 10*3 Joules- The k i n e t i c energy of the plasma sheet has been estimated at 2 x 10 9 Joules and to represent t h i s energy by a capacitor (where E = 0,i5CVZ) charged to 3 x 10 v o l t s would require a capacitance of 4 Farads. 26 E=104-105V Solar wind Tail Neutral sheet R,*0.03n instability Magnetosphere Birkeland currents . Ionosphere LflOOH I«1CTA 4 2 u ' C « 4 F L 2*50H A / W V 1- L,1 J «5«10 1 3 J T, = L , /R ,«50 min CV « 2 x)0 J X =R2C=0.4sec x 2 = L 2 / R 2 « 8 m i n 1000 2000 Time,, sec 3000 a) equivalent c i r c u i t f o r the substorm current system. b) time variations of the current to the ionosphere (Bostrcm, 1972) . 27 At the onset of a substorm the t a i l resistance i s postulated to increase so that the current must flow through the ionospheric r e s i s t o r and the inductance due to the increased dimensions of the circuit*.Bostrom (1968) suggested values of 0.1 ohm and 50 hehrys for these components which would give a time constant for the growth of the e l e c t r o j e t current, L/B, of 8 minutes which i s consistent with the observed rapid onset of bays. The capacitance of 4 farads has l i t t l e e f f e c t on the gross development of the e l e c t r o j e t current, tut combined with the L and L of the e l e c t r o j e t c i r c u i t causes o s c i l l a t i o n s with a period of about 70 seconds. This value i s t y p i c a l of the period of Pi2 pulsations which frequently accompany magnetic bays* The decay of the e l e c t r o j e t current i s associated, by Bostrom (1972), with the time constant of the magnetotail c i r c u i t , L/R«*50 minutes. The temporal variations of the current through the ionosphere' r e s i s t o r , caused by a sudden increase i n t a i l resistance, are shown i n f i g 11b. An actual e l e c t r o j e t with such temporal c h a r a c t e r i s t i c s would c e r t a i n l y be consistent with most ground magnetic observations during substorms. 28 i - . IONOSPHERIC SUBSTORMS Abnormal absorption can be subdivided into Polar-Cap Absorption (PCA), Sudden Cosmic Noise Absorption (SCNA) and Auroral Absorption (AA) (Hultquist, 1966) . PCA events cover the polar cap as well as auroral l a t i t u d e s and are produced by energetic protons (Bailey, 1964) . SCNA events extend to lower latit u d e s and the magnitude of the absorption i s related to the solar zenith angle (Holt, 1963).. Both SCNA and PCA events are seen rarely i n sunspot minimum years (Hultquist, 1966; Piggott and Hurst, 1976) and are disregarded i n t h i s study which i s concerned with auroral absorption. It i s generally accepted that auroral absorption i s produced by electron p r e c i p i t a t i o n . The incoming electrons also produce bremsstrahlung X rays which themselves produce io n i z a t i o n and hence absorption* This contribution to the t o t a l absorption, however was found by Hultquist (1966) to be an order of magnitude l e s s than that due to electrons with E<5keV and as thi s i s i t s e l f l e s s than the absorption produced by higher energy electrons, the bremsstrahlung—produced absorption can be regarded as negligable* . Observations of bremsstrahlung X ray energy can, however, be used to provide information about the energy of the electron p r e c i p i t a t i o n ; and using such measurements Bewersdorff et a l (1966) showed that the average energy cf precipitated electrons i s between 30keV and 50keV..This agrees with the re s u l t s of J e l l y et a l (1964) who found a good cor r e l a t i o n between the d i s t r i b u t i o n of auroral absorption and electron fluxes of E>40keV. These electrons are of higher energy than those 29 producing vis u a l aurora (Ansari, 1964) and so w i l l penetrate deeper in t o the ionosphere than the height at which aurora are usually seen (100km or above)*. Bailey (1957) has shown that electrons with energies of 30-50keV w i l l produce i o n i z a t i o n between 80 and 100km. Rocket measurements of electron density during polar substorms at Syowa Station* Antarctica, show s i g n i f i c a n t i o nization down to 70km ( f i g 12 a f t e r Nagata et a l . 1975) . HEIGHT km 120h 100 80 \ .i 60! 1 ELECTRON DENSITY cm .-3 Fig. 12. The v e r t i c a l p r o f i l e s of electron density above Syowa sta t i o n during 4 polar substorms (Nagata et a l , 1975). 30 3. 1..Morphology Of Auroral Absorption On the nightside, auroral absorption extends to lower latit u d e s than the auroral arcs and e l e c t r o j e t with which i t i s associated (Nagata et a l , 1975), but i t s temporal v a r i a t i o n i s the same as that of magnetic bays. The extent of the absorption i s shown by results from the- Canadian chain of stations, reported by J e l l y (1970), (fig 13). Dayside absorption, however has d i f f e r e n t c h a r a c t e r i s t i c s which w i l l be outlined i n t h i s section. The d i s t r i b u t i o n of absorption recorded at a north-south chain of stations i n Canada had a maximum at 08.00 geomagnetic time (Hartz et a l , 1963). Comparison with similar r e s u l t s from Norwegian stations shows that the two patterns were comparable i n geomagnetic time but not i n l o c a l time. The seasonal variation of absorption, that Hultquist (1966) quotes as t y p i c a l of auroral zone stations, features a maximum in winter and minimum i n summer..However the seasonal v a r i a t i o n of auroral blackout, reported by C o l l i n s et a l (1961) shows a peak occurrence i n the equinoxes at a l l geomagnetic l a t i t u d e s up to 80°. An examination of the i n d i v i d u a l stations i n the work of Hartz et a l (1963) shows that at Ottawa and Val d'Or most absorption occurred i n the autumn whereas at the higher l a t i t u d e stations. Cape Jones and C h u r c h i l l , absorption was most freguent in the winter. Thus reports of the diurnal v a r i a t i o n of absorption are reasonably consistent but, as yet, there i s no cl e a r picture of the seasonal v a r i a t i o n . 31 / R E S O I U T C BAY A - 1 4 * . l»17 C O M l X A M O I I I A - 7 1 ' , 1 ' 1 7 r i O I I S N E I A - 7 5 - . L - 1 S \—Y CHIt lCHILL A - 7 0 * . l - l . l C I C A T WHALE I IVE I A - t r . I -7 .0 j C A F E JOKES A - 1 7 ' . l - i . l I HOOSOXEE A ' S 4 * , l - 9 . 2 ) » A l D'OI A > I I ' , 1-4.1 OTTAWA A ' S I M O . I 00 I I Fig- - 13. - Absorption recorded by the Canadian north-south chain of riometers during a substorm on May 15, 1964 ( J e l l y , 1970) . 32 A complete survey of absorption produced by substorms at stations round the whole of the northern auroral zone was made by Berkey et a l (1974).. They produced synoptic maps of absorption for 60 substorms from IQSY (1964-1965) and IASY (1969) and found that i n the majority of cases the absorption region expanded eastward during a substorm, as shown i n f i g 14. The rate of expansion , on average, corresponded to the d r i f t v e l ocity of electrons with energies 50-300 keV, but the v a r i a b i l i t y of the expansion rate suggested that other processes, apart from simple l o n g i t u d i n a l d r i f t , might be involved. As well as the eastward motion, i n about half the substorms examined, Berkey et a l also found evidence of a simultaneous westward motion of the absorption region into the evening sector.. This work f i n a l l y confirmed that dayside absorption was related to polar substorms. However the observed variations i n dayside absorption cannot be explained simply by p r e c i p i t a t i o n due to scattering of d r i f t i n g electrons; and Berkey et a l (1974) proposed that either changes i n the response of the ionosphere to p r e c i p i t a t i o n must occur, or that an independent p r e c i p i t a t i o n mechanism i s involved,. 33 T.con F i g . 14* The average temporal development of the auroral absorption associated with 60 substorms studied by Berkey et a l (1974),. The coordinate system i s one of corrected geomagnetic l a t i t u d e and time. 34 Ji2s. Besponse Of The Ionosphere To Pre c i p i t a t i o n Measurements of auroral absorption and the causative precipitated f l u x of electrons with E>40keV responsible f o r i t have been compared by J e l l y et a l (1964). They derived an average relationship between the two and found that the same flux produced twice as much absorption during the day as at night. That i s In daytime A = 2.6 . 10-3 . j i / z In night-time A = 1. 3 . 10- 3 . J i / 2 where A i s absorption i n dB and J i s electron f l u x i n cm~2 s e c - 1 s t e r a d - 1 . . T h i s day/night asymmetry may be due to either differences i n the energy spectrum of the precipitated electrons or changes i n the ionosphere. . Evidence for a va r i a t i o n with l o c a l time i n the precipitated electron spectra was fcund by Bewersdorff (1966) from measurements of the energy of bresstrahlung X rays. Differences of the electron spectrum have also been reported by Parks et a l (1968). Changes i n the ionosphere have been indicated from models of the D region proposed by various authors, eg Bailey (1957), Zmuda and Potemra (1972).. According to these models a s i g n i f i c a n t process i s the loss of electrons by attachment to neutrals to form negative ions. During the night t h i s reduces the electron density but during the day the electron density i s restored because there i s photodetachment of the electrons by sunlight: t h i s w i l l a f f e c t absorption values; and riometer measurements during PCA events have indicated day-night r a t i o s between 3 and 6 (Eeid, 1969),. Ionospheric changes were also 35 suggested by Saito et a l (1974) as an explanation f o r the day/night asymmetry of absorption at Syowa. However Hultquist (1966) claims that photodetachment i s cegligable f o r auroral absorption and Berkey et a l (1974), i n t h e i r study of auroral absorption, found no e f f e c t produced by the absorption region moving between the day and night sectors, so the problem i s s t i l l c ontroversial. 36 3.4, P r e c i p i t a t i o n Mechanisms The electrons involved in p r e c i p i t a t i o n into the dayside ionosphere a r r i v e by l o n g i t u d i n a l d r i f t from the nightside. Direct measurements during substorms of electrons with energy>30keV have been made by the ESEO 1A s a t e l l i t e (Smith and Thomasj 1976) which showed that counts of trapped electrons have a maximum coincident with maximum magnetic substorm a c t i v i t y , on the nightside, but, on the dayside, have a maximum 1-2 hours l a t e r than maximum substorm a c t i v i t y * Trapped electrons s p i r a l round f i e l d l i n e s and bounce back and forth from one end of the f i e l d l i n e to the other, the mirror height depending, i n t e r a l i a , on their pitch angle. Hence c o l l i s i o n s of electrons which a l t e r t h e i r pitch angle can'result i n t h e i r p r e c i p i t a t i o n (pitch angle d i f f u s i o n ) , Other processes which a l t e r the pitch angle of the electrons w i l l also r e s u l t i n th e i r p r e c i p i t a t i o n , tor instance electron cyclotron i n s t a b i l i t y . This involves a resonance e f f e c t whereby k i n e t i c energy of the electrons i s transferred to wave energy which can be detected as VLF emissions, Only energy from the transverse component of the electron v e l o c i t y i s transferred so the process r e s u l t s i n a change of the electron pitch angle with the consequent increased p r o b a b i l i t y of pr e c i p i t a t i o n (see Kennel and Petschek, 1966, f o r rigorous treatment of electron cyclotron i n s t a b i l i t y ) . . Pulsations are also believed to be related to p r e c i p i t a t i o n of txapped electrons since s a t e l l i t e measurements have shown that f l u c t u a t i o n s in electron counts i n the radiatio n b e l t often coincide with certain types of micropulsation..McPherron et a l 37 (1968) found that, i n the l o c a l time period 02*00 to 10.00, Pi1 pulsations correlated with 5-10 second period fluctuations of electrons with energy>15keV*.In the l o c a l time period 10.00 15.00, Pc3 pulsations correlated with 20-40 second period fluctuations of electrons with energy > 30keV. When micropulsations and electron fluctuations occur simultaneously s i g n i f i c a n t energy i s present i n the common frequency band. McPherron et a l (1968) concluded that a causal r e l a t i o n s h i p between the pulsations and the modulated electron p r e c i p i t a t i o n was a d e f i n i t e p o s s i b i l i t y . They argued that due to the more continuous nature of the pulsations these would have the fundamental r o l e in such a r e l a t i o n s h i p . More recently, Southwood (1974) has shown t h e o r e t i c a l l y that pulsations can be generated by the Kelvin Helmholtz i n s t a b i l i t y at the surface of the magnetosphere.* These pulsations set up o s c i l l a t i o n s of the f i e l d l i n e s inside the magnetopause and these o s c i l l a t i o n s propagate down the f i e l d l i n e s . These waves are r e f l e c t e d back at the lower boundary of the magnetosphere and when the f i e l d l i n e path length i s an exact multiple of the micropulsation wavelength a resonance condition occurs. Normally the energy f o r resonance i s fed i n from the i n s t a b i l i t y at the magnetopause but the energy can also be derived from trapped electrons* Such coupling of electrons to f i e l d l i n e o s c i l l a t i o n s may r e s u l t i n p r e c i p i t a t i o n of the electrons into the ionosphere and consequent production of enhanced absorption.. 38 Also, Sato(1965) found a c o r r e l a t i o n between fluctuations in absorption at College, i n the northern auroral zone, and Pc5 pulsations observed at both College and Macquarie Island, i n the southern auroral zone ( f i g 15)-RIOMETER RECORD OF 27.6-MC COSMIC NOISE 1 ' ' lOOr MACQUARIE ISLANO ; 501 2100 2115 2130 2145 2200UT Fig; 15. . Auroral absorption at College, Alaska, and Pc5 pulsations recorded at College and Macquarie Island i n the southern auroral zone (Sato, 1965) . 39 4. MAGNETIC OBSERVATIONS AT HALLEY BAY This section presents an analysis of the magnetic bays noted by the author at Halley Bay during 1974 and 1975.. Comparison i s also made with magnetic disturbances at Halley Bay during the IGY, reported by MacDowell and Blackie (1961)« Data on Pi2 pulsations i s also presented (after Hamilton, • 1979) together with a comparison made by Green and Hamilton (1978) of Pi2 pulsations observed at Halley Bay and i t s conjugate point, St. Anthony* Also i icluded i s a study of radio aurora at Halley Bay by Shipstone (1S7 1).. 4, 1. Magnetic Bay Results Magnetic bay disturbances were i d e n t i f i e d from the recordings of a fluxgate magnetometer* The only c r i t e r i a for selection of bays from amongst other disturbances were whether the bay was clear enough fo r a s t a r t time to be determined as well as an amplitude and duration measured. In ccnseguence there is a bias i n the r e s u l t s towards bays recorded during magnetically guiet conditions.. Measurements were made of the magnitude of the disturbance i n the H, D and Z components AH, AD and AZ, and the results grouped according to the sign of the disturbance (see table 3). Of 294 bays studied 222 are contained i n group A f o r which t y p i c a l values of disturbance are AH = 365*", AZ = 2758 and &D = 260*. This group of bays occurred between 22.00 and 06.00 l o c a l time, ie roughly centred about magnetic midnight (01.00 l o c a l time) whilst bays i n group B occurred between 16.00 and 21.00 l o c a l time (fig 16). Bays i n group C occurred during Group Sign of Disturbance Percent of Total H Z D a) - + + 71*1 + 4.4 + + 0 16.3 c) 1.7 6l5 a) + + + + o 0 Table 3._ Freguency of magnetic bays i n 1974-75, c l a s s i f i e d according to the sign of the H and Z disturbances* A t o t a l of 294 bays are included, recorded between January 1974 and Decenber 1975. both the evening and night-time periods, though those with a negative bay i n the D component occurred p r i n c i p a l l y i n the evening period, whilst those with a positive D bay occurred i n the night-time period.. Each magnetic bay disturbance was analysed to define the angle of azimuth (tan AD/4H) and the angle of elevation (tan AZ/AH) of an equivalent disturbing vector. 41 ~ \ — i — i — i — I — l — i — i — r - i — i — i — i — i — i — i — i — i — i — i — r 30 20 Neg H bays Pos Z bays CO L U o ,0 L U a: o O o o -z. 0 10 0 Pos H bays Neg" Z bays Neg H bays Neg Z bays L O C A L TIME OF O C C U R R E N C E S Fig. .16. The diurnal variation of occurrence of magnetic bays at Halley Bay i n 1974-75. 42 Bays are produced by the auroral e l e c t r o j e t and the magnetic f i e l d i t sets up i s analogous to that produced by a current i n a long wire*.Thus examination of the orientation of the disturbing vectors for each group of bays allows d e t a i l s of the eguivalent current systems responsible to be determined™ For group A the disturbing vectors are inclined downwards towards the south of the station and would be produced by a westward e l e c t r o j e t to the south of the s t a t i o n . The disturbing vectors of group B would be produced by an eastward e l e c t r o j e t , also to the south of the sta t i o n . . I t i s notable that the disturbing vectors of groups A and B are each limited to a narrow range of azimuth and elevation.. Disturbing vectors in group C however have two d i s t i n c t l y d i f f e r e n t azimuths; one which occurs i n the morning sector and the other i n the night-time sector » Those i n the night-time sector point south-east, the same as those i n group A and likewise are produced by a westward e l e c t r o j e t , The angle of elevation however indicates that i n these (few) cases the e l e c t r o j e t i s to the north of the station. The disturbing vectors of group C that occur in the evening sector are presumably due to an eastward e l e c t r o j e t . Referring to the th e o r e t i c a l latitude p r o f i l e ( f i g 7) of Kisabeth (1972) would suggest that the station i s a long way from the e l e c t r o j e t such that the H bays are produced by f i e l d - a l i g n e d currents. These extreme cases of group c though account for only 1 percent of bays-For groups A and B the orient a t i o n of the disturbing vector i s used to define the azimuth from Halley Bay at which the mid-point of the e l e c t r o j e t would be observed. Comparison with the 8 0 45 This i s unexpected since, according to Akasofu (1968), the e l e c t r o j e t flows along the auroral oval where the auroral arcs are situated* A separate study (Appendix B), made of the azimuths of disturbing vectors and auroral arcs has shown that the r e s u l t s are consistent with induced currents flowing i n the sea, adjacent to the station. The i n t e n s i t y of such currents i s dependent on the depth of the sea and so near to the coast they are deflected to follow the contours of the sea f l o o r . These induced currents modify the magnetic f i e l d s of the e l e c t r o j e t to give the azimuths of the disturbing vector that are observed. The elevation of the disturbing vectors i s closer to horizontal than would be expected f o r an e l e c t r o j e t located at the s i t e of vi s u a l aurora ( f i g 17). induced currents i n the sea would have the effect of rotating the disturbing vector closer to the v e r t i c a l so cannot account for t h i s anomaly. Other induced currents may exist deep i n the earth but i t i s not possible to assess t h e i r e f f e c t s on the disturbing vector. These induced currents may be responsible for the apparent difference between the elevation of auroral arcs and the e l e c t r o j e t . A l t e r n a t i v e l y t h i s difference may a r i s e because the e l e c t r o j e t does i n f a c t l i e equatorward of v i s u a l aurora, although t h i s i s contrary to the results of e a r l i e r studies; 46 A study of magnetic bay a c t i v i t y was made by MacDowell and Blackie (1961) at Halley Bay during the IGY (1957-58). This covered sunspot maximum, i n contrast to the 1974-75 study which is at sunspot minimum. 525 bays were observed in the 20 month period from May 1957 to December 1958, compared with 297 bays from January 1974 to December 1975. Comparison of chart records from the two periods show that the same phenomena are being classed as bays and t h i s i s confirmed by the breakdown of resu l t s shown i n table 4. Types A and B correspond to the same cl a s s of bays as i n the 1974-75 r e s u l t s and occur at the same l o c a l times* For types A and B the bays i n D were usually positive and negative respectively at sunspot maximum, consistent with the findings at sunspot minimum. However as no magnitudes were given for the bays i t was not possible to compare the disturbing vectors from the two periods. . Disturbing vectors f o r i n t e r n a t i o n a l disturbed days during 1957-58 were plotted by MacDowell and Elackie and interpreted i n terms of a l i n e current i n the ionosphere, to the south of the station. The magnitude of the disturbing vector, throughout the day, and the elevation and azimuth from Halley Bay of the centre of a l i n e current are shown i n f i g 18. These elevation and azimuth angles are t y p i c a l l y 15° greater than the angles of elevation and azimuth of an equivalent l i n e current shown i n f i g 17.. On the IGY disturbed days the occurrence of a ring current could possibly increase the apparent angles of elevation and azimuth of a westward e l e c t r o j e t * However, with an eastward e l e c t r o j e t the effect of a rin g curxentfcould be to decrease the apparent angles of elevation and azimuth. 47 Group Sign of Disturbance Percent of Total H Z D a) + + 54.4 + 15. 8 b) + - + 0.6 + 13.6 c) - - + 1.9 . - 6*4 d) + + + + + -3. 8 3.5 Table 4. Frequency of Bay-like phenomena i n 1957-58, c l a s s i f i e d according to the sign of the H and Z disturbances. A t o t a l of 525 phenomena recorded between May 1957 and December 1958 are c l a s s i f i e d . 48 F i g . . 18... Magnetic disturbance on in t e r n a t i o n a l disturbed days in winter 1957 and 1958 represented as l i n e currents (after MacDowell and Blackie, 1961) a) magnitude of t o t a l vector change from disturbed days daily mean b) azimuth from Halley Bay of centre of l i n e current c) elevation from Halley Bay of centre of l i n e current 49 4.2. Pi2 Pulsation Results Hamilton (1979) tabulated pulsations recorded at Halley Bay using a Rubidium Vapour Magnetometer, i n 1975 and 1976. The normal d e f i n i t i o n of Pi2 was relaxed so that i r r e g u l a r pulsations with periods (from 45 seconds) up to 240 seconds have been accepted as Pi2. The period of the Pi2 pulsations was found to be dependent on the time of occurrence and on the l e v e l of magnetic a c t i v i t y , The period of Pi2 steadily decreases from evening, through midnight, to morning; and for any p a r t i c u l a r l o c a l time i s found to decrease l i n e a r l y with the K index of magnetic a c t i v i t y . This i s presented i n table 5, Pi2 pulsations occur at night with a d i s t r i b u t i o n roughly centred around l o c a l midnight, Analysis of the polarization c h a r a c t e r i s t i c s of Pi2 showed that clockwise rotation of the Pi2 disturbing vector i n the horizontal plane occured predominantly before l o c a l midnight while counterclockwise rotation occurs after l o c a l midnight (fig 19). The times of occurrence of clockwise and counterclockwise Pi2 are very s i m i l a r to the times of occurrence of positive and negative H bays respectively. . 50 20. 15 21.45 23.15 00,45 02. 15 03.45 05- 15 06.4f 0 215 175 150 135 126 120 119 118 1 180 155 134 118 108 103 100 98 2 163 137 116 101 93 85 81 78 3 145 120 100 85 74 67 63 59 4 128 103 83 68 58 51 45 40 5 105 85 67 53 43 Table 5. Typi c a l values of the period (in seconds) of Pi2 for va rious values of the index of magnetic a c t i v i t y (K) i n di f f e r e n t 90 minute i n t e r v a l s centred around the l o c a l times shown. Local time = UT - 2 hours* K i s the sum of the three values of Kp i n the i n t e r v a l 18.00 03.00 UT. 51 U.T. 18 00 06 10 CO LU CJ «Z LU 0 or 8 io o o o 0 T — T — i — i — i — i — i — i — ( — i — i — i — i — I — i — i — I — I — r Counter-clockwise Pi2 12 ~ t — i — r 12 Clockwise Pi2 J I I l I— L _l I I I I 1 L J I I L 18 00 06 12 LOCAL TIME OF OCCURRENCES f i g . 19,. The diurnal variation of occurrence of Pi2 pulsations at Halley Bay i n 1976, Plots of the azimuth of the pola r i z a t i o n e l l i p s e , from the data presented by Hamilton, in f i g s 20a and 20b showed a d i s t r i b u t i o n with a mean NW orient a t i o n f o r counter-clockwise rotating Pi2, but an even d i s t r i b u t i o n in azimuth f o r clockwise rotating Pi2« The NW orientation i s the same as that of the disturbing vectors of magnetic bays and would indicate that these Pi2 are related to the auroral e l e c t r o j e t and are modified by the presence of induced currents i n the sea. 52 A a) Fig. 20.. Distribution i n azimuth of p o l a r i z a t i o n of Pi2 pulsations observed at Halley Bay i n 1976 a) pulsations with counterclockwise rotation b) pulsations with clockwise rotation 53 Whether the different azimuth d i s t r i b u t i o n s for counter-clockwise and clockwise rotating Pi2 i s s i g n i f i c a n t cannot be determined from the present small amount of data, but t h i s point i s c e r t a i n l y worth further i n v e s t i g a t i o n . The p o l a r i z a t i o n c h a r a c t e r i s t i c s of Pi2 at Halley Bay and i t s conjugate point, St. Anthony, have teen examined by Green and Hamilton (1978),* Spectral analysis was used to determine the e l l i p t i c i t y (e) and azimuth (<*) measured clockwise from the meridian, of the horizontal p o l a r i z a t i o n e l l i p s e of Pi2 events recorded simultaneously at the two stations,. A positive e l l i p t i c i t y represents counter-clockwise rotation looking downwards i n the northern hemisphere* For the same sense of rotation around the f i e l d l i n e , e l l i p t i c i t i e s at conjugate stations would be expected to be equal i n magnitude and opposite i n sign* A plot of the e l l i p t i c i t i e s at Halley Bay against those at St. Anthony showed that t h i s i s approximately true,. A similar plot of the azimuths showed that the angles at Halley Bay are equal but opposite i n sign to those at St.. Anthony as the conjugate rel a t i o n s h i p would require. A simple conjugate relationship generally seems to e x i s t between Pi2 at the two stations with the mean phase differences being 10 degrees for H and 170 degrees for D. However towards the end of the year t h i s r e l a t i o n s h i p becomes confused and Green and Hamilton (1978) have attributed t h i s to the presence of a night-time E layer at Halley Bay (due to the 24 hour so l a r illumination that occurs i n the au s t r a l summer) which a f f e c t s the propagation of the pulsations to the ground. This i s another feature requiring further examination. 54 4.3* Correlation With Other Phenomena A study was made of radio aurora (Shipstone, 1972) at Halley Bay during IQSY, a comparable epoch of the sunspot cycle to 1974-75..Echoes from radio aurora were received from south of the s t a t i o n , t y p i c a l l y at a range of 300km (approx 2.7 degrees in latitude) which i s , on average, one degree eguatorward of v i s u a l aurora..Observations, by Shipstone, of movement of the echoing region ( f i g 21), showed a change from westerly v e l o c i t i e s i n the evening (before 01.00 U.T.) to easterly v e l o c i t i e s i n the morning..According to Shipstone the v e l o c i t i e s involved (200-400m/sec) are too great to be produced by mass motions and he interpreted them as consistent with the d r i f t v e l o c i t i e s of electrons i n the e l e c t r o j e t current system, The times of eastward-moving radio aurora agree with the times of occurrence of westward e l e c t r o j e t s as determined from magnetic bay r e s u l t s ; and there i s a s i m i l a r agreement between the times of occurrence cf westward-moving radio aurora and eastward e l e c t r o j e t s . This indicates that the e l e c t r o j e t i s comprised of a flow of electrons* I t i s believed that polar substorms are related to the acceleration of p a r t i c l e s i n the magnetospheric t a i l towards the earth. .Lezniak and Winkler (1970) found that p a r t i c l e i n j e c t i o n from the t a i l occurred between 22.00 and 06.00 l o c a l time, which i s consistent with the time of occurrence of negative H bays at Halley Bay,.The injected p a r t i c l e s are guided into both the north and south auroral zones and so substorm e f f e c t s should be observed simultaneously i n both hemispheres.. 55 Fig..21.. Diurnal variations i n the number of radio aurora with eastward and westward v e l o c i t i e s (Shipstone, 1972) 56 This i s shown to occur by the c o r r e l a t i o n between enhanced absorption, at a northern and southern station* and magnetic bays at Halley Bay (fig 22) . Also the c o r r e l a t i o n of these features with increased electron counts in the t a i l , observed by s a t e l l i t e , i s consistent with the theory of p a r t i c l e i n j e c t i o n from the t a i l during substorms. The l i n k with absorption i s i l l u s t r a t e d further i n the next section* Fig. 22. Electron bursts i n the t a i l region together with increased absorption, detected by riometer, i n the north and south auroral zones, during the occurrence of a magnetic substorm at Halley Bay (after Akascfu, 1968). 57 5..ABSORPTION BESOLTS FROM HALLEY BAY This section presents a study of absorption at Halley Bay during 1974-75, made by the author using riometer and ionosonde measurements; Comparisons are made with the ionosonde and A1 measurements made at Halley Bay during the IGY (1957-58) by Bellchambers et a l (1962). Also, d e t a i l s of VLF emissions at Halley Bay are presented from a s t a t i s t i c a l study of whistler and VLF a c t i v i t y during 1972 made by Thomas (1975),. The riometer used at Halley Bay i s a commercially produced model from the design by L i t t l e and Leinback (19 59) and operates on 27.6MHz,. The riometer records the signal i n t e n s i t y of cosmic noise received; and the antenna i s directed at the south c e l e s t i a l pole to minimise the s i d e r i a l variation i n the received signal* In practice, however, t h i s cannot be e n t i r e l y eliminated and i t i s necessary to determine a guiet-day curve representing the unabsorbed s i g n a l l e v e l . Any drop i n sign a l l e v e l below t h i s curve then gives a measure of the absorption suffered by the cosmic noise i n passing through the ionosphere. In the r e s u l t s presented here short period absorption occurrences were i d e n t i f i e d , by inspection, from the o r i g i n a l cosmic noise record* However, to id e n t i f y longer periods of enhanced absorption, I used an hourly sampling of absorption values, obtained a f t e r removing the quiet-day curve* An absorption event was then defined as the occurrence of absorption greater than 1dB for one or more consecutive hourly values. . An ionosonde makes radio soundings of the ionosphere and receives both ordinary (o) and extraordinary (x) r e f l e c t i o n s 58 from the E and F layers.; The c h a r a c t e r i s t i c frequency of a layer i s designated by " f , mode, layer name": thus the c h a r a c t e r i s t i c frequencies for the F2 layer are foF2 and fxF2.. The top frequency at which echoes are received from the F layer i s labelled fxl*.When there i s no spread F present f x l = fxF2, but commonly there i s spread F and as a resu l t f x l i s greater than fxF2. The condition when no echoes are received i s known as "blackout'',. , This occurs when the absorption at the highest echo frequency, normally f x l , exceeds the threshold l e v e l of the ionosonde* The occurrence of blackout on ionoscndes has been used as an indication of abnormal absorption. However evidence i s presented i n Appendix A to show that the occurrence of blackout i s seriously affected by changes i n f x l values. This makes blackout occurrence an unreliable indicator of enhanced absorption.)p the A1 technigue involves making radio soundings of the ionosphere on certa i n fixed frequencies. The frequencies are chosen so that the signals are re f l e c t e d by the E region. These signals w i l l be p a r i a l l y absorbed i n the D region and so variations i n the received s i g n a l strength give a measure of the absorption suffered by the si g n a l i n the D region. 59 5.1. Rapidly Varying Absorption For the majority of magnetic bays recorded i n 1974 and 1976 the riometer operated at Halley Bay showed an increase in D region absorption which commenced at the same time as the bay, In many cases the magnetic and riometer v a r i a t i o n s appeared nearly i d e n t i c a l . For seventy f i v e percent of bays there was also blackout on the ionosonde but t h i s often persisted beyond the duration of the bay. Blackout results from a variety of causes and occurs f a r more frequently than magnetic bays so i t would appear unlikely that i t i s d i r e c t l y related to substorms.. However cases have been i d e n t i f i e d where the occurrence of blackout i s limited to the duration of a s p e c i f i c bay ( f i g 23). Thus absorption occurs simultaneously with magnetic bays but the p o s i t i o n and extent of the affected zone are d i f f i c u l t to assess. The ionosphere makes v e r t i c a l soundings of the ionosphere but also frequently receives echoes obliquely,. Thus for blackout to occur there must be enhanced D region i o n i z a t i o n v e r t i c a l l y above the s t a t i o n and often over a wider area as well. The e l e c t r o j e t occurs poleward of the s t a t i o n and, from the orientation of the disturbing vector, was estimated to l i e at an angle of elevation between 40 degrees and 70 degrees, The riometer uses a d i r e c t i o n a l a e r i a l (3dB bandwidth of 15 degrees) pointing due south at an elevation of 75 degrees so views an area of the ionosphere eguatorward of the e l e c t r o j e t ; Thus although the poleward extent of the absorption region cannot be assessed, the results indicate that the absorption region does extend eguatorward of the e l e c t r o j e t * 60 HALLEY BAY f-PLOT Fig- 23. Magnetic and riometer records and an f - p l o t of ionosonde measurements for twc magnetic bay events: i) at 01-00 showing above average foEs on the f - p l o t i i ) at 05*00 showing blackout bn the f - p l o t 61 5. 2. . Slowly Varying Absorption Absorption events were found to occur p r i n c i p a l l y on the dayside. This can be seen from the percentage occurrence at each hour of absorpticn>1dB and absorption>2dE (fig 24) which shows that the former peaks at 14.00 l o c a l time while the l a t t e r peaks s l i g h t l y e a r l i e r . The occurrence of blackout at Halley Bay has a maximum, at 06.00 l o c a l time, considerably e a r l i e r than the maximum occurrence of absorption events. CD a •c a> i _ ' i _ O o O CD a> +-> c CD a i _ CD 0_ 40 i—i—i—i—i—r 20 T—I—I—r T—I—i—i—i—i—i—i—r T — i — r A R > ldB A R > 2dB 0 I—i—i—i—i—i—i i i i i i i i 00 06 12 18 » i i 00 L O C A L T I M E F i g . .24. . The percentage of hours i n 1974 with absorption > 1dB and > 2dB at Halley Bay. 62 To examine the seasonal v a r i a t i o n of absorption events the diurnal v a r i a t i o n of absorption events was f i r s t produced for each month i n 1974 and 1975. The res u l t s for the summer months were then combined to give the mean diurnal variation of absorption events for the summer season; and the same procedure was also used to give the mean diu r n a l variation of absorption events f o r the equinoctual and winter seasons; These re s u l t s (fig 25) show that absorption events are most frequent at the equinoxes and occur l e a s t often i n winter. The shape of the diurnal v a r i a t i o n i s also shown by f i g 25 to change with season, with the peak i n occurrence e a r l i e r i n winter than i n summer. In order to determine the significance of t h i s , e r r or bars are drawn which show the standard deviation of the mean* Even allowing f o r the deviations shown, i t would appear that the diurnal v a r i a t i o n i n summer peaks at 13.00 -18.00 l o c a l time while the diurnal variation i n winter, although not so pronounced, peaks between 09.00 and 12.00 l o c a l time. The data suggest that the peak of the diurnal v a r i a t i o n becomes progressivly e a r l i e r as the seasons change through summer and eguinox to winter. 63 40 h 1 Summer 1 1 1 1 1 06 12 18 Local Time f i g , 25. The mean monthly percentage occurrence of absorption events i n 1974-75 for a) Summer: Nov, Dec, Jan, Feb b) Equinox: Mar, Apr, Sep, Oct c) Winter: May, Jun, J u l , Aug Therefore each curve i s the mean of 8 months' data and the error bars show the standard deviation* 64 5.3. Comparison With IGY Results During the IGY, measurements were made at Halley Bay of the absorption at 2,. 2MHz and 4.0MHz using the A1 technique, as well as the recordings of frain values and the occurrence of blackout. Fmin was found to be clos e l y related to solar zenith angle but was also affected by the l e v e l of magnetic a c t i v i t y (Bellchambers et a l , 1962) . The occurrence of blackout during IGY shows the same diurnal v a r i a t i o n as i n 1974-75. The maximum blackout occurrence i n IGY i s only 15 percent compared with 30 percent i n 1974-75, but during IGY f x l was consistently twice the corresponding values i n 1974-75. Blackout occurrence has been shown to be dependent on f x l values (Appendix A) so these r e s u l t s do not indicate any s i g n i f i c a n t differences i n the occurrence of enhanced absorption i n the two periods.. Comparison can be made between the A1 measurements on 2 . 2 M H z and 4.0 M H z during the IGY and the riometer measurements on 27.6MHz i n 1974-75 by presenting the data i n terms of an absorption index. A, given by A = L . (f+ f L ) Where L = absorption observed at freguency f and f L = electron gyro frequency. r i g 26 shows the monthly median values of A for 1957, 58, 74, and 75 and the mean seasonal curves f o r the two periodsi. I t can be seen that these seasonal variations are remarkably s i m i l a r , the only difference being that the general l e v e l of absorption appears to be higher i n 1974-75. However t h i s may only be due to an increase i n the number of absorption events.. I t should be noted that an absorption event of 1dB on the riometer 65 corresponds to an absorption index of about 800dB, so the occurrence of only a few more absorption events would s i g n i f i c a n t l y increase the mean absorption index- Thus the data does not indicate any major change i n the l e v e l of absorption or the number of absorption events between 1974-75, sunspot minimum, and 1957-58, sunspot maximum. 66 C 0'—1 1 1 1 1 1 1 1 1 1 L 0 I I——I : I I I I I I i i i i J F M A M J J A S O N D Month f i g . 26. The seasonal v a r i a t i o n of the absorption index at noon a) calculated from monthly medians of A1 measurements b) calculated from monthly medians of riometer measurements. . 67 5. 4, Correlation With VLF Emissions This section i s concerned with VIF emissions because of t h e i r possible association with electron cyclotron i n s t a b i l i t y and the p r e c i p i t a t i o n responsible for dayside absorption. Synoptic VLF recordings have been made at Halley Bay f o r Dartmouth College, USA, since 1971 and a s t a t i s t i c a l study of the whistlers and VLF emissions so recorded during 1972 has been made by Thomas (1975)*. Thomas distinguished between hiss and discrete emissions, in h i s study of VLF emission a c t i v i t y , and found that both occur predominantly during the day..The recordings were s p l i t into two frequency ranges; a low range from 100Hz to 1.5KHZ and a high range from 1.5KHz to 10KHz..The majority of hi s s occurred i n the low frequency range whereas discrete emissions were found i n both the low and high frequency ranges. . Both VLF emissions and absorption events are dayside phenomena with a maximum occurrence i n the middle of the day, but there i s no close s i m i l a r i t y between the shapes of the diurnal v a r i a t i o n of the two phenomena. .At the time cf maximum a c t i v i t y 60 percent of the recordings contained hiss and 40 percent had discrete emissions compared with absorption events which have a maximum occurrence of les s than 30 percent. However no information i s given about the number of emissions per recording of about t h e i r i n t e n s i t y so the comparison with absorption may be misleading. Hence a closer examination of the recordings i s reguired to determine whether or not there i s any direct c o r r e l a t i o n between absorption and VLF emissions 68 6. . DISCUSSION This chapter presents the authors contributions to the theory of substorms. F i r s t , the local time d i s t r i b u t i o n of substorm phenomena observed at Halley Bay i s examined and t h i s information i s used to derive a new unified picture of substorm phenomena and their effects on the ground. Most of the Halley Bay r e s u l t s conform to the picture of substorm processes presented in chapters 2 and 3. However i n order to provide a complete explanation of the observed phenomena there are two substorm features that reguire further inves t i g a t i o n . The f i r s t i s the occurrence of an eastward e l e c t r o j e t ; and i n section 6.2 a model i s developed to account for t h i s feature; The other problem i s the uncertain r o l e of photodetachment with regard to auroral absorption and i n section 6.3 the unique location of Halley Bay i s used to c l a r i f y the si t u a t i o n . . 69 Local Time Disturbance Pattern 6>iJ-._li Magnetic Disturbances Throughout the data analysis i t was clear that phenomena had d i f f e r e n t c h a r a c t e r i s t i c s i n the evening sector compared to in the midnight morning sector. This i s e s p e c i a l l y obvious i n the magnetic bay data where a change from positive H bays to negative H bays occurs at 22.00 l o c a l time. This feature of magnetic bays was f i r s t noted by Harang (1946) and i t i s now recognised that many features change at the "Harang Discontinuity". The fundamental feature of the discontinuity i s now i d e n t i f i e d as a change from a poleward to an eguatorward e l e c t r i c f i e l d across the auroral zone (fig 27) . Radio aurora at Halley Bay also showed a change at the Harang discontinuity which i s associated with the d i f f e r e n t e l e c t r o j e t s responsible for the change in the magnetic bays. The d i s t r i b u t i o n of Pi2 overlapped that for both p o s i t i v e and negative H bays however the p o l a r i s a t i o n changed from clockwise before 00.00 l o c a l time to counterclockwise afterwards. Whether t h i s change i s associated with the Harang discontinuity i s uncertain. However the Harang discontinuity i s located at d i f f e r e n t l o c a l times i n di f f e r e n t l a t i t u d e s so the " l a t e r " Pi2 time might indicate that the Pi2 source region i s at lower la t i t u d e s than the e l e c t r o j e t responsible for magnetic bays. 70 ELECTRIC FIELD SCHEMATIC o MLT fig,. 27. E l e c t r i c f i e l d s i n the polar cap and the position of the Harang discontinuity where the e l e c t r i c f i e l d across the auroral zone changes from eguatorward to poleward. (Maynard, 1974),.. 6.1.2. Ionosperic Disturbances The absorption r e s u l t s show a discontinuity at 04.00 l o c a l time analagous to the Harang discontinuity i n the magnetic results... Before 04.00 the absorption i s rapidly varying and of short duration while af t e r 04.00 longer periods of slowly-varying absorption occur.. This effect i s evident i n the d i s t r i b u t i o n of substorm phenomena reported by Hartz and Brice (1967) . They c l a s s i f y r a p i d l y varying auroral absorption, together with discrete aurora and auroral Es, as "discrete" events; and they l a b e l slowly varying absorption, along with di f f u s e aurora, as " d i f f u s e " events. The discrete events are produced by impulsive bursts of "splash" p r e c i p i t a t i o n , and occur along the auroral oval, with peak occurrence just before 71 l o c a l geomagnetic midnight. In contrast, diffuse events are associated with slowly varying " d r i z z l e " p r e c i p i t a t i o n with a maximum at 09.GO mean geomagnetic time. F ig 28 shows the d i s t r i b u t i o n of events for the Northern hemisphere. As explained i n section 3, the splash p r e c i p i t a t i o n i s caused by d i r e c t i n j e c t i o n of p a r t i c l e s from the t a i l , while the d r i z z l e p r e c i p i t a t i o n comes from electrons trapped i n the radiation belt d r i f t i n g round into the morning sector, The diurnal v a r i a t i o n of absorption at Halley Bay can b a s i c a l l y be explained by t h i s precipitation pattern. 12 _L Fig. 28. Idealized representation of the d i s t r i b u t i o n of •diffuse* events (dots) and 'd i s c r e t e 1 events (triangles) a f t e r Hartz and Brice (1967),. An auroral zone station w i l l see a change from discrete events to dif f u s e events at about 04.00 l o c a l time. 72 6 .1.3. Overall Picture Of Substorm Effects To summarise: the p r i n c i p l e polar substorm processes can be pictured ( f i g 29) as p a r t i c l e i n j e c t i o n on the nightside dir e c t into the ionosphere* together with penetration of higher energy p a r t i c l e s across f i e l d - l i n e s to become trapped i n the outer radiation b e l t . Once i n the r a d i a t i o n belt the protons d r i f t west into the evening sector and ion cyclotron turbulence causes th e i r p r e c i p i t a t i o n . „The electrons, conversely, d r i f t east into the morning sector and pr e c i p i t a t e into the ionosphere as a r e s u l t of pitch angle d i f f u s i o n or electron cyclotron turbulence;. The disturbance pattern seen on the ground as a r e s u l t of the above p r e c i p i t a t i o n pattern i s shown schematically i n f i g 30. ..It must be emphasised that t h i s disturbance pattern i s fixed in space and that the earth rotates underneath i t . Thus an auroral zone station, such as Halley Bay w i l l observe a s p e c i f i c type of disturbance depending on where i t i s when a substorm occurs* To i l l u s t r a t e t h i s , suppose substorms occur at 22..00, 0'4.00 and 10.00 U.T. Local time at Halley Bay i s U.T. - 2hrs and so the r e s u l t i n g magnetic and absorption records would appear as shown in f i g 31. A local-time disturbance pattern can be used to provide several hours warning of disturbances. 73 Fig. 29,. Schematic i l l u s t r a t i o n of the polar substorm precipitation._ 74 F i g . . 3 0 i . Schematic i l l u s t r a t i o n of the e f f e c t s i n the ionosphere of the p r e c i p i t a t i o n pattern of Fig 29. 75 U. T. 20 02 08 14 20 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 I 1 1 1 1 — 1 — Particle Injection on Nightside Mag. Disturbance HF Absorption I 1 I 1 1 1 1 1 ] i i i i i i i i I 18 00 06 12 18 L O C A L T I M E f i g . 31, Schematic i l l u s t r a t i o n of the di f f e r e n t disturbances, depending on l o c a l time* produced at a ground observatory by a burst of p a r t i c l e i n j e c t i o n on the nightside of the earth. The nightside events are dominated by the e f f e c t s of p a r t i c l e precipitation*.However the dayside absorption i s affected by the solar i l l u m i n a t i o n of the ionosphere and so evidences a seasonal variation (see f i g "25 and section 6;3). The number of disturbances also shows a seasonal variation with a peak i n the equinoxes, 76 6.2. „ The Cause Of The Eastward. E l e c t r o j e t The eastward e l e c t r o j e t cannot he simply explained l i k e the westward e l e c t r o j e t . The low occurrence of positive bays compared to negative bays at Halley Bay suggests that substorm i n j e c t i o n of p a r t i c l e s alone i s not s u f f i c i e n t to give r i s e to an eastward e l e c t r o j e t . ,It i s therefore worthwhile to examine the r e l a t i o n s h i p between the eastward e l e c t r o j e t and two other evening features: the p a r t i a l r i n g current, and the plasmasphere bulge* §.s.Iz.ls. Association With The P a r t i a l fling Current There i s considerable evidence that a p a r t i a l ring current occurs at the same time as the eastward e l e c t r o j e t . For instance Kamide and Kukushima (1972) investigated the l a t i t u d e p r o f i l e of magnetic disturbance i n the evening sector and found that the H decrease at middle and low l a t i t u d e s i s much greater than that expected from the return current of an eastward e l e c t r o j e t . S a t e l l i t e observations nave also detected a p a r t i a l ring current and Frank.(1970) found that the current i s carried by low energy protons. These prctons would be injected from the t a i l during a substorm, become trapped on closed f i e l d l i n e s , and d r i f t westward under the influence of the magnetic f i e l d . However the intensity of the p a r t i a l ring current shows l i t t l e r elationship with substorm magnitude, and instead i s well correlated with the previous integrated southward IMF (Clauer and McPherron, 1978). Clauer and McPherron also report a strong c o r r e l a t i o n between the p a r t i a l ring current magnitude and the AD index which 77 supports the hypothesis that closure of the p a r t i a l ring current occurs through the eastward e l e c t r o j e t . This i s consistent with the substorm current system proposed by Kamide and Fukushima (1972) which i s shown i n f i g 32. The Halley Bay p o s i t i v e H bays (fig 18) suggest that the eastward e l e c t r o j e t (and p a r t i a l r i n g current) should be centred at 18.00 l o c a l time rather than as shown,; The occurrence pattern of positive H bays may be misleading because an eastward e l e c t r o j e t extending further i n t o the nightside may not produce positive H bays at these l a t e r l o c a l times because of the occurrence of an overlapping westward e l e c t r o j e t . However the time of occurrence of p o s i t i v e H bays has not yet been s a t i s f a c t o r i l y accounted f o r by any of the theories proposed (Nishida, private communication, 1979). Fig..32;. Schematic i l l u s t r a t i o n of a model current system for a t y p i c a l polar magnetic substorm, showing the occurrence of a p a r t i a l ring current (Kamide and Fukushima, 1972)., 78 6.2. 2. Evening Bulge In The Plasma pause The plasmasphere has a bulge in the evening sector coincident with the location of the eastward electrojet,.Whether there i s a connection between the two i s uncertain however t h i s p o s s i b i l i t y i s c e r t a i n l y worth consideration. Experimental evidence indicates that the inner boundary of the ring current i s clo s e l y associated with the plasmapause (Russell and Thorne, 1970); and i t has been suggested (eg Thorne, 1972) that t h i s i s due to l o c a l enhancement of the proton loss rate at the plasmapause.. However Bernstein et a l (1974) report that p r e c i p i t a t i o n of ring current protons actually occurs over a wide region outside the plasmapause, A popular theory f o r proton p r e c i p i t a t i o n involves ion cyclotron i n s t a b i l i t y (eg Thorne, 1972)* However Cor o n i t i et a l (1972) have suggested that e l e c t r o s t a t i c ion i n s t a b i l i t y might be more s i g n i f i c a n t . A review of the possible p r e c i p i t a t i o n mechanisms has been made by Ashour-Abdalla and Kennel (1978) and they conclude that e l e c t r o s t a t i c ion i n s t a b i l i t y appears the most l i k e l y process, but that i t i s c r i t i c a l l y dependent on the cold electron density and temperature* The cold electron density inside the plasmaphere i s markedly different from that outside and the plasmasphere may prove to be a region that i n h i b i t s e l e c t r o s t s t i c ion i n s t a b i l i t y . . I n consequence the evening bulge of the plasmasphere could a f f e c t the d i s t r i b u t i o n of proton p r e c i p i t a t i o n and as a r e s u l t give r i s e to the eastward el e c t r o j e t * This p o s s i b i l i t y w i l l be considered in the next section. 79 6.2.3. Proposed Mechanism For The Eastward E l e c t r o j e t During a substorm there i s electron p r e c i p i t a t i o n into the nightside ionosphere* , Also enhanced convecton i n j e c t s protons into the 18.00-24.00 l o c a l time sector (Cornwall et a l , 1970) and these become trapped i n the outer radiation belt. These trapped protons d r i f t westward under the influence of the magnetic f i e l d , and so form a p a r t i a l ring current. Closure of th i s current system could be achieved by a c i r c u i t featuring a flow of electrons up f i e l d l i n e s . However fo r the system proposed here, i t i s assumed that a s i g n i f i c a n t portion of the downward f i e l d - a l i g n e d current i s carried by p r e c i p i t a t i n g protons* In d r i f t i n g round to the afternoon sector some of the protons w i l l pass through the evening bulge of the plasmasphere. It i s proposed that the mechanism f o r proton p r e c i p i t a t i o n i s e l e c t r o s t a t i c ion i n s t a b i l i t y , and that t h i s i n s t a b i l i t y i s i n h i b i t e d inside the plasmsphere. This would r e s u l t i n an uneven d i s t r i b u t i o n i n the p r e c i p i t a t i o n of d r i f t i n g protons. Greater p r e c i p i t a t i o n would occur on the day-side of the plasmasphere bulge, and i n a simplified picture, t h i s can be considered as a downward f i e l d — a l i g n e d current. The electron p r e c i p i t a t i o n on the nightside constitutes an upward f i e l d - a l i g n e d current. Closure of the current system in the ionosphere by a westward flow of electrons thus produces the eastward e l e c t r o j e t . . 80 6.3..The Bole Of Photodetachment In Auroral Absorption 6.3,1..D Region Chemistry Ionisation of the atmospheric constituents i n the D region i n i t i a l l y produces free electrons and positive ions. However, i n contrast to the higher regions of the ionosphere, there also exist negative ions. Negative ions are p r i n c i p a l l y observed below 80km, however negative ions have also been detected at greater heights (Mitra, 1 978). Negative ions are formed by an i n i t i a l attachment of free electrons to 0 and Oa t c form 0" and 0a.„ Charge exchange c o l l i s i o n s result i n the production of a series of ions with increasing electron a f f i n i t y u n t i l stable negative ions such as NO~ and X~(Ha0) are formed. At intermediate steps the main process of loss of negative ions are through photodetachment by solar radiation or by c o l l i s i o n a l detachment involving neutral constituents of the atmosphere (Mitra, 1978), The rate of photodetachment i s dependent on the solar zenith angle and so w i l l occur more in summer than i n winter giving r i s e to a seasonal e f f e c t . Also, i f the major ion i s 0", the wavelength required for photodetachmemt i s 2870nm. This i s in the v i s i b l e spectrum so the occurrence of photodetachment would be controlled by sunrise and sunset* Whether photodetachment i s a s i g n i f i c a n t process with respect to auroral absorption depends on whether negative ions occur at the heights where auroral i o n i s a t i o n occurs. At most auroral zone stations the seasonal change i n solar radiation i s not large enough for this problem to be resolved* However the 81 uniquely high geographic l a t i t u d e of Halley Bay should make any effect of photodetachment more easy to detect. 6 ..3 ..2.. Halley Bay. Results At Halley Bay the changes with season i n the diurnal variation of absorption events (fig 25) suggest that photodetachment i s a s i g n i f i c a n t process. At the equinox, the diurnal v a r i a t i o n , roughly centred about noon and with rapid changes i n absorption at 06.00 and 18,00 l o c a l time, indicates that photodetachment i s the c o n t r o l l i n g process. While i n winter the absence of photodetachment should allow the p r e c i p i t a t i o n pattern to be mcst e a s i l y recognised; and, i n f a c t , the maximum occurrence i n winter, between 09:. 00 and 12.00 l o c a l time, i s consistent with the maximum time of " d r i z z l e " p r e c i p i t a t i o n i n f i g 28. Further evidence of the competing e f f e c t s of p a r t i c l e p r e c i p i t a t i o n and photodetachment can be seen in f i g 24, which shows that the peak occurrence of the larger absorption events (abs > 2dB) i s shifted more towards the peak " d r i z z l e " time than the peak occurrence of the standard absorption events (abs > 1dB) . This would indicate that the great p r e c i p i t a t i o n , responsible f o r abs > 2dB, i s overcoming the photodetachment control that af f e c t s smaller absorption events* 82 6,3.3. Production Of Auroral Absorption Auroral absorption i s primarily attributable to the D region i o n i s a t i c n produced by electron precipitation.. The continual l o s s of free electrons, by attachment, to form negative ions, however reduces radio wave absorption. This i s the s i t u a t i o n at Halley Bay i n winter where l i t t l e absorption i s observed* However when there i s solar illumination of the ionosphere photodetachment of electrons counteracts the loss by attachment and the absorption i s correspondingly greater. The Halley Eay re s u l t s are not conclusive and so the above outline of auroral absorption i s only tentative. However the Halley Bay r e s u l t s are consistent with an ionospheric model i n which photodetachnent i s s i g n i f i c a n t with regard to auroral absorption* 83 CONCLUSIONS The wide range of data c o l l e c t e d by the author at Halley Bay has been used to examine the v a l i d i t y of present theories regarding many aspects of polar substorms. The d i v i s i o n of substorm features by Hartz and Brice (1967) i n t o 'discrete* and •diffuse* phenomena i s judged to be reasonable..However a t h i r d class, for evening features, should be added; and a substorm disturbance pattern i s developed with t h i s included. This features d i r e c t p a r t i c l e p r e c i p i t a t i o n into the nightside ionosphere which causes rapid variations i n absorption* as well as the westward ele c t r o j e t which i s responsible for the magnetic disturbances i n that sector* Also some p a r t i c l e s become trapped in the outer radiation b e l t : protons d r i f t into the evening sector and give r i s e to the eastward e l e c t r o j e t , while electrons d r i f t round to the morning and day side where they produce long period v a r i a t i n s i n absorption. Detailed examination of the observations revealed several previously unexplained features. The diurnal and seasonal variations i n the occurrence of enhanced absorption, detected by the riometer, indicate that photodetachment of negative ions by sunlight has a s i g n i f i c a n t e f f e c t on auroral absorption. Also i t has been shown that the occurrence of blackout on the ionosonde i s seriously affected by f x l values and so i s misleading as an indicator of enhanced absorption: a better indicator being the occurrence of absorption at 27,6MHz, Theories f o r polar substorm features i n the night and morning hours are reasonably s a t i s f a c t o r y ; However for evening phenomena the theories are inadaguate. I t i s proposed (see 84 section 6*3) that the plasmasphere bulge may modify the proton p r e c i p i t a t i o n pattern i n the evening sector and as a r e s u l t give r i s e to the eastward e l e c t r o j e t . However t h i s requires further i n v e s t i g a t i o n , i d e a l l y as part of a study encompassing a l l evening sector features. The analysis of the ef f e c t s of induced currents i n the sea, in Appendix B, shows that they can seriously a f f e c t the magnetic bays i n the D component. 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H. .Ward Studies of the magnetospheric substorm 2, Correlated magnetic micropulsations and electron p r e c i p i t a t i o n occurring during auroral substorms* J*_ Geophys. Res. , 73, No, .5, 1697, 1968.. Mitra, A.P., The D region of the ionosphere. Endeavour 2, No 1, 92 12, 1978.. Nagata, T., T„ Hirasaw, M. Takizawa and T, Tohmatsu, Antarctic substorm . events. observed by sounding rockets, ionization of the D and E regions by auroral electrons.. Planet. . Space S c i * , 23, No. 9, 1321*1327, 1975. Nishida, A. Geomagnetic Diagnostics Of The Magnetosphere. Springer Verlag, New York, 1978. for modelling d i s t r i b u t i o n s * Oldenburg, D.W. -Ionosperic current structure as determined from ground-based magnetometer data, Geophysi. J. Boy. Astron. Sod, 46, 41, 1976. Parkinson, W.D., The influence of continents and oceans on geomagnetic var i a t i o n s , Geophys, J* R. Astr. Soc*, 6, 441, 1962. . Parks, G. K., F.V. C o r o n i t i , R»L. 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Rev;* Geophys. 10, No. 4, 981, 1972, 95 APPENDIX h± THE EFFECT OF FXI ON BIACKCUT OCCUBBENCE (sub. to the Journal of Atmos. and Terr™ Physics, Oct,. 1979) Introduction The occurrence of blackout on ionosondes has long been used as an i n d i c a t i o n of enhanced absorption (eg. Thomas and Piggott, 1960). Analysis of riometer and ionosonde records from Halley Bay (75.5°S, 26.6°W) fo r 1974, allows comparison between actual measurements of absorption and the ionosonde data. At Halley Bay the diurnal variation of blackout occurrence peaks at 06.00 l o c a l time i n contrast to the occurrence of absorption at 27.6MHz>1dB which has a maximum at 14.00 l o c a l time (fig A,*1)i Some workers believe that blackout i s generally an i n d i c a t o r of smaller absorption events than those seen by the riometer, and that f i g A. 1 indicates that small absorption events, in general, occur at a time shifted relevant to the time of large absorption events (Piggott, private communication, 1979) . . Blackout occurs when the absorption at the highest frequency at which echoes are received exceeds the threshold l e v e l of the ionosonde. The highest ordinary component echo from the F region i s f o l , while the highest extraordinary component echo i s f x l * Noroal observatory practice i s to scale only f x l but f o l can e a s i l y be obtained from the r e l a t i o n f x l = f o l + f t / 2 The value of f x l (and fol) i s affected by diurnal and seasonal F region variations and by the occurrence of spread F. (for f u l l explanations of terms f x l and f o l see Piggott and Eawer, 1972). This study examines the e f f e c t of variations i n f x l on the 96 occurrence of blackout i n order to assess the r e l i a b i l i t y of blackout occurrence as an i n d i c a t o r of enhanced absorption. 00 06 12 18 00 LOCAL TIME f i g A.I. Comparison between the occurrence of blackout on the ionosonde and the occurrence of absorption at 27.6MHz > 1dB as seen by the riometer i n 1974 at Halley Bay, Antarctica. 97 A.2. Conditions Required For Blackout Absorption at Halley Bay i s p r i n c i p a l l y produced by D region i o n i s a t i o n above 70km, except during PCA events which are comparatively rare during sunspot minimum, the time considered (Piggott and Hurst, 1976)*.Hence the freguency dependence of the absorption can be expressed by an inverse square law relationship (Piggott, 1953). An absorption index, A, for a single path can therefore be defined i n terms of the absorption, L, measured at any freguency f : A = L(f+ f L ) 2 Where f t i s the electron gyro-freguency, and the sign i s positi v e for ordinary rays and negative for extraordinary rays* Thus under conditions that produce an absorption index. A, the absorption suffered by ordinary and extraordinary rays of freguency, f, i n passing once through the D region w i l l be respectively L 0 = A/(f+f L ) 2 L„ = A / ( f - f t ) 2 Ionosonde signals, r e f l e c t e d by the E or F region, pass twice through the D region and so the absorption suffered by the o and x components w i l l be L 0 = 2A/(f+f t ) 2 L„ = 2A/(f-f L ) 2 The minimum freguency at which an echo i s seen on the ionosonde w i l l occur where the absorption, L, at that frequency equals the threshold l e v e l of the ionosonde, T. For ordinary rays fnin i s given by fmin 0 + f t = (2A/T)o-s For extraordinary rays fmin i s given by fmin x - fL = (2A/T)o.s S8 Combining the above two equations shows that fmin 0 = fmin* + 2f L Thus when fmin i s close to the top echo frequency seen on the ionosonde, that top frequency w i l l be due to an ordinary ray, not an extraordinary ray: i e . the tcp frequency w i l l be f o l not f x l . Therefore blackout w i l l occur when fmin exceeds the ordinary component top echo frequency. That i s when fmin > f o l Substituting f o r fmin , from equation (v) gives (2A/T) o. s > f o l + f t Fol i s not usually scaled and so i t i s preferable to use f x l . Using equation ( i ) , the above inequality can be written as (2A/T) o. 5 > f X i + f t/2 Thus the requirement f o r blackout can be expressed as A > 0.5T(fxI + f t/2) 2 99 hs.2i. Blackout Occurrence At Halley Bay At Halley Bay the electron gyro-f requency, f = 1, 2MHz. )p by comparing fmin values with ricmeter measurements of absorption as described by Kressman and Piggott (1976), the threshold l e v e l of the ionosonde at Halley Bay compared to the amplitude of an unabsorbed r e f l e c t i o n was calculated to be 80 + 10 dB- This value was then used i n expression (xi) to calculate the absorption index. A, required to qive blackout f o r di f f e r e n t values of f x l as well as the absorption values, L«, for a riometer operating on 27.6MHz..(table A*1). Fxl (MHz) A (dB) 100 270 520 850 1250 1750 L R (dB) 0. 12 0. 32 0. 62 U02 1.50 2.10 table A.1.. Absorption index. A, required to give blackout for different values of f x l at Halley Bay, calculated from the eguation: A > 0.5T (fxl+f t/2) 2, with T=80dB and f L =1. 2MHz. . Also shown i s the associated value of absorption at 27.6MHz, 1 R, calculated from A=LR (27. 6+f L) 2 . 100 A.4* Comparison Between Ionosonde And Biometer Measurements Biometer and ionosonde measurements are not always comparable i n i n d i v i d u a l cases. The riometer gives a measure of the average absorption within the acceptance cone of i t s antenna, about 60°., whereas the ionosonde for a near v e r t i c a l r e f l e c t i o n i s affected only by the absorption which i s present in a much smaller angle from the v e r t i c a l * Also the ionosonde sometimes receives echoes from obligue angles and i n such cases blackout would net be recorded. Both these factors can be expected to cause some scatter i n the r e l a t i o n s between riometer and ionosonde measurements when the absorbing zone i s not unifornu .Also during blackout, by d e f i n i t i o n , no echoes are received by the ionosonde, and so actual values of f x l are not available. Therefore monthly median values, which are available for each hour, w i l l be used instead of individual f x l values. . As a f i r s t test to determine the effect of f x l on blackout occurrence, because enhanced absorption occurs p r i n c i p a l l y at the equinoxes, the median f x l values for the equinoctual months were used* Comparison between the average median f x l value (fig A.2) and table A.I shows that at 06.00 l o c a l time blackout should occur when LR>0.9dB while at 14.00local time blackout should be produced when Lft > 1. 9dB. . Examination of a l l 1974 riometer and ionosonde records for 06.00 l o c a l time shows that absorption at 27.6MHz>1dB i s accompanied by blackout i n 90% of cases. A s i m i l a r study shows that at 14.00 l o c a l time absorption at 27.6MHz>2dB i s accompanied by blackout i n 85% of cases, tests show that these r e s u l t s are s i g n i f i c a n t at the 0.1% l e v e l . This indicates that i o n i s a t i o n producing L R> 1dB i s usually 101 s u f f i c i e n t to produce blackout at 01.00 l o c a l time but that at 14.00 i o n i s a t i o n producing Lft > 2dB i s required. f x l ( M H z ) 5 1 i i i i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r J — « — i — i — i — i — i — i — i — i — i — i i i i t i i J L 00 06 12 L O C A L T I M E 18 Fig. A. 2. The median value of f x l , normally the highest freguency at which echoes are seen on the ionosonde, for the eguinoctual months Of 1974. 102 As a further check on the e f f e c t of f x l , the median values of f x l for each month i n 1974, were used to determine the equivalent absorption at 27. 6MHZ that would be expected to give blackout. The actual riometer records were then examined to determine the number of times at each hour that LR exceeded these values. Comparison of t h i s "predicted" blackout occurrence with the actual blackout produced a goodness of f i t to a straight l i n e of C.82. This i s r e f l e c t e d i n the close s i m i l a r i t y of the diurnal variation of occurrence of "predicted" and actual blackout as shown in f i g A.3. 40 I I I . — I — I — I — i — I — I — | — I — | — I — i — i — | — i — i | i — i — | — i — | — I 0 '— 1—L_I— '—i—i—i—i—i—i—i—i—l i i i i i i i i i i i 1 00 06 12 18 00 L O C A L T I M E Fig. A.3.. Percentage occurrence of blackout 'predicted* by comparing riometer records with the absorption index A = 0.5T(fxI + f t/2) 2 where T i s the threshold l e v e l of the ionosonde. Together with the percentage occurrence of actual blackcut 103 A.5. . Discussion I t i s generally accepted that ionosonde fmin values provide a more s e n s i t i v e measure of small absorption events than a riometer,i . However table A.I shows that the absorption required to produce blackout i s heavily dependent on the value of f x l . Thus s t a t i s t i c s of blackout occurrence are unreliable as a guide to the occurrence patterns of enhanced absorption. Diurnal changes i n f x i , at Halley Eay, seriously a f f e c t the occurrence pattern of blackout..As shown in f i g A.I, the peak occurrence of blackout i s considerably e a r l i e r than the peak occurrence of absorption > 1dB as detected by the riometer. This difference i s adaquately accounted for when variations i n f x l are taken i n t o account ( f i g A.3). Thus there i s no evidence from Halley Bay blackout r e s u l t s to i n d i c a t e that smaller absorption events occur e a r l i e r than large absorption events. Seasonal variations i n f x l also affect the occurrence of blackout..In 1974, enhanced absorption at Halley Bay was most frequent at the equinoxes and there was a correspondingly high occurrence of blackout. . However during the winter, although enhanced absorption was comparatively rare, there was considerable blackout. . This was often associated with only 0,. 5dB absorption at 27.6MHz because f x l values were usually below 3MHz. Conversely during the summer, because of high values of f x l (about 6MHz) there was l i t t l e blackout despite s i g n i f i c a n t occurrence of enhanced absorption. 104 Appendix AAj_ The Threshold Level Of TJie Halley Bay Ionosonde Assuming an inverse square law re l a t i o n s h i p i t has been shown that for ordinary rays fmin * fL = (2A/T) 0.5 Which can be written A = 0.5T(fmin + f L ) 2 Thus i n a plot of A versus (fmin+fj^) 2 the slope, T/2, gives a measure of the threshold l e v e l of the ionosonde, Following the method of Kressman and Piggott (1976), I used fmin values, from December 1976, for 14.00 l o c a l time, a time when no added attenuators were i n use. These fmin values were plotted against absorption index values derived from riometer measurements (f i g A.4)« The slope, T/2, of the l i n e drawn by eye through the points, i s 41dB.. Thus the threshold l e v e l of the ionosonde at Halley Bay i s taken as T = 80 ± 10 dB 105 f i g A. 4. Relation between riometer absorption measurements, the equivalent absorption indices, and fmin f o r 14.00 l o c a l time, December 1976. 106 APPENDIX B£ THE EFFECT OF INDUCED CUBBENTS IN THE SEA ON MAGNETIC BAYS OBSERVED AT A COASTAL CBSERVATORY (pub. i n the Journal of Atmos. and Terr. Physics, May, 1978) B. 1. Introduction The magnetic signature of a polar substorm i s a departure of the magnetic f i e l d from i t s normal value* by several hundred gammas l a s t i n g for an hour or so and normally referred to as a magnetic bay^.Such magnetic bays are primarily due to an intense current flowing along the auroral oval, c a l l e d the auroral e l e c t r o j e t (see review paper by Anderson and Vondrak, 1975). Near the westward e l e c t r o j e t , a negative bay i s observed i n the horizontal component of the magnetic field,H i n the v e r t i c a l component, Z, positive bays are observed to the north of the westward e l e c t r o j e t while negative bays are observed to the south. The c h a r a c t e r i s t i c s of H and Z bays are well established but there remains considerable confusion about the bays in the east-west component, D, and most authors have i n the past ignored them* In a s t a t i s t i c a l study of bays observed at Scandinavian stations Harang (1946) showed that negative bays i n H were usually acconpanied by an eastward deflection i n D. Such bay events would indicate a magnetic disturbing vector that pointed south-east throughout the night hours;. l a t e r workers, however, found cases where the disturbing vectors were directed to the south-west i n the morning sector (Akasofu and Meng, 1969),. The observed d i s t r i b u t i o n of disturbing vectors i s often expressed in terms of an equivalent current system i n the ionosphere 107 which, as Akasofu (1968) points out, i s a mathematically correct method of representation. . The actual current system i s now believed to involve f i e l d - a l i g n e d currents which connect the ends of the e l e c t r o j e t to the magnetosphere where the c i r c u i t i s completed by the current generator. (the l a t t e r i s believed either to involve s h o r t - c i r c u i t i n g of part of the magnetotail current (e,;g. McPherron et a l . , 1973), or connection to the p a r t i a l r i n g current (Bonnevier et a l , , 1970), This current system i s consistent with e a r l i e r models since i t produces the same magnetic disturbance on the ground as a purely ionospheric current system (Fukushima, 1970). According to Kisabeth and Rostoker (1971), bays i n the H and Z component are produced by the e l e c t r o j e t i t s e l f whereas the bays i n the D component are caused by the f i e l d - a l i g n e d currents. Rostoker (1972) , constructed latitude p r o f i l e s of the magnetic disturbance produced by an e l e c t r o j e t and reported that the D bay configuration i s determined by whether the meridian of the l a t i t u d e p r o f i l e i s east or west of the central meridian of the e l e c t r o j e t . For the simple model, i n which the current flows down the f i e l d l i n e s on the morning side, along the auroral oval and back up to the magnetosphere on the evening side, the sign of the D bay would be expected to change as the s t a t i o n moved from east to west of the central meridian of the e l e c t r o j e t , However, a more complex model, recently proposed by Kamide et a l (1976), which i s composed of pairs of f i e l d - a l i g n e d currents, suggests that the sign of the D bay should not change in keeping with Harang 1 s results.. At Halley Bay, Antarctica (75°S, 27°W), a change i n the D 108 bay with position of the s t a t i o n r e l a t i v e to the e l e c t r o j e t , occurs i n only a few cases and the D bays are predominantly of the same sign* However, there i s another factor which can e f f e c t magnetic bays recorded at coastal stations, namely currents induced i n the neighbouring sea by the auroral e l e c t r o j e t . Neglecting the effect of such currents may lead to incorrect conclusions about the form of the ionosphere magnetospheric current system. This paper examines the influence of induced currents and demonstrated the e f f e c t they can have on the magnetic bays observed,. B. 2j_ Induced Currents An e l e c t r i c a l current system i n , or above, the ionosphere, such as the auroral s l e c t r o j e t system, w i l l induce currents i n the earth. The intensity of the induced currents w i l l be much greater i n the oceans than i n the continental land mass because the conductivity of sea water (3-4 ohm"' m"' ) i s several orders of magnitude greater than that of rock (10"' -10~4 ohm ' m"' ) . Ice has a conductivity of 1-2. 10"* ohm-' m"' , so induced currents i n the polar ice or adjoining i c e shelf w i l l also be i n s i g n i f i c a n t compared with those i n the ocean. An induced current near the surface w i l l reduce the observed Z disturbance, 4Z, and increase the observed H disturbance, AH. If the depth of the region where the induced currents are situated i s s u f f i c i e n t l y great, 4Z w i l l be reduced to zero and H w i l l be doubled. The skin depth, f , at which the induced current density drops to 1/e of i t s surface value i s given by Garland (1960) as 109 where T i s the period, i n seconds, of a sinusoidal inducing current and <r i s conductivity, i n e.n.u. . For T = 1000s, corresponding to the duration of a short bay, and <r = 4 ohm m (4 x 10~" e.m. u. ) the skin depth i s about § knu. Using a formula which takes into account the e f f e c t of a non-uniform inducing f i e l d upon skin depth, Price (1967) calculated the e f f e c t s of induced currents on the H and Z parameters measured at the surface of an i n f i n t e l y large ocean of depth D, Table B» 1. D, km 0. 5 1 2 3 4 5 AH/flH0 1.56 1.71 1. 83 1. 88 1.91 1.93 AZ/AZ, 0. 44 o,29 0. 17 a. 12 6.09 0.07 Table B. 1. The effects of induced currents on H variations at the surface of an ocean of depth D. The wavelength of the inducing f i e l d i s taken as 1000,km, the conductivity, er as 4 i 10~M e.mi.u., and the period T as 1000s (after Price, 1967), 110 Clearly the r a t i o s of the resultant f i e l d s , to the inducing f i e l d s , dH/AHo, and AZ/AZo, are s i g n i f i c a n t even f o r depths of 0.5km. Therefore sizeable induced currents are expected to occur in the Weddell Sea which w i l l a f f e c t the magnetic disturbances at Halley Bay..Using measurements from d r i f t i n g i c e islands i n the A r c t i c Ocean, Zhigalov (1960) found that the r a t i o 4Z/4H, of the magnetic disturbances i n the Z and H components, d>ecreased considerably as the sea depth increased (Fig. B.l.) due to the effect of induced currents. 0 Distance drifted, km Fig..B*%.Values of the r a t i o AZ/Ati and of ocean depth of d r i f t i n g i c e station North Pole 6 (after Zhigalov, i960). 1: Ocean depth, 2: Mean value of &Z/AE for a l l hours of day 3: Mean value of AZ/AE f o r variations of period 10 min. 111 Ms.is. Analysis Of Magnetic Bays Quiet arcs, which define the position of the auroral oval (Feldstein, 1963), are seen well to the south of Halley Bay, at angles of elevation below 7 (Seivwright, 1969). Thus i t should be possible at Halley Bay to i n t e r p r e t magnetic bays in terms of an equivalent line current flowing along the auroral oval, although such an approach would not be valid at a station close to the e l e c t r o j e t . . Of 294 bays examined, during sunspot minimum (1974-75) at Halley Bay, 222 featured a negative bay in the H component and a positive bay i n the Z component.. These bays occurred between 22.00 LZT and 06.00 LZT and are compatible with a westward e l e c t r o j e t to the south of the station* The other major c l a s s of bays was consistent with an eastward e l e c t r o j e t to the south and contained 48 examples occurring between 16.00 LZT and 21.00 LZT. For both classes, bays should be evenly d i s t r i b u t e d between those caused by an e l e c t r o j e t to the east of the station and those caused by an e l e c t r o j e t to the west. As the e l e c t r o j e t i s fixed i n space while the Earth rotates underneath, the position of the station r e l a t i v e to the e l e c t r o j e t changes steadily with l o c a l time. Therefore for bays with a symmetrical d i s t r i b u t i o n of occurrence, the station should, on average, cross the.central meridian of the e l e c t r o j e t at the median time of occurrence,. For a westward e l e c t r o j e t t h i s i s at 02.00 LZT and f o r an eastward e l e c t r o j e t i t i s near 18.00 LZT. As mentioned, the sign of the D bay should change at these times, however, of the 222 bays produced by a westward e l e c t r o j e t , a l l but 6% featured a positive bay i n the D component. The negative bays in D that 112 occurred were evenly d i s t r i b u t e d through the period of negative H bays. For the 48 bays produced by an eastward e l e c t r o j e t , a l l of the D bays were of the same sign; For each magnetic bay disturbance the r a t i o of the H and D bays, AH and AD, was used to define the angle of azimuth (tan 4D/AH) of the equivalent disturbing vector. A disturbing vector produced by an e l e c t r o j e t along the auroral oval should l i e normal to the oval, however, both classes of bays produced disturbing vectors rotated 20°-30° anticlockwise from the normal. This i s i l l u s t r a t e d i n Fig. B. 2 which, taking the azimuth of midpoints of guiet arcs as the normal to the oval, shows that the disturbing vectors for the bays produced by a westward e l e c t r o j e t point 20(>-30a east of the normal. 113 No of occurences IO 20 30 40 50 8 0 L Fig. B. 2. D i s t r i b u t i o n i n azimuth of disturbing vectors responsible for magnetic bays at Halley Bay, 1974-75. The dotted l i n e s represent the range of position of the normal to the oval, as given by the mean azimuth of guiet arcs (after Sievwright,1969). 114 B. 4. Discussion Parkinson (1962), i n a study of disturbing vectors, observed at statcns throughout the world, found that a large proportion of them pointed i n the di r e c t i o n of the nearest deep ocean. . Induced currents increase as the depth of the ocean increases so i t i s reasonable to assume that near the coast the currents are deflected to follow the contours of the ocean f l o o r . Price (1967) notes that there i s probably an important coast effect extending over the shallower parts near the edge of an ocean* The effect of induced currents on magnetic variations near the C a l i f o r n i a coast was studied by Schmucker (1964) who concluded that the currents flowed p a r a l l e l to the coast and tended to concentrate near the edge of the continental shelf* Halley Bay i s a coastal observatory, so the most important currents w i l l flow in the sea. A bathymetric contour map of the Weddell Sea (Fig. .B. 3) shows that the contour l i n e s immediately adjacent to Halley Bay l i e at an angle of azimuth of approximately 35°, as does the coast. The actual e f f e c t of the coast on the intensity of the induced currents i s uncertain, however an assessment can be made by assuming that they produce t h e i r maximum eff e c t , i . e . doubling of the horizontal disturbance* During the night the mean azimuth of the mid-point of guiet arcs,fi* varies smoothly between 162° and 175°, from 18.00 LZT to 08.00 LZT (Sievwright, 1969). The disturbing vector solely produced by the el e c t r o j e t would be expected to l i e between these values. Assuming that the induced currents flow p a r a l l e l to the contour l i n e s , i * e . at an angle of azimuth Of = 35° ± 5°, 115 the expected azimuth of the observed disturbing vector «*, can be calculated*. The two l i m i t cases, when & = 162° with ~6 = 30° and when & = 175° with 8= 40°, give values for oc of 141° and 153° respectively. This i s consistent with the median azimuth of the actual disturbing vector observed which l i e s between 140° and 150°. 30°W 0° Fig. B.3. Bathymetric chart of the Weddell Sea adjacent to Halley Bay (after Heazen et a l , 1972). 116 B. 5. Conclusion Induced currents i n the sea can have a s i g n i f i c a n t e f f e c t on magnetic bays at coastal observatories. The magnetic bays at Halley Bay correspond to an e l e c t r o j e t to the south of the station modified by the effects of induced currents i n the adjacent ocean. These currents flow p a r a l l e l to the edge of the continental shelf and, because i f i t s alignment, have a greater e f f e c t on bays i n the D component than on those i n H. I t i s possible that induced currents have a mere s i g n i f i c a n t e f f e c t on D bays than f i e l d - a l i g n e d currents at other coastal observatories. Whether t h i s occurs depends on the r e l a t i v e magnitudes of the D perturbation a r i s i n g because of the coastal e f f e c t , Dc, and the normal perturbation* Dn, that would otherwise be obser ved; _ Any e f f e c t s of f i e l d - a l i g n e d currents (such as the reversal i n the D bays, proposed by Eostoker (1972)) would be r e f l e c t e d i n Dn and whether or not they were observed would depend on the r a t i o Dc/Dn.. The magnitude of Dc/Dn i s dependent on the orientation of the coastline: being small f o r an E-W coast and large when the coast runs N-S. Thus for coastal observatories the - alignment of the coast and the e f f e c t of induced currents should be investigated before using the magnetic bays to i n f e r overhead current systems..For instance Harang's (1946) analysis i s of magnetic disturbances at Scandinavian observatories which may be influenced by induced currents i n the sea. 117 APPENDIX C i THE EEOBLEM OF SOLAB IN EUCEE CUEEENTS (presented at the International S c l a r - T e r r e s t r i a l Predictions Workshop, Boulder, A p r i l 1979) C. 1. Introduction Power system disturbances have been known to occur during geomagnetic storms for nearly 30 years,* The disturbances are due to quasi-d. c. Currents, induced i n the earth by geomagnetic f i e l d v a r i a t i o n s , flowing through transformer neutral-ground connections into the power system. Because the geomagnetic storms originate with disturbances on the sun the guasi-d. c. . currents were called Solar Induced Currents (SIC), however recently the . more appropriate term Gecmagnetically Induced Currents (GIC) has also been used. The l e v e l of in t e r e s t of e l e c t r i c a l engineers i n geomagnetic phenomena shows a marked cor r e l a t i o n with the sunspot c y c l e ! Interest was f i r s t aroused by power system disturbances during the geomagnetic storm of March 24, 1940 (McNish, 1940) and extensive SIC e f f e c t s were noted during the storm of February 1958 (Slothower and Albertson,1967) and the storm of August, 1972 (Albertson et a l , 1974),* By the time of the l a s t sunspot maximum (1968-1970) a major research e f f o r t had been mounted under the sponsorship of the Edison E l e c t r i c I n s t i t u t e , and the r e s u l t s of the study (Albertson et a l , 1973; Albertson and Thorsen, 1974) represent the pr i n c i p l e contribution to cur knowledge of SIC. Si g n i f i c a n t contributions have been made i n p a r a l l e l f i e l d s by Anderson et a l (1974) , who studied the effect of geomagnetic 118 disturbances on cable communication systems, and by Campbell (1978) who analysed the induced currents in the Alaska pipeline. Campbell used the geomagnetic a c t i v i t y index, Ap, to determine the expected levels of induced currents i n the Alaska pipeline, and t h i s technigue was adapted by Goddard and Boerner (1978) to the SIC problem. Apart from t h i s , there has been an increased awareness of the effect of geomagnetic phenomena on man-made systems (e*g. . Lanzerotti, 1978) and of the effect cf power system ra d i a t i o n on the space environment (Helliwell et a l , 1975; Hayashi et a l , 1978) but l i t t l e work s p e c i f i c a l l y related to the problem of SIC* . The facts to date about SIC are that they fluctuate with a period of several minutes, ie,« . are guasi-d.c. compared to 60Hz; and t h e i r occurrence correlates with that of geomagnetic storms. . Areas of igneous rock geology give r i s e to higher SIC values and anamously-high SIC are experienced at a location i n Newfoundland (Albertson and Thorsen, 1974). SIC are generally more severe at higher l a t i t u d e s and because of t h i s are believed to be due to the geomagnetic disturbances produced by the auroral e l e c t r o j e t . . Possible values of the surface e l e c t r i c f i e l d (and conseguent l e v e l s of SIC) due to the auroral e l e c t r o j e t were calculated by Albertson and Van Baelen (197C) and shown to be consistent with observed SIC..Albertson and Van Baelen also showed that SIC should be greater i n power l i n e s running E-W compared to those running N-S, although no observation of this e f f e c t i n p r a c t i c e has been reported i n the l i t e r a t u r e . . The e f f e c t s of SIC on e l e c t r i c power systems and the 119 prediction needs of the power industry have been well covered by Albertson and Thcrsen (1974) and Albertson and Kappenman (1979) . The. problem confronting geophysicists i s to explain the production of surface e l e c t r i c f i e l d s by geomagnetic disturbances and combine the e l e c t r i c f i e l d information with power system parameters to produce a quantitative understanding of SIC. This knowledge should then be coupled with improvements in p r edicting geomagnetic disturbances to obtain forecasting of SIC l e v e l s i n any particular power system. C.2.„Production Of Surface E l e c t r i c Fields Geomagnetic disturbances induce currents i n the earth and the p o t e n t i a l drop produced by the flow of these currents can be detected at the surface as an e l e c t r i c f i e l d E, also call e d the earth surface potential, ESP. In the simplest case the geomagnetic disturbance can be considered as a downward propogating wave of freguency u/, incident on a homogeneous earth of conductivity <r, . . The r e l a t i o n s h i p between the e l e c t r i c and magnetic f i e l d s at the surface i s given by and the depth of penetration of the wave to 1/e of i t s surface value i s the skin depth It i s obvious that the skin depth increases with the period of the geomagnetic variation and so, i n a re a l earth, conductivity changes with depth w i l l a f f e c t the freguency response of the function Ex/Hy. This was recognised by Kato and Kikuchi (1950) and Kato and Yokoto (1953) who expounded the i n i t i a l theory 120 which, with further developments by Cagniard (1953), led to the technique now known as the magnetotelluric method,. The magnetotelluric method uses measurements of Ex/Hy versus frequency, , to determine the conductivity structure of the earth, usually by comparison of the experimental r e s u l t s with r e s u l t s from calculations f o r a range of earth models. In the SIC study we are concerned with the reverse problem of (hopefully) knowing the conductivity structure below the 'area concerned and wishing to calulate the surface e l e c t r i c f i e l d Ex produced by geomagnetic variation of magnitude Hy and freguency .. However, the mathematical analyses of induced current developed f o r magnetotelluric studies (e.g. P r i c e , 1967; Jones and Price, 1970) are s t i l l applicable to the SIC problem. The th e o r e t i c a l treatment of induced currents developed by Cagniard (1953) has been extended by l a t e r workers to include ef f e c t s due to the scale of the source f i e l d . For example, the formula produced by Wait (1962) f o r a 3 layer earth with layer thicknesses h, , hx and o o , and c o n d u c t i v i t i e s <r, , <rx a»J <rs is where the 1st term i s the formula for a homogeneous earth of conductivity o;; the 2nd term includes the e f f e c t of the source size L, with B= S/^La ; and the 3rd term contains the layered earth e f f e c t s . Price (1962) has argued that Waits formulation contains some simplifying assumptions, regarding the e f f e c t of the source size , that a f f e c t both Q and S ; and there i s s t i l l some confusion as to the true e f f e c t of the source size on the 121 d i s t r i b u t i o n of induced currents; Determining the source size i s also a problem- Campbell (1978) used Waits* (1962) formulation and derived the formula L=0.2T f o r the scale length, i n km, of a geomagnetic variation with period T sec. This formula i s based on the concept that a geomagnetic variation with a period of 24 hours has a scale equal to the circumference of the earth round the auroral zone, and that shorter period variations have a correspondingly smaller scale length; .However i f one considers a geomagnetic bay with T=1800 sees (30 min.), produced by the auroral e l e c t r o j e t , Campbell's formula gives a scale length eguivalent to 7,. 5 degrees of longitude, whereas the auroral e l e c t r o j e t i s known to extend up to 60 or more i n longitude. The scale length of other geomagnetic variatons whose o r i g i n i s attributed to the auroral e l e c t r o j e t (eg. Pc5 pulsations) are l i k e l y to be s i m i l a r l y underestimated by Campbell*s formula. Wait's formulation was applied to the SIC problem by Goddard and Boerner (1978) who used i t to compute the probable freguency spectrum of e l e c t r i c f i e l d variations during a geomagnetic disturbance observed i n Manitoba.. This e l e c t r i c f i e l d freguency spectrum was then used f c r comparison with the frequency spectrum of Sic observed during the same disturbance (fi g C.I). The spectra of the e l e c t r i c f i e l d and S i c should be highly correlated so the discrepancy shewn i n f i g . C.1 i s l i k e l y due to the ca l c u l a t i o n of the e l e c t r i c f i e l d . Goddard and Boerner used Campbell's formula for scale length and, as indicated above, this i s an approximation and may produce a different freguency dependence than occurs i n practice, The 122 transfer function Ex/Hy used by Goddard and Boerner, as shown by fi g s C.1a and C«1b, i s obviously independent of frequency; however, Campbell (1978) using the same formulation, but a different earth model, obtained a transfer function dependent on freguency which he approximated by the expression thus the frequency dependence of the transfer function, Ex/Hy i s very dependent on the earth model used-122 transfer function Ex/Hy used by Soddard and Boerner, as shown by f i g s C.1a and C.1b, i s obviously independent of frequency; however, Campball (1978) using the same formulation, but a di f f e r e n t earth model, obtained a transfer function .dependent on freguency which he appraximated by the expression <\o\ = 3-d — 0-1,2 L^T thus the frequency dependence of the transfer function, Ex/Hy i s very dependent on the earth model used. 123 T 1 i 1 r a . 10J 1 ' « " « • I 01 02 05 1 2 5 10 Period in Kiloseconds F i g . ,.C. 1. . Average power spectra of (a) X component of magnetic f i e l d at Whiteshell, Manitoba, (b) expected Y component of surface e l e c t r i c f i e l d (calculated from the magnetic f i e l d variations) , and (c) solar induced currents at LaVerendrye, Manitoba, (after Goddard and Boerner, 1978).. 124 I t should be remembered that the parameters c\t <rx/ k,, used in the earth models are the mean values for an area comparable to the scale of the inducing f i e l d . The s i g n i f i c a n c e of the conductivities at depth i s i n how they a f f e c t the d i s t r i b u t i o n of currents with depth and consequently the value of the surface current* The e l e c t r i c f i e l d at a p a r t i c u l a r location can then be determined by knowledge of the surface current and the l o c a l conductivity. Igneous rock areas are an example of the e f f e c t of l o c a l conductivity because th e i r low conductivity, compared to other rock types, gives r i s e to higher e l e c t r i c f i e l d s and i n consequence a greater p r o b a b i l i t y of SIC problems. In Newfoundland the Sic magnitudes are too large to be simply accounted for by low conductivity and evidence i s presented i n section C.5 to show that the severity cf SIC at Ccrnerbrcok, Newfoundland i s due to channelling of currents induced i n the sea through a region adjacent to the power l i n e . C.3. System Considerations The resistance of a power l i n e , although t y p i c a l l y a few ohms* i s considerably greater than the resistance of the earth between the two ends of the power l i n e * Thus i t i s applicable to calculate the surface e l e c t r i c f i e l d ignoring the presence of the power l i n e , and then examine the e f f e c t cf that e l e c t r i c f i e l d on the power l i n e as a seperate problem.. The e l e c t r i c potential applied across the ground points at the ends of a power l i n e i s simply the product of the component of the e l e c t r i c f i e l d p a r a l l e l to the power l i n e and the distance spanned by the power l i n e . . However the magnitude and 125 distributuon of the currents, in the power system, produced by t h i s earth potential depend on the r e l a t i v e magnitudes of the resistances of different parts of the system. An analysis of part of the B.C. Hydro power system showed that for guasi-d.c. currents the system could be represented by a network of "line resistances" and "station resistances" as shown i n f i g C.2a. The station resistance i s the resistance between the high voltage bus to which the power l i n e s are connected and the ground mat of the substation.. This station resistance could be simply the resistance of two transformers i n p a r a l l e l or, where autotransformers are used, a more complicated network of resistances as shown in f i g C*2b. There i s also a resistance between the station ground mat and "true earth", which i s c a l l e d the "station ground resistance", but t h i s i s usually small compared to the s t a t i o n resistance and can be negl-ected, . The 3-phase power system can then be considered as 3 i d e n t i c a l resistance networks i n p a r a l l e l and i t i s s u f f i c i e n t to consider one network only. This w i l l enable calculations to be made of the quasi-d.c. currents expected i n any part of the system; except that the current through the neutral-grcund connector of a transformer (where SIC are usually measured) w i l l have currents from each of the 3 p a r a l l e l networks and so w i l l have 3 times the current calculated for an i n d i v i d u a l winding of the transformer. To determine the quasi-diC. Currents i n the network shown i n f i g C. 2a eguate voltages around loop I where I 126 -VA'AWvW-F i g . 2 a) 1-phase diagram of a power l i n e , i l l u s t r a t i n g the n e t -work of l i n e r e s i s t a n c e s & , R 2 . . .R , and s t a t i o n ^ r e s i s t a n c e s S , ,S , . . . S 4 . , 0 1 1 2 n,n+l. b) Diagram o f the components of the s t a t i o n r e s i s t a n c e betve-• en a 500KV l i n e and ground, ( i n t h i s case at W i l l i s t o n s u b s t a t i o n i n B.C.) v 330KV 60 KV s i - i . i ( I i - I i - i ) + V i - s i , i + i 'WV - vt (5) C o l l e c t i n g terms gives -S. , . I . - + (R. + S. .. . + S. . ,) I - S I = V i-1,1 l - l i i - l , i i,i+lJ i i . i + l ^ i + l v i (6) (7) For an i s o l a t e d loop , . x Co) = V. 1 R.+S. . .+S. . i i-l»i i , i + l and t h i s can be taken as a f i r s t approximation to the s o l u t i o n of the g e n e r a l case, i e . equation (6). Employing an i t e r a t i v e method one can w r i t e [ - I <°> + i CD + i i i ** (8) (1) where the f i r s t order c o r r e c t i o n 1± i s given by s I ( o ) + <? T Co) <!) - i+1 R. + S. , . + S. . i i-l»i i , i + l (9) x - l , i V. ,+ S j , i + 1 R.+S. . +S. ... } R. ,+S. 0 . ,+S. , . i - l R. ,+S ,+S , l i - l , i , i + l I i - l i - 2 , i - l i - l , i ; i+1 i , i + l i+1 .1+2 V i + 1 j Therefore I, = (10) 1 R i + S i - l , i + S i ) i + l l i R i - l + S i - 2 ( i - l + S i . l , i i , i + l R i + 1 + S i , i + 1 + S i + 1 (ID .1+2 v3 127 Of the three terms i n brackets i n equation 11 the c o e f f i c i e n t s of V and V w i l l be les s than one and i n the many cases i n which l i n e resistances are qreater than the station resistances the c o e f f i c i e n t s w i l l be small so that these terms can be ignored. The isolated loop approximation (equation 7) then represents a reasonable solution to the problem of calculating loop currents. To determine the current through a pa r t i c u l a r station resistance S^+i i t i s necessary to consider the currents i n loops I and i + 1. It w i l l be seen from equation 5 that the currents I; and 1;+, tend to cancel each other at th e i r common station* and so higher currents w i l l be experienced i n the l i n e s than at the stations,* The st a t i o n resistance i s actually comprised of a network such as i n f i g C.2b and the station current w i l l naturally divide up between the d i f f e r e n t paths according to the r e l a t i v e resistances of the di f f e r e n t transformers. Hence the current through an i n d i v i d u a l transformer w i l l be a f r a c t i o n of the current It-„., - I i flowing to ground through the station.. Fig C,2b also shows that where autotransformers are used higher currents w i l l flow in the high voltage part of the winding than i n the low voltage part. 128 C.4..Prediction Of SIC I f forecasting of SIC i s to be improved i t i s no longer s u f f i c i e n t to describe SIC as being correlated with geomagnetic storms. I t i s desirable to ascertain which mechanisms are responsible for SIC and then prediction of these mechanisms can be related to prediction of SIC. .The increase of SIC magnitudes with l a t i t u d e , as mentioned e a r l i e r , points to the auroral e l e c t r o j e t as the cause of the f i e l d variations responsible and t h i s has been assumed to be the case by most authors. However i t i s debatable whether the SIC seen at lower l a t i t u d e s (eg. In the southern USA) are due.to the auroral e l e c t r o j e t ; and Anderson et a l (1974) have presented evidence tc show that magnetopause currents were responsible for the geomagnetic disturbance that so badly affected some communications systems i n August 1972. Resolution of the question would be greatly aided by knowledge of the diurnal occurrence pattern of SICu . Many geomagnetic phenomena have d i s t i n c t average v a r i t i o n s of occurrence with local time which are well documented (e.g. Hartz and Brice, 1967). An eguivalent representation i s a l o c a l time perturbation p r o f i l e such as that shown in f i g 1 of Clauer and McPherron (1979). Their p r o f i l e indicates that, for the current system s p e c i f i e d (which involves a p a r t i a l ring current), the maximum perturbation at mid-latitudes w i l l occur i n the afternoon. At auroral l a t i t u d e s the majcr source of disturbances i s the westward e l e c t r o j e t and t h i s i s located i n the region extencing from 22.00 to 06.00 l o c a l time. Other disturbances can be s i m i l a r l y associated with a range of l c c a l times and the l o c a l times of occurrence of SIC should match those of the 129 phenomena responsible..Also knowledge of SIC occurrence patterns at a number of locations could show whether or not the responsible phenomena change with l a t i t u d e . Should a SIC mechanism be i d e n t i f i e d which has a well defined l o c a l time dependence t h i s knowledge would greatly aid SIC prediction. Even i f SIC are produced by each and every type of geomagnetic disturbance they may show a local-time occurrence pattern simply because of the greater frequency cf occurrence or greater magnitude of one s p e c i f i c mechanism. In the majority of cases such a local-time dependent probability of SIC occurrence could be used to provide several hours warning to e l e c t r i c u t i l i t y operators of potential SIC problems. Suppose, for example, that SIC are predominantly due to geomagnetic diaturbances caused by the westard auroral e l e c t r o j e t and so occur mainly between 22.00 and 06.00 l o c a l time. By continuously monitoring t h i s time zone, warnings of geomagnetic disturbances, i n many cases, could be given before they had a s i g n i f i c a n t e f f e c t in N,. America. This method, of course, provides no warnings of disturbances that commence while Ni _ America i s i n the 22.00-06.00 l o c a l time zone. Major disturbances w i l l l a s t f o r a day or more so the time zone specified could be considered as a "disturbed" zone,fixed w. r. t. the sun (fig C. 3), through which N*America passes as the Earth rotates. Continuous observation of t h i s "disturbed" zone e f f e c t i v e l y reguires world-wide real-time monitoring of the surface geomagnetic f i e l d and relaying of the information to a centre i n N. America. . (A major undertaking). 130 1 8 0 0 Fig- C.3. View of the earth from above the N. pole showing N. America being brought in t o a 'disturbed 1 zone by the earth's rotation. 131 However n o t i f i c a t i o n of disturbed conditions at European observatories could be used to provide forecasts, 4 hours or more i n advance, of potential SIC conditions i n N. America. Even when geomagnetic disturbances can be predicted, sections 2 and 3 have shown that there ate several factors l i m i t i n g the translation of th i s to a forecast of the SIC magnitude to be expected at any p a r t i c u l a r location, Fortunately e a r l i e r experimental work has provided us with an empirical measure of the re l a t i v e magnitudes of SIC at d i f f e r e n t locations on the form of Albertson and Thorsen fs (1974) cumulative severity index (CSI) . SIC predictions could be broadcast for a station with a CSI of 1 and, providing a suitable scale could be devised f o r the prdictions, the e l e c t r i c a l u t i l i t y operators at each location could multiply t h i s by t h e i r own CSI to obtain a prediction relevant to their own system, 132 The Case Of Newfoundland The severity of SIC at Cornerbrook, Newfoundland i s too great to be simply another example of an igneous rock area and the case requires closer inspection* Fisher (1970) documented some of the problems due to SIC experienced with the Cornerbrook power system and he also reported that other e l e c t r i c u t i l i t i e s in Newfoundland did not experience noticeable SIC effects* The Cornerbrook power l i n e i s coincident with a zone joining carboniferous sediments to the s a l t water of the Gulf of St Lawrence (Wright, 1978); and Wright has suggested that t h i s zone may be a conductive channel f o r t e l l u r i c currents and be responsible for the great severity of SIC. The i n t e n s i t y of induced currents i n the oceans w i l l be much greater than those on land because the conductivity of sea water (3-4 ohm-"' m**' ) i s several orders of magnitude greater than that of rock (10"' -10 - 4 ohm"1 m~' )..Price (1967) has shown that s i g n i f i c a n t induced currents with a period of 15 minutes can be expected i n depths as small as 500m; therefore, for shorter periods, sizeable currents can be expected i n the depths of 300m t y p i c a l of the continental s h e l f alcng the east coast of N. America. A t h e o r e t i c a l analysis of induced currents near a conductivity discontinuity (such as a coastline) by Jones and Price (1970) shows that the currents should be concentrated on the high conductivity side of the discontinuity; and evidence for such an e f f e c t has been presented by Schmucker (1964) and Boteler (1978).. Thus the i n d i c a t i o n s are that high induced current d e n s i t i e s occur along the east coast of N. America during geomagnetic disturbances. . 133 Newfoundland and Nova Scotia represent low conductivity anomalies i n the path of the co a s t a l induced currents (fig D.4) and although the currents w i l l tend to flow around the land the current across Newfoundland and Nova Scotia w i l l be greater than the currents experienced on the continent proper. The currents w i l l obviously concentrate across the narrowest parts of Newfoundland (see insets i n f i g C. 4) and the surface e l e c t r i c f i e l d i n these locations w i l l thus be considerably greater than elsewhere on the island. Over distances for which the potential drop in the sea i s snail the coastlines on opposite sides of the i s l a n d can be considered as eguipotentials. Hence any power l i n e running across the i s l a n d and grounded on each shore w i l l have the same pot e n t i a l applied across i t during geomagnetic storms. Should such power l i n e s present the same resistance to SIC the magnitudes of SIC would be the same i n each power l i n e * However shorter l i n e s have lower resistances and consequently higher SIC l e v e l s . Thus the Cornerbrook power l i n e i s i n one of the worst possible situations for experiencing SIC problems, Another location prsumably s i m i l a r l y affected i s the Amherst area of Nova Scotia: a fact that may be s i g n i f i c a n t considering the plans for power generation i n the Bay of Fundy. 134 F i g . .C.4i Map of the E coast of N. America (geographic coordinates) . The i n s e t s show the channelling of induced currents across Newfoundland and Nova Scotia that i s postulated to occur during geomagnetic disturbances. 135 C. 6 . Conclusions I t has been demonstrated that the magnitude of SIC at different locations on l o c a l conditions ans t h i s i s most marked in the case of the Cornerbrook power system i n Newfoundland, At a p a r t i c u l a r location, the l e v e l of SIC i n i n d i v i d u a l transformers (which i s the c r i t i c a l parameter f o r SIC effects) cepends on the r e l a t i v e magnitude of the currents i n the adjacent parts of the power system as well as on the number and size of the transformers at the substation. The determination of the l o c a l time dependence of SIC occurrence would help i d e n t i f y the phenomena and could p o t e n t i a l l y enable several hours warning to be given of SIC conditions. 

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