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Prompt world-wide geomagnetic effects of high-latitude nuclear explosions Caner, Bernard 1964

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PROMPT WORLD-WIDE GEOMAGNETIC EFFECTS OF HIGH-ALTITUDE NUCLEAR EXPLOSIONS by BERNARD CANER B.Sc, University of Alberta, i 9 6 0 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of GEOPHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1964 In presenting this thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of • B r i t i s h Columbia, I agree that the Library shall'make i t f r e e l y available for reference and study, I further agree that per-mission for extensive copying- of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that;copying or publi-cation of this thesis for f i n a n c i a l gain shall, not be allowed without my written permission* Department of GEOPHYSICS The University of B r i t i s h Columbia, Vancouver 8, Canada Date i ABSTRACT A b r i e f summary of observational data i s presented, covering the disturbances recorded within seconds of high-a l t i t u d e nuclear detonations, with p a r t i c u l a r emphasis on the "phase B" signal recorded at H+2 seconds following the " S t a r f i s h " test of July 9, 1962. The s a l i e n t character-i s t i c s of t h i s signal are specified, and a number of suggested models are analysed i n d e t a i l . Although no con-clusive decision can be reached on the basis of presently available data, the most l i k e l y mechanism appears to be hydromagnetic waves along the f i e l d l i n e through the deton-ation point, with energy conversion into electromagnetic modes at the mirror points. V ACKNOWLEDGEMENTS Work on t h i s report was begun at V i c t o r i a Magnetic Observatory (Dominion Astrophysical Observatory). I should l i k e to thank Prof. J.A. Jacobs f o r providing the f a c i l i t i e s f o r i t s completion. Valuable suggestions and c r i t i c i s m have been received from Prof. T. Watanabe of the Department of Geophysics, University of B r i t i s h Columbia, Dr. K. Whitham of the Dominion Observatory, and Dr. J.P. Kenney and Dr. H.R. Willard of the Boeing S c i e n t i f i c Research Laboratories. I should l i k e to thank the Director of the Signals Research and Development Establishment (U.K. Ministry of Aviation), and Dr. C.P. Sechrist of HRB-Singer Inc., f o r supplying unpublished data. i i TABLE OP CONTENTS ABSTRACT i TABLE OP CONTENTS i i LIST OF FIGURES i v ACKNOWLEDGEMENTS v INTRODUCTION 1 OBSERVATIONAL DATA 4 INTERPRETATION - "PHASE A" 14 INTERPRETATION - "PHASE B" . . . , . . . . 15 "PHASE B" - DIRECT EFFECT MODELS 1) Hydromagnetic waves i n Ionosphere 19 2) Hydromagnetic waves i n high al t i t u d e ducts . . . . 20 3) Neutron-decay betas 21 4) Fission-product betas 24 5) Setting up of local conditions . . . 25 "PHASE B" - SECONDARY FOCUS MODELS General comments . . 26 1) Neutron-decay betas 28 2) Fission-product betas 29 3) Hydromagnetic-Electromagnetic conversion i n the lower ionosphere 30 i i i 4) Hydromagnetic impulse guided along the f i e l d - l i n e . 3 3 5) Hydromagnetic wave along the f i e l d - l i n e 38 6) Protons guided along the f i e l d - l i n e 40 CONCLUSION 44 APPENDIX I - ADDITIONAL LINES OF APPROACH . . . .' . . . 4 5 APPENDIX II - X-RAY IONIZATION AND ALFVEN VELOCITIES BELOW THE DETONATION POINT 51 . REFERENCES 59 i v LIST OF FIGURES F i g . 1 Geomagnetic recording at V i c t o r i a , B.C., July 9, 1962, 0900-0901 UT 7 F i g . 2 Geomagnetic recording at State College, .Pa., July 9, 1962, 0859:50-0900:45 UT 8 F i g . 3 Geomagnetic recording at Christchurch, England, July 9, 1962, 0900:09-0900:26 UT 9 F i g . 4 Frequency-response c h a r a c t e r i s t i c s of instrumentation used at V i c t o r i a , State College, and Christchurch 10 F i g . 5 Summary of geomagnetic recordings from Argus III (after Berthold, Harris and Hope, i960) 17 F i g . 6 F i e l d l i n e through the detonation point -" S t a r f i s h " test of July 9, 1962 34 F i g . 7 Energy spectrum of f i s s i o n neutrons 4 l Pig. 8 x-Ray i o n i z a t i o n below the detonation point 54 F i g . 9 Alfven v e l o c i t i e s below the detonation point 55 INTRODUCTION This report examines i n d e t a i l one narrow aspect of geomagnetic disturbances set up by high-altitude nuclear tests - the world-wide signals recorded within a few seconds of detonation, with p a r t i c u l a r emphasis on the major o s c i l -l a t o r y signal, the st a r t of which was recorded at many locations about 2 seconds a f t e r the detonation, and which we have designated "phase B". Slower geomagnetic a r r i v a l s as well as radio propagation e f f e c t s have been reported and analysed by several observers (for example, Maeda et a l i a , 1 9 6 4 ) . Although a l l these e f f e c t s are obviously related i n as f a r as the source i s concerned, the actual mechanisms involved appear to be e n t i r e l y d i f f e r e n t , thus j u s t i f y i n g the separate treatment of an i s o l a t e d feature such as the "phase B" sig n a l . The main inter e s t of t h i s signal l i e s i n i t s - broad s i m i l a r i t y with c e r t a i n types of natural geomagnetic pulsations (pearls, or type-A o s c i l l a t i o n s ) . Having the advantage of a controlled source (at l e a s t In time and lo c a t i o n ) , i t therefore provides an i n t e r e s t i n g opportunity f o r complementing the study of possible natural micropul-sation mechanisms - even though the analogy with natural phenomena can only be followed to a l i m i t e d extent. Although no e x p l i c i t conclusions are drawn i n t h i s respect, 2. the proposed mechanisms have been narrowed down to essent-i a l l y the same as those which are being considered as explanations f o r pearl-type micropulsations. This report w i l l be l i m i t e d to a discussion of global geomagnetic e f f e c t s recorded within a few seconds of the detonation, and i t w i l l not include i s o l a t e d l o c a l e f f e c t s i n the v i c i n i t y of the launch zones or conjugate areas. The observational data w i l l f i r s t be presented f a i r l y b r i e f l y . A number of possible mechanisms w i l l then be considered on a q u a l i t a t i v e basis, with rough order-of-magnitude quantitative support. It should be mentioned at the outset that no f u l l y acceptable unique solution Is pro-posed. In p a r t i c u l a r , t h e o r e t i c a l work i s required on hydromagnetic-electromagnetic energy conversion at very low frequencies. However, t h i s i n v e s t i g a t i o n should provide a useful basis f o r more detailed work on any p a r t i c u l a r mech-anism. In view of the large release of energy i n various forms, a unique solution may not be possible on the basis of the presently available data, since combinations of several d i f f e r e n t mechanisms may be involved. The energy output of a nuclear explosion i s 105 Joules per Kiloton. For an unshielded f i s s i o n explosion i n space t h i s i s d i s t r i b u t e d roughly as following (Glasstone, 1963; L a t t e r and LeLevier, 1964): Approximately 30-70$ i n x-rays (1 to a few Kev energy range), 0.01-1$ i n prompt 7-rays (mean energy about 1 Mev), 0.1-1$ In prompt neutrons ( 0.1 Mev to a few Mev), and the remainder i n k i n e t i c energy of f i s s i o n products, with a few percent i n delayed radia t i o n . 4. OBSERVATIONAL DATA The relevant tests f o r which information i s a v a i l -able are the following: Aug. 1958: two te s t s In the P a c i f i c above Johnston Island, "Teak" at an al t i t u d e of 70-80 km, "Orange" at 30-40 km. The announced y i e l d was " i n the Megaton range". Sowle (1961) estimated the y i e l d as about 4 Megatons f o r "Teak" and 2-4 Megatons f o r "Orange". Aug.-Sept. 1958: three low-yield (1-2 Kilotons) explosions over the South A t l a n t i c , code-named Argus I, II and III, at al t i t u d e s of about 480 km. July 9, 1962: Johnston Island, y i e l d estimated at 1.4 Megatons, a l t i t u d e 400 km, code name " S t a r f i s h " . Oct. 20, Oct. 26, Nov. 1, 1962: low and medium al t i t u d e ("tens of kilometers") tests, with sub-megaton y i e l d s . None of the lower a l t i t u d e (under 100 km) te s t s produced major global geomagnetic e f f e c t s . Most of them produced both f a s t (onset time within 1 second - McNish, 1959) and slower following bay-type disturbances at locations within about 1000-2000 km of the launch area, f a l l i n g o f f rap i d l y with distance. Some sharp e f f e c t s were also observed at the conjugate points. Lawrie, Gerard, G i l l (1959) and Obayashi (1963) have summarized and interpreted these e f f e c t s . However, no major world-wide e f f e c t s were observed, i n spite of the high y i e l d s . For the higher-altitude Argus tests, e f f e c t s were observed at magnetic stations at widely separated locations (Eschenbrenner et a l i a , i 9 6 0 ) , i n spite of the low y i e l d s . Amplitudes were very small ( f r a c t i o n to a few gammas), and recordings were obtained only on sensitive equipment - i n some cases barely above the background l e v e l . Berthold, Harris and Hope (i960) have summarized t h i s data i n d e t a i l f o r Argus I I I (Sept. 6, 1958). The a r r i v a l times were plotted against station to detonation point distances. The slope of a straight l i n e f i t t e d to the f i r s t a r r i v a l points indicated a ground-level propagation v e l o c i t y of 3050 km/sec. No probable error was s p e c i f i e d f o r t h i s r e s u l t . Similar global e f f e c t s were recorded following the " S t a r f i s h " test of July 9, 1962, but with considerably higher amplitudes - presumably due to the higher y i e l d . The test was announced well i n advance, and a countdown trans-mitter could be monitored. Nevertheless, a s u r p r i s i n g l y small amount of high-quality u n c l a s s i f i e d data has been co l l e c t e d i n geomagnetism. With a few notable exceptions very few stations improved t h e i r operating techniques f o r t h i s p a r t i c u l a r period i n order to obtain timing accuracies of ±0.1 seconds or better ( i n p a r t i c u l a r accurate absolute timing on the record i t s e l f and speed-up of recorders). Also, the hour time-mark ob l i t e r a t e d the f i r s t a r r i v a l at many locations, p a r t i c u l a r l y on slow-speed magnetograms. F i n a l l y , the amplitude of the signals was unexpectedly large. At almost a l l locations the instrument settings were too sensitive and the traces went o f f - s c a l e . Con-sequently very l i t t l e v a l i d amplitude data has so f a r be-come available, and t h i s report i s based almost e n t i r e l y on timing considerations, p a r t i c u l a r l y from the 4 following stations: V i c t o r i a , B.C., Canada - Dominion Observatory College, Alaska - University of Alaska State College, Pa., U.S.A. - HUB Singer Inc. Christchurch, England - Signals Research and Development Establishment The Christchurch data were p a r t i c u l a r l y valuable In view of the long distance coverage, and i n view of the high q u a l i t y of the available recordings - complete three-componentsets of high-speed h i g h - s e n s i t i v i t y as well vas broad-band l o w e r - s e n s i t i v i t y recordings. Figures 1, 2 and 3 show the high-speed recordings f o r V i c t o r i a , State College and Christchurch, and Figure 4 the frequency response of the instrumentation used. The relevant data f o r the 4 stations i s summarized i n Table 1 below. Consecutive l i n e s contain the following information: a) great c i r c l e distance (km) to Johnston Island; b) delay time (seconds) f o r the f i r s t a r r i v a l . FIG. I VICTORIA MAGNETIC OBSERVATORY DECLINATION (MAGNETIC E-W) 0.5 GAMMA/DIVISION 0900 0900:11.4 0900:30 UNIVERSAL TIME STATE COLLEGE, PENNSYLVANIA FIG. Z - GEOMAGNETIC MICROPULSATIONS - 9 JULY 1962 (HORIZONTAL COMPONENT) AFTER SECHRIST (1962) COURTESY HRB-SINGER INC. FIG. 3 CHRISTCHURCH, ENGLAND COURTESY SRDE 2 UJ CO U . O Hi co o a. CO ui a: a a. UJ > - J UJ a: 1.0 0.5 0.1 0.05 0.0! 0.001 CHRIS CHUFlCH x, s i - / ; v COLLEGE 3 r 0.01 0.1 FREQUENCY (CPS) V I — 1 o 10 FIG. 4 MAGNETOMETER CHARACTERISTICS: VICTORIA, STATE COLLEGE, CHRISTCHURCH 11. The detonation time (H=0) i s 0 9 0 0 : 0 9 . 0 2 5 ± 0 . 0 2 5 sees, as deduced from the sharp cut-off of the countdown transmitter and other recorded radio e f f e c t s (Hanley, 1962 ; Caner and Whitham, 1962); c) probable and maximum errors f o r the delay time - the l a t t e r includes the uncertainty i n o r i g i n time; d) components recorded, and estimated peak-to-peak amplitude (in gammas) of the f i r s t movements ( a l l o f f - s c a l e ) ; e) reference. TABLE 1 V i c t o r i a College State College Christchurch 5400 5600 9600 12000 2.0 2.1 2.4 1.94 ± 0 . 1 ( 0 . 2 ) ±0.06 ( 0 . 1 ) ± 0 . 1 ( 0 . 3 ) D >20 P >5 NS >10 Caner and Wilson and Sechrist Whitham Suglura (1962) (1962) (1963) ± 0 . 0 1 ( 0 . 0 4 ) EW >10-15 NS > 5 Z < 0 . 5 Stevens (unpublished) 12. An important group of observations comes from the network of the U.S. Army El e c t r o n i c s Research and Development laboratory. These have not been published, but a summary of the r e s u l t s was presented In a paper at the IUGG meeting (Bomke et a l i a , 1963). A strong o s c i l l a t o r y signal was re-corded at H+1.9 seconds simultaneously at a l l the stations of t h i s network (Florida, Maine, South Carolina, New Jersey as well as Hawaii and Samoa). No probable error was given f o r t h i s f i g u r e ; ±0.1 seconds i s probably a reasonable estimate. A sharp broad-band pulse containing higher f r e -quencies (but of much lower amplitude) was recorded at the instant of detonation at Samoa and Hawaii. A t h i r d group of observations comes from the net-work of the " I n s t i t u t de Physique du Globe" of the Univer-s i t y of Paris (Roquet, Schlich and Selzer, 1962). Two of the stations, Chambon-la-Foret i n France and Kerguelen i n the Indian Ocean, reported the instantaneous a r r i v a l at H+0 (±0.1 to ±0.2 seconds), followed by a very strong and sharp reinforcement of the perturbation at Chambon about 2 seconds l a t e r (Roquet et a l i a , 1963). The t h i r d station, Dumont d ' U r v i l l e i n the Antarctic, reported a major a r r i v a l at H+2 ± 1.5 seconds. F i n a l l y , high-resolution ELF recordings obtained at Byrd i n the Antarctic (Lokken, private communication) and at Westford, Mass. (Balser and Wagner, 1963) show both the 13. instantaneous pulse and the higher-amplitude, lower-frequency signal at H+2, with d i s t i n c t separation between the two signals. Because of the high-frequency passbands of these systems (5-35 cps f o r Westford, 2-30 cps f o r Byrd) no con-clusions can be drawn regarding the waveform of the second signal on these recordings. There are many other recordings, but since absolute timing accuracies are ± 0 . 5 to ± a few seconds they do not add any useful information from the point of view of timing -although they are important f o r an examination of geographic coverage. As pointed out by Roquet, Schlich and Selzer (1963) the evidence f o r global synchronism i s overwhelming. We can i d e n t i f y the following d i s t i n c t e f f e c t s : "phase A": an instantaneous pulse, low-amplitude, high-frequency content (> 2 cps), at H+0 (± milliseconds to ± 0 . 1 seconds). "phase B": a second, major o s c i l l a t o r y signal s t a r t i n g at H+2 simultaneously ( ± 0 . 1 to 0 . 2 sees) a l l over the globe. The period i s i n i t i a l l y about 3 . 5 to 4 seconds, and appears to decrease to about 2 - 2 . 5 seconds by the f i f t h o s c i l l a t i o n . This period decrease i s not d e f i n i t e l y proved, since at most stations the traces went o f f - s c a l e and the zero reference s h i f t e d . In view of the rapid amplitude decay a r e l i a b l e frequency measurement becomes d i f f i c u l t . 14. PHASE A - INTERPRETATION Interpretation of the "phase A" signal does not present any p a r t i c u l a r d i f f i c u l t i e s ; there are two possible mechanisms: a) a spheric, presumably excited by the prompt 7-ray emission (which occurs within microseconds and deposits most of i t s energy at a l t i t u d e s of about 20-30 km), or possibly by e l e c t r o s t a t i c (charge separation) e f f e c t s . > I t i s prob-ably reinforced by Schumann o s c i l l a t i o n s of the earth-ionosphere cavity. This i s confirmed by the Byrd ELF re-cording which has excellent time resolution - the fundamental Schumann frequency can be c l e a r l y i d e n t i f i e d on the f i r s t signal (Lokken, private communcation). The f i r s t signal on the Westford ELF recording i s reported to be si m i l a r to that observed f o r l i g h t n i n g strokes (Balser and Wagner, 1 9 6 3 ) . b) the second possible mechanism i s e f f e c t s of neutron-decay betas. This w i l l be discussed i n more d e t a i l at a l a t e r stage. The reason why not a l l stations recorded the "phase A" signal i s almost c e r t a i n l y a combination of i n s t r u -mental l i m i t a t i o n s and rapid amplitude f a l l - o f f with distance. The frequency response of most of the detectors used f a l l s o f f very rapi d l y beyond a few cps; f o r example the 3 db high-frequency cut-off points f o r V i c t o r i a , State 15. College and Christchurch (broadband record) are 5 cps, 1.5 cps and 2 cps respectively, A very low-amplitude signal at 8 cps would therefore be masked by background a c t i v i t y and/or instrument noise, whereas i t shows up on records obtained eithe r at close locations (Hawaii, Samoa), or with s p e c i a l -ized ELP equipment (Byrd, Westford)., or with low-noise high-frequency sensitive equipment such as that used by the French network. PHASE B - INTERPRETATION Interpretation of the "phase B" signal i s f a r more complicated and controversial. The sa l i e n t c h a r a c t e r i s t i c s of t h i s signal are summarized below, and any suggested mech-anism must take these points into account: a) global synchronism ( ± 0 . 1 - 0 . 2 sees) b) . 2 second delay a f t e r explosion ( ± 0 . 1 - 0 . 2 sees) c) extremely sharp r i s e (over 20-30 gamma/sec) d) i n i t i a l period 3 . 5 - 4 sees ( i . e . frequency 0 . 2 8 - 0 . 2 5 cps) e) amplitude heavily damped f) large i n i t i a l amplitude (about 30-50 gamma) g) altitude-dependence (occurs only when the source i s above F layer) h) decreasing period, to about 2 - 2 . 5 sees (?) 16. Before beginning a discussion of possible mech-anisms, i t i s useful to reconsider the Argus III data to check whether i t could f i t some of the above c h a r a c t e r i s t i c s . The relevant figure from Berthold et a l i a ( i 9 6 0 ) i s repro-duced i n Figure 5« I f we v i s u a l i z e the f u l l v e r t i c a l scale ( i . e . extended to zero distance) i t becomes obvious that the slope of a l i n e based on a number of points bunched over a narrow range i s extremely sensitive to errors i n i n d i v i d u a l point positions. The Azores observation i s not very r e l i a b l e - to quote from Newman's (1959) o r i g i n a l paper: "there may be a signal between plus 4 and plus 6 seconds". This leaves 5 points, a l l i n the range 12000 - 13700 km, i . e . covering only about 15$ of the t o t a l range. Consider-ing the probable errors of some of the observations, i t becomes obvious that any slope derived on t h i s basis i s not too r e l i a b l e . In f a c t , i t can be c l e a r l y seen that a ver-t i c a l l i n e (I.e. global synchronism) would f i t the data just as well or better (see dotted, l i n e on Figure 5 ) . This i s strengthened by Troitskaya (1961), who reported that the a r r i v a l s recorded at the Soviet t e l l u r i c stations (extending over 1 2 0 ° i n longitude) were simultaneous to within 1 second. The exact delay time i s unfortunately not known, since the e f f e c t i v e o r i g i n time has not been published. It would be very i n t e r e s t i n g to know whether the delay i s of the order of 2 seconds also, or whether i t i s dependent on y i e l d and a l t i t u d e . It i s not c l e a r whether the papers by Berthold 9.800 AZORES 3050 430 2t UJ o ? I2,000f^ 5 12.200 J ^KM/SEC" KM/SEC ( KM/SEC I 12.500 12.9001 { NEW JERSEY MAINE SUGGESTED ALTERNATIVE 13.700 .-•-H r UPPSALA REYKJAVIK W1^ - ARIZONA o 10 . 1 ..... I .'.J I . 20 30 40 50 TIME AFTER ZERO. SECONDS FIG. 5 Replotted recordings, giving an indication of velocities of two signals from Argus III. AFTER BERTHOLD, HARRIS, HOPE (I960) 18. et a l i a ( i 9 6 0 ) and Bomke et a l i a ( i 9 6 0 ) were based on access to unpublished o r i g i n time data, or whether the o r i g i n time was derived from extrapolation of the distance-time l i n e s f o r l a t e r a r r i v a l s . In the l a t t e r case, the same objections apply as previously discussed f o r the 3050 km/sec velocity, and the o r i g i n time can be considered unknown to ±2 sees, i . e . a 2 second delay i s possible. However, i f these papers were based on access to r e l i a b l e o r i g i n information (which i s a l i k e l y assumption), the delay indicated by the data i s about 4 ± 0 . 5 seconds. Troitskaya ( i 9 6 0 ) reached a sim i l a r tentative conclusion, based on the time spacing between a f i r s t impulse-type a r r i v a l (assumed at H+0) and the second ( o s c i l l a t o r y ) s i g n a l . A number of d i f f e r e n t mechanisms w i l l now be d i s -cussed i n d e t a i l ; some of these are obviously unsuitable to explain the "phase B" signal, but since at one time or another they have been considered, we have included them b r i e f l y . We have attempted to present a number of p o s s i b i l -i t i e s with a minimum of personal bias, i n order to provide a broad basis f o r a more detailed discussion of one or two s p e c i f i c mechanisms which we consider as most l i k e l y . No claim i s made that a l l p o s s i b i l i t i e s have been considered -others can almost c e r t a i n l y be proposed, p a r t i c u l a r l y i f complex combinations of d i f f e r e n t mechanisms are considered. However, i n view of the sharp and c l e a r l y defined nature of 19. the signal we f e e l that a r e l a t i v e l y simple explanation i s more l i k e l y - preferably one which t i e s i n with "prompt" detonation e f f e c t s rather than with delayed fission-product decay. The l i s t i n g of mechanisms has been divided into two groups; I: d i r e c t "overhead" e f f e c t s ; I I : secondary focus mechanisms. I. DIRECT EFFECTS l ) Hydromagnetic waves, at or near detonation a l t i t u d e : The f i s s i o n "bubble" expands at a rate of about 100-1000 km/sec (Latter and LeLevier, 1963). I t i s highly d i a -magnetic and therefore acts as a piston on the earth's mag-netic f i e l d , s etting up hydromagnetic waves i n the d i f f e r e n t modes. Some of these can almost c e r t a i n l y be i d e n t i f i e d among the l a t e r a r r i v a l s , but i n as f a r as "phase B" Is con-cerned they do not f i t the requirement of global synchron-ism, since the relevant propagation v e l o c i t i e s are of the order of a few hundred to a few thousand km/sec - f a r too low to account f o r the simultaneous a r r i v a l s at distant locations. Caner and Whitham (1962) proposed hydromagnetic shock wave i n the ionosphere as one possible explanation. 20. This was based on records from one station (Victoria) only, and on the assumption that the 2 second delay represented a propagation time. As soon as i t became evident that the two second Interval represented a "fixed delay" t h i s mechanism had to be ruled out. 2) Hydromagnetic waves i n high-altitude ducts: This was proposed by Bomke et a l i a ( i 9 6 0 ) f o r the Argus III r e s u l t s and assumes that the energy i s propagated within "ducts" at the a l t i t u d e s of maximum Alfven ve l o c i t y , i . e . 2000-3000 km. The delay time f o r the energy to reach these ducts was assumed n e g l i g i b l e , and the actual nature of the i n j e c t i o n mechanism i s not clea r - hydromagnetic wave propagation to the ducts, or expansion of the diamagnetic buble,, would both required delays of the order of seconds. The only possible i n j e c t i o n mechanism which would not introduce s i g n i f i c a n t delay i s the prompt x-ray or r-ray pulse. However, i n the absence of any sharply defined "duct" boundary i t i s d i f f i -c u l t to see how s i g n i f i c a n t energy could be converted into hydromagnetic waves along the ducts. Quite apart from the time-delay ojection, i t i s not cl e a r why s i g n i f i c a n t amounts of hydromagnetic energy should be refracted into such ducts. Some energy could be trapped (even though i n e f f i c i e n t l y ) i n such a high-velocity 21. layer, but most of the energy would be conducted i n the lower-v e l o c i t y ducts ( p a r t i c u l a r l y since the detonation occurred almost inside such a low-velocity duct). We would therefore expect an almost continuous e f f e c t , with amplitudes increas-ing as the slower signals a r r i v e . This does i n f a c t apply to the Argus III test, where l a t e r a r r i v a l s , although some-times d i s t i n c t l y separate, were recorded with larger ampli-tudes (Berthold et a l i a , i 9 6 0 ; Troitskaya, i 9 6 0 ) . However, i t does not f i t the S t a r f i s h data. The main argument against t h i s mechanism as f a r as the S t a r f i s h data i s concerned i s of course the global syn-chronism and 2 second delay. Propagation v e l o c i t i e s at these a l t i t u d e s are f i n i t e , even though very high, and pro-pagation delays over the distances involved would vary between 2 and 6 seconds. 3) Ionization e f f e c t s due to neutron-decay betas: The prompt neutron emission can account f o r some e f f e c t s d i r e c t l y , but these must be l i m i t e d to the geometrically accessible regions, since neutron paths are not bent or deflected. However, beta p a r t i c l e s which have decayed from these neutrons can be guided down to ionospheric a l t i t u d e s by magnetic f i e l d l i n e s , and therefore produce i o n i z a t i o n e f f e c t s i n regions not d i r e c t l y accessible to the neutrons 22. themselves. For a f i s s i o n explosion, the neutron-decay betas have terminal energies of O.78 Mev, with a very broad maximum centered at about 0.25 Mev (Zmuda et a l i a , 1963b). This mechanism was f i r s t proposed by Crain and Tamarkin (1961) to explain sudden changes i n i o n i z a t i o n following the Aug. 1958 t e s t s . I t has been further devel-oped recently by Sechrist (1962)., Zmuda et a l i a (1963a,b), Willard and Kenney (1963), Kenney and Willard (1963), and others. There seems very l i t t l e doubt that t h i s mechanism accounts f o r at least some of the observed prompt VLF e f f e c t s . The known existence of such a mechanism (as contrasted with the mainly hypothetical nature of some of the other pro-posals) makes i t very tempting to apply i t to the geomag-netic data as well, but there are two major objections: a) The f i x e d time delay. Over the distances involved the onset times at d i f f e r e n t locations should vary between m i l l i -seconds and f r a c t i o n s of a second (on the assumption of mean neutron energy of 1 Mev, i . e . e j e c t i o n v e l o c i t i e s of 1.4 x 109 cm/sec). However, should the o r i g i n spectrum contain an unexpectedly large f l u x of very low energy neut-rons, the delay times could indeed reach 2 seconds at some remote locations, but i n that case there i s no global syn-chronism. For the predicted f i s s i o n spectrum, the onset time would be p r a c t i c a l l y instantaneous (within the time resolution used i n t h i s report, i . e . ±0.1 to 0.2 seconds). 23. This mechanism could therefore account f o r some of the "phase A" a r r i v a l s , but not f o r "phase B". For the "Orange" test i n 1958? Benioff (quoted by Hodder, i960) reported small ( f r a c t i o n a l gamma) signals instantaneous with the detonation (i?) at two locations remote from the detonation point (> 7000 km), but on f i e l d l i n e s accessible to neutron-decay betas. F i e l d (1962) attributed these signals to neutron decay betas. b) The second major objection i s amplitude. For a 1.4 Megaton f i s s i o n bomb, the number of ion p a i r s formed by deposition of betas at the relevant a l t i t u d e s (80-100 km) i s about 75 per cm3 i n one second. A rough estimate of the amplitude of the re s u l t i n g geomagnetic e f f e c t s can be ob-tained by comparison with the diurnal f l u c t u a t i o n s ( F i e l d , 1963). For mid-latitude stations, H I >~20-507, N0~lo4/cm3, the resultant signal would be of the order of 0.1 to 0.5 gamma. I f the prompt deposition (at the f i r s t pass) alone i s considered, the amplitudes would be even lower. Using Kenney and W i l l a r d 1 s (1963) figures, the number of ion pa i r s formed i n a 1 cm^ f l u x tube over V i c t o r i a would account f o r an increase of only about 7 ion pa i r s per cm3.in the r e l e -vant region, i . e . a 0.01 to 0.05 gamma signal. Recent estimates (Kenney, private communication) indicate a possible increase i n these values by a fa c t o r of 10. However, they would s t i l l be too low (by a fact o r of about 100) to account f o r the observed 3O7 signals. 24. 4) Fission-product decay betas: The decay of f i s s i o n pro-ductsprovides a f a r more copious source of betas than the decay of the prompt neutrons. The bulk of t h e i r energy i s deposited at a l t i t u d e s of 80 km and below (mean energy about 1 Mev). However, deposition i s spread over a period of seconds following the detonation. Also these electrons cannot be r a p i d l y transported across the f i e l d l i n e s and t h e i r prompt e f f e c t s must be confined to the f i e l d l i n e s d i r e c t l y accessible to f i s s i o n products. The upwards-expanding diamagnetic bubble confining the f i s s i o n products provides a means f o r spreading these beyond the immediate f i e l d , l i n e through the detonation point. Since at these a l t i t u d e s the magnetic f i e l d i s the dominant f a c t o r f o r stop-ping expansion of the bubble, i t s terminal dimensions can be estimated. Latter and LeLevier ( 1 9 6 3 ) have shown that the terminal cross section area A of the flux-tube containing the bubble i s A ( k r a 2 ) = 3 x 1 G 4 ( 1 + ~ T ) 4 Y 2 / 3 where Z - a l t i t u d e i n km, Rg - earth radius i n km, and Y -explosion y i e l d i n Kilotons. The corresponding deposition area A 0(km 2) at a l t i t u d e 80 km i s For the " S t a r f i s h " test (Z = 4 0 0 km, Y = 1 4 0 0 Kilotons), A = 480 x 1 0 ^ km2 (Radius = 1 2 4 0 km) and A Q = 4 0 0 x 10^ km2 (Radius 1 1 3 0 km). These are maximum values, based on the 25. assumption that the entire energy output goes into expanding-the bubble. A more reasonable proportion would be about 0.5 to 0.7 of t o t a l y i e l d . Even though a small proportion of the decay elec-trons (about 5$ - Colgate,. 1963) i s ultimately injected into higher L - s h e l l s through magnetic i n s t a b i l i t i e s , the bulk of the deposition i s l i m i t e d to the area Ao. The prompt ground e f f e c t s due to i o n i z a t i o n are therefore l i m i t e d to a range of about 20° at each conjugate area ( i . e . about 15°S to 35°S and 15°N to 35°N), and the mechanism i s inadequate to explain the geographic extent of the "phase B" signals - quite apart from other objections (timing, sudden onset). Transverse d r i f t would of course extend the longitudinal coverage, but even f o r very high energy electrons the d r i f t rate i s f a r too slow to account f o r "phase B" a r r i v a l s . 5) Setting up of l o c a l conditions: One possible mechanism suggested by Selzer (private communication) i s that the 2 second delay represents a "setting-up" time of conditions at the point of observation rather than a propagation or o r i g i n delay. In other words, the actual t r i g g e r i n g impulse reaches a l l over the globe simultaneously (within the a v a i l -able time resolution) and "..the two second delay would have been the time necessary f o r the process, involved i n each of 26. these l o c a l e xcitation, to be performed" (Roquet, Schlich and Selzer, 1963). The t r i g g e r i n g mechanism i s not speci-f i e d , but eith e r the "phase A" pulse or neutron-decay betas could f i t the timing requirements. However, i t would appear to be almost impossible to reconcile the sharp sudden com-mencement with t h i s hypothesis. Unless some sort of break-down e f f e c t can be formulated, any such process would be gradual from the instant of impulse a r r i v a l , rather than sharply defined at H+2. I I . SECONDARY FOCUS MECHANISMS We are now considering a group of possible mech-anisms which we have c a l l e d "secondary focus" type f o r want of a better name. These imply energy i n some form imping-ing on an interface (or other c r i t i c a l region) and setting up a secondary focus of disturbance from which energy i s re-emitted at (or close to) the speed of l i g h t . The two-second delay would be accounted f o r by propagation delay between primary source and secondary focus, or by the time required to set up the necessary conditions f o r emission at the secondary focus, or a combination of both. We f e e l that In view of the close synchronism of the signals recorded at widely separated locations, such a "secondary focus" mechanism 27. i s f a r more l i k e l y than a d i r e c t primary "overhead" e f f e c t at each station. The actual nature of the energy conversion and of the speed-of-light transmission mode are as yet unspecified -the required t h e o r e t i c a l work i s beyond the scope of t h i s preliminary inve s t i g a t i o n . It should be pointed out that the term "e.m. wave radiated from a secondary focus" must be used only with reservations, p a r t i c u l a r l y I f the disturbance i s to be propagated i n the earth-Ionosphere wave guide. At the frequencies involved (0.25-0.30 cps) the free-space wave-lengths are over 10^ km, i . e . orders of magnitude greater than the dimensions of the wave guide or the propagation distances. The s t a t i c (l/ d 3 ) and induction (1/d2) e f f e c t s both become comparable to radiatio n e f f e c t s at distances l e s s than 1/6 of a wavelength. On the other hand we could consider the signal as a series of impulses, with recovery In between. The observed sine-shaped fl u c t u a t i o n s would then be explained by f i l t e r i n g through the transmission medium and/or instrumentation. This would In f a c t f i t some of the proposed mechanisms better than a monochromatic source emission, but i t s t i l l leaves un-spec i f i e d the mode i n which these impulses are propagated at the speed of l i g h t . 28. 1) Neutron-decay betas. We have p r e v i o u s l y c o n s i d e r e d (and r e j e c t e d ) the d i r e c t overhead i o n i z a t i o n e f f e c t s of neutron-decay betas at each s t a t i o n . However, because of the f o c u s s i n g e f f e c t of the f i e l d - l i n e geometry, a h i g h pro-p o r t i o n of them w i l l be d e p o s i t e d In the conjugate areas and i n the a u r o r a l zones (Kenney and W i l l a r d , 1963). T h i s c o u l d p r o v i d e a p o s s i b l e t r i g g e r i n g mechanism f o r secondary d i s t u r b a n c e s . The main o b j e c t i o n s to t h i s t h e o r y are: a) the 2 second time d e l a y - the d e p o s i t i o n I s p r a c t i c a l l y Instantaneous at the t i m e - s c a l e s c o n s i d e r e d , I.e. m i l l i -seconds to about 0.2 seconds, and b) the p r e v i o u s l y con-s i d e r e d amplitudes. T h i s mechanism i s almost c e r t a i n l y r e s p o n s i b l e f o r the (instantaneous?) a u r o r a l d i s p l a y s r e -p o r t e d from a u r o r a l zone l o c a t i o n s ^ and p r o b a b l y a l s o f o r the abnormally high-amplitude h o r i z o n t a l component d e f l e c -t i o n r e p o r t e d from an a u r o r a l zone magnetic o b s e r v a t o r y (Meanook; Cook, p r i v a t e communication). I t I s however u n l i k e l y to be r e s p o n s i b l e f o r the g l o b a l "phase B" s i g n a l . I t should be p o i n t e d out t h a t i f our assumed neutron spectrum ( i . e . f i s s i o n e x p l o s i o n ) i s I n c o r r e c t , t h i s mechanism may have to be r e c o n s i d e r e d . In p a r t i c u l a r , i f a very l a r g e f l u x of very low energy neutrons i s produced, a l o n g e r d e l a y (2 seconds) c o u l d be accounted f o r i n the case of an a u r o r a l zone secondary f o c u s , and the mechanism may be t h e o r e t i c a l l y p o s s i b l e as f a r as the t i m i n g i s concerned. 29. However, the available amplitude data point towards a source i n the detonation area rather than In the auroral regions (Bomke et a l i a , 1963). 2) Fission-product decay betas. We have previously d i s -cussed the e f f e c t s of fission-product decay betas, and con-cluded that they do not f i t "phase B" observations i n as f a r as d i r e c t overhead i o n i z a t i o n e f f e c t s are concerned. How-ever, the r e l a t i v e l y concentrated deposition of these betas i n the conjugate area could provide a possible secondary-focus mechanism. The t r a n s i t time of t h e betas along the f i e l d l i n e s i s p r a c t i c a l l y instantaneous at t h e time r e s o l -utions considered, which raises the question of the 2 second delay. The expansion time of the bubble i s of the right order of magnitude. Colgate (1963) estimated 2.5 seconds as the time required f o r the bubble to reach i t s terminal dimensions R = 1000 km. .A precise figure would be d i f f i c u l t to obtain i n view of the uncertainties i n parameters - i n p a r t i c u l a r , the proportion of the t o t a l y i e l d which i s a v a i l -able f o r expansion of the bubble could vary between 30$ and 70$. A continuous process rather than a sudden release could be expected, since energetic electrons would be contin-uously escaping from the bubble during expansion. However, 30. the expansion of the bubble may a c t u a l l y p r o v i d e some s o r t of energy " f i l t e r i n g " mechanism - h i g h e r energy e l e c t r o n s would l o s e energy i n expanding the bubble a g a i n s t the pressure of the magnetic f i e l d , u n t i l at the time the expansion i s stopped (H+1.9 sec?) a reasonably monoenergetic f l u x i s r e l e a s e d t o s p i r a l t o the conjugate areas and to t r i g g e r the secondary d i s t u r b a n c e . T h i s mechanism does not e x p l a i n the o s c i l l a t o r y nature of the s i g n a l ( s i n c e the m i r r o r i n g e l e c -t r o n s have much s h o r t e r p e r i o d s ) , but i t appears to f i t the t i m i n g requirements f o r the i n i t i a l impulse of "phase B". The s p e c t r a l c h a r a c t e r i s t i c s of f i s s i o n decay decay betas have been d i s c u s s e d by Zmuda e t a l i a (1963a), but the g e n e r a l arguments f o r t h i s p a r t i c u l a r mechanism are p r a c t i c a l l y u n a f f e c t e d by the p r e c i s e s p e c t r a l d i s t r i b u t i o n , s i n c e the beta t r a n s i t time to the conjugate area i s short compared w i t h 2 seconds, even f o r low energy e l e c t r o n s . 3) Hydromagnetic E l e c t r o m a g n e t i c c o n v e r s i o n at the lower  Ionosphere boundary. iA very simple, and t h e r e f o r e a t t r a c t i v e , model was t e n t a -t i v e l y proposed by Bomke e t a l i a ( 1 9 6 3 ) . A hydromagnetic wave propagates v e r t i c a l l y downward from the d e t o n a t i o n p o i n t , and on r e a c h i n g the lower ionosphere boundary s e t s up a secondary d i s t u r b a n c e which i s then propagated at the speed of l i g h t . The c o n v e r s i o n e f f i c i e n c y between hydro-31. magnetic (h.m.) and electromagnetic (e.m.) waves at a sharp interface (which i s s a t i s f i e d at the wavelengths involved) i s of the order of one to a few percent. Kahalas (i960) obtained a figure of 1$, but pointed out that h i s derivation was based on assumptions which are not necessarily v a l i d f o r the lower ionosphere. The propagation time f o r a hydromagnetic wave between the detonation point and 80 km a l t i t u d e could possi-bly account f o r the 2 second delay. A precise computation of t h i s propagation delay would be meaningless, since the properties of the medium below the detaonation point would have been d r a s t i c a l l y altered by the prompt radiati o n (x-rays i n p a r t i c u l a r ) . An estimate of l i m i t i n g values has been worked out i n Appendix I I . A 2-second delay appears to be at the extreme l i m i t of the acceptable range f o r radiating temperatures up to 2 kev, but would be well within the acceptable range f o r higher-temperature devices (fusion bomb?). There are two objections to t h i s model: a) The monochromatic nature of the observed signal does not f i t the e s s e n t i a l l y impulsive ( i . e . broadband) o r i g i n d i s t u r -bance. Although selective propagation could explain the predominance of c e r t a i n fequency bands (Jacobs and Watanabe, 1962), i t i s u n l i k e l y that t h i s could be e f f e c t i v e enough over the short pathlength involved to account f o r a sharp frequency selection as observed. b) The main objection against t h i s mechanism i s the f a c t that lower a l t i t u d e tests ( i n p a r t i c u l a r "Teak", at or just below 80 km) do not produce similar e f f e c t s . Admittedly the conversion of explosion k i n e t i c energy into hydromag-netic forms i s considerably more e f f i c i e n t at the higher a l t i t u d e s . Nevertheless, considering how close the "Teak" detonation was to t h i s hypothetical "secondary focus" a l t i t u d e , some comparable e f f e c t s (even If with much lower amplitudes) should have been observed, with a reduced or zero delay. I t should be mentioned that t h i s argument i s somewhat complicated by the p o s s i b i l i t y that shielding experiments may have been included i n some of the t e s t s . For example, r e l a t i v e l y simple devices can reduce the prompt 1 1 r a d i a t i o n f l u x by f a c t o r s of 100 or more, and the mean energy l e v e l s by factors of 10-20 (Latter, Herbst and Watson, I 9 6 I; Latter and LeLevier, 1963). Also the actual type and construction of the bomb, i n p a r t i c u l a r the casing material, has a strong influence on the spectrum of the emit-ted prompt radia t i o n . Although shielding experiments may have been included i n some of the smaller tests, i t Is very u n l i k e l y that major ones such as "Teak" included any such complications. Nevertheless, I t should be kept i n mind that not only do we have a minute sample, but we do not even have r e l i a b l e o r i g i n information on these few t e s t s . Consequent-l y any arguments as to why only c e r t a i n tests do or do not produce c e r t a i n r e s u l t s must be used with caution. 33. 4) Hydromagnetic Impulse guided along the f i e l d - l i n e . In t h i s mechanism a hydromagnetic Impulse i s guided along the f i e l d - l i n e through the detonation point, with p a r t i a l r e f l e c t i o n s at each end. The mechanism i s e f f e c t i v e l y a variant of the previous one, but might also explain the o s c i l l a t o r y nature of the signal, the observed period, and the absence of sim i l a r e f f e c t s from lower-altitude t e s t s . It would also f i t the damped amplitude e f f e c t , and the "bounce" period i s of the right order (3-4 seconds). A simple consideration of the geometry involved indicates a major timing vs. period objection. Figure 6 shows the relevant f i e l d - l i n e , drawn to scale. I t i s obvious that i f t (the i n i t i a l delay, H-»P) i s 2 seconds, T (the f i r s t bounce period, P-*R) cannot be as low as 3 .5-4 seconds. There i s , however, a possible explanation f o r the discrepancy - the " r e f l e c t i o n point" P i s i n the di r e c t r a d i a t i o n zone, and the medium around i t undergoes vast i n -stantaneous changes i n i o n i z a t i o n due to prompt radiation, i n p a r t i c u l a r x-rays (which f o r the spectrum of a f i s s i o n bomb deposit most of t h e i r energy at a l t i t u d e s of 70 to 90 km) - see Appendix I I . The r-rays deposit energy down to lower a l t i t u d e s (about 20-30 km f o r the Mev energy range). Although the i o n i z a t i o n due to the prompt 7-ray pulse decays within microseconds, some p e r s i s t i n g net i o n i z a t i o n at these lower a l t i t u d e s (over a few seconds) r e s u l t s from delayed SCALE 1000 KM FIG. 6 FIELD LINE THROUGH T H E DETONATION POINT "STARFISH" T E S T OF JULY 9 , 1 9 6 2 . 35. fission-product r-rays. I t Is therefore possible that f o r the f i r s t pass (H-*P) the medium was p r a c t i c a l l y opaque to h.m. waves, or that the necessary (interface sharpness?) conditions at point P did not e x i s t . The i n i t i a l "secondary focus" would therefore be set up at point R i n the southern hemisphere. This removes the timing-geometry objection: fo r t (H-»R) = 2 seconds, T (R-+P) = 3-4 seconds i s quite reasonable. Crain (1963) has shown that at the relevant a l t i t u d e s (75 km) the x-ray induced i o n i z a t i o n decays with time-constants of the order of a few seconds. Consequently, when the r e f l e c t e d wave reaches P ( i . e . aft e r 5-6 seconds, H-»R-*P) the "int e r f a c e " may have s u f f i c i e n t l y recovered to permit e f f i c i e n t energy conversion (h.m. -* e.m.) and re-f l e c t i o n . The very s l i g h t reverse movement which preceded the main movement on several recordings (e.g. V i c t o r i a , Figure 1, or Jicamarca - see Figure 5 of Casaverde et a l i a , 1963) may represent the e f f e c t of the "aborted" f i r s t H-* P pass. The fac t that the delay H-*P was only s l i g h t l y l e s s than H - R would be reasonable, i n view of the large x-ray induced i n i t i a l increase i n i o n i z a t i o n over the H-» P sec-t i o n - see Appendix I I . There i s an in t e r e s t i n g ambiguity i n t h i s model. Does energy emission occur at both mirror points ( i . e . are there 2 secondary sources), or does only the southern focus exist, with the northern end having recovered s u f f i c i e n t l y 36. to permit passage of the hm wave and r e f l e c t i o n , but not f o r e f f i c i e n t h.m. e.m. energy conversion? In as f a r as timing data are concerned, t h i s question can be restated as: does the observed 3.5-4 second period represent the t r a n s i t time f o r a one way bounce (R-* P or P-* R), i . e . focus at each end, or does i t represent a f u l l mirror period (R-*P—R), I.e. focus at southern end only? Even under normal conditions the Alfven v e l o c i t i e s are not known to better than a f a c t o r of 1.5-2, and with the a r t i f i c i a l i o n i z a t i o n changes over parts of the path i t becomes impossible to compute the mirror period with s u f f i c i e n t accuracy to resolve the above ambi-guity. The l i m i t i n g cases f i t just about equally the two p o s s i b i l i t i e s . For undisturbed night-time conditions, the mean Alfven v e l o c i t i e s at a l t i t u d e s of 350, 500* 750 km are 800, 1000 and 2000 km/sec respectively (Jacobs and Watanabe, 1962), and a f u l l mirror period of 4 seconds i s possible. Under disturbed conditions these v e l o c i t i e s are reduced by a fa c t o r of 2 or more, and the 4-second period would have to be the time per one-way t r a n s i t , i . e . focus at each end. The ambiguity might be resolvable on t h e o r e t i c a l grounds once the exact conditions f o r r e f l e c t i o n and energy conver-sion are established. In any case, the o v e r a l l v a l i d i t y of t h i s model i s i n no way affected by t h i s ambiguity. We are well aware of the weaknesses i n t h i s q u a l i -t a t i v e treatment, and considerable t h e o r e t i c a l work on the 37. c o n d i t i o n s f o r h.m. -* e.m. energy c o n v e r s i o n must be c a r r i e d out b e f o r e such a model I s f u l l y a c c e p t a b l e . I t does, how-ever, f i t the experimental t i m i n g data extremely w e l l - i t i s the o n l y theory so f a r which f i t s a l l the s p e c i f i e d con-d i t i o n s , i n c l u d i n g the decrease i n p e r i o d . T h i s c o u l d be e x p l a i n e d by the f a c t t h a t on the f i r s t pass the i o n d e n s i t y over p a r t s of the path would have been s i g n i f i c a n t l y i n c r e a s e d by d e t o n a t i o n products t r a v e l l i n g f a s t e r than the h.m. wave, and the A l f v e n v e l o c i t y (which i s I n v e r s e l y p r o p o r t i o n a l to the square r o o t of the Ion d e n s i t y ) would be lower than normal. As the a r t i f i c i a l i o n i z a t i o n decays (time c o n s t a n t s of the order of seconds) the A l f v e n v e l o c i t y I n creases and the bounce p e r i o d decreases. The observed 20$-30$ decrease i n bounce p e r i o d over about 15 seconds would i n d i c a t e an e f f e c t i v e i n i t i a l i n c r e a s e In i o n d e n s i t y of 50$-70$ - not unreasonable f o r t h i s p a r t i c u l a r path which i s p a r t l y i n the , prompt r a d i a t i o n zone and a l s o along the guided path of f a s t b e t a s . There i s some independent (though very weak) con-f i r m a t i o n f o r a "source" of 3-second p e r i o d o s c i l l a t i o n s i n the southern hemisphere. P o l e t t i and Gadsden (1962), from amplitude comparisons of t e l l u r i c r e c o r d i n g s at two New Zealand s t a t i o n s with I d e n t i c a l i n s t r u m e n t a t i o n , conclude t h a t the source of 3-second p e r i o d p u l s a t i o n s must have been "a few hundred k i l o m e t e r s , at most, to the n o r t h of Lauder 38. (50° geomag. l a t . ) " , i . e . at about 40°S geomag. , This i s of course much further south than point R (by about 20°, i . e . 2000 km). Also the pulsations referred to are not those of "phase B" (which were unfortunately o f f - s c a l e at both stations), but some l a t e r a r r i v a l s . Consequently no great amphasis can be put on t h i s evidence i n support of the proposed model. Although i t may be relevant, i t i s prob-ably due to some e n t i r e l y d i f f e r e n t conjugate-point e f f e c t . 5) Hydromagnetic wave along the f i e l d - l i n e . Some of the points discussed f o r the previous model can also be applied to a long-period hydromagnetic wave propagating along the f i e l d - l i n e through the detonation point. The 2--second delay would represent the propagation time of the wave front to the conjugate point. The d i s t i n c t i o n between t h i s model and the preceding one should be c l e a r l y understood. In the preceding model we considered a hydromagnetic impulse ( i . e . broadband "wave-packet") which i s propagated along the f i e l d - l i n e . 0.25 p.p.s. represents the r e p e t i t i o n rate of the impact of t h i s impulse on the interface, not a wave f r e -quency. The higher component frequencies of the pulse are eit h e r not converted into electromagnetic forms, or are f i l t e r e d out by propagating media and/or instrumentation. In t h i s model we consider a single hydromagnetic standing wave, frequency 0.25 cps, with d i r e c t coupling to a 0.25 cps 39. electromagnetic wave at an int e r f a c e . The main objection to t h i s mechanism i s the mono-chromatic nature of the sign a l . I t i s however possible that p r e f e r e n t i a l e x c i t a t i o n of the eigen periods, combined with selective h.m. to e.m. conversion of c e r t a i n frequency bands, could provide a suitable explanation. The fundamental char-a c t e r i s t i c period can be estimated by numerical integration T = j" ( 2 / / V"A) • ds over the f i e l d - l i n e (Obayashi and Jacobs, 1958). It l i e s between about 6 and 15 seconds (using the Alfven v e l o c i t i e s f o r night time during periods of maximum and minimum sunspot a c t i v i t y r e s p e c t i v e l y ) . In view of the increase i n Ionization due to radiation, and of the extreme d i s t o r t i o n of the f i e l d - l i n e by the diamagnetic bubble, a precise determination of eigen-periods i s impossible. The observed periods are therefore well within an acceptable range of values f o r the f i r s t harmonic. This mechanism f i t s the observational data i n a l l respects. The decrease i n period i s also obtained - as the a r t i f i c i a l l y - i n d u c e d increase i n i o n i z a t i o n decays, the Alfven v e l o c i t y increases and the eigen-period decreases. There appears to be no obvious way of dist i n g u i s h i n g between t h i s and the preceding model on the basis of the observational data, but we f e e l that the standing-wave model i s preferable, even i f only on aesthetic grounds alone. I t does not involve any elaborate considerations of geometry or focus 4o. location, and i t does not require complex f i l t e r i n g mechan-isms to explain the observed quasi-sinusoidal signals. 6) Protons guided along the f i e l d - l i n e . The major source of prompt protons from a f i s s i o n process i s neutron decay. The protons have e s s e n t i a l l y the same k i n e t i c energy as the parent neutrons. The energy d i s t r i b u t i o n of prompt neutrons from a f i s s i o n process has been discussed i n d e t a i l by Bonner, P e r r e l , Rinehart (1952), H i l l (1952), and Watt (1952). In the energy range 0.075 Mev to 17 Mev i t can be roughly represented by the semi-empirical formula given by Watt (1952): N(E) = 4.75 x 10 6 sinh ( 2 E) X /2 exp (-E) No information i s available f o r energies below 75 Kev. Figure J shows a plot of the energy d i s t r i b u t i o n up to 7 Mev. The spectrum has a very broad maximum centered at 0.8 Mev -emission i s p r a c t i c a l l y constant (within a few percent) between 0.4 Mev and 1.3 Mev. Even though energies range to above 18 Mev, most of the neutrons are below 2 Mev, the f a l l -o ff being p r a c t i c a l l y exponential above 2 Mev. For t h i s p a r t i c u l a r f i e l d - l i n e the two-way mirror period of 1 Mev protons i s about 2 seconds (Zmuda et a l i a , 1963a), 3 seconds f o r 0.4 Mev, and 4 seconds f o r 0.25 Mev, which i s of the right order f o r the observed "phase B" ~t 1 1 I 1 1 1 * • 0 I 2 3 4 5 6 Neutron Energy (MEV) FIG. 7 ENERGY SPECTRUM OF FISSION NEUTRONS 4 2 . periods. In t h i s connection i t should be mentioned that Zmuda et a l i a (1963a) reported 10-second period c y c l i c VLP e f f e c t s over the NPG-APL/JHU path, which they t e n t a t i v e l y attributed to 0 . 4 Mev protons mirroring over NPG. In view of the uncertainties i n f i e l d - l i n e geometry and other para-meters, i t i s possible that the two e f f e c t s could be recon-c i l e d to f i t the mirroring of protons with the same energy (say C . 3 - 0 . 4 Mev) along d i f f e r e n t f i e l d - l i n e s . The same objections of timing-geometry ( i n i t i a l delay vs. period) which were discussed i n one of the preced-ing mechanisms, can be. made to t h i s mechanism and the same solution can be proposed, v i z . the f i r s t r e f l e c t i o n and energy conversion occurs at the southern end. Here again the same ambiguity as i n the previous model a r i s e s : does a "focus" ex i s t at each end, or only at the southern end. In t h i s case, however, the ambiguity can be solved. The relevant r e f l e c t i n g a l t i t u d e s are higher (about 100 km f o r 0 . 4 Mev protons), and the x-ray induced i o n i z a t i o n decays f a r more slowly at these a l t i t u d e s than at 80 km. For al t i t u d e s 95 to 115 km the decay constant varies between 4 0 0 and 3000 seconds (Crain, 1963), depending on the exact a l t i -tude and magnitude of the Ionizing pulse. I f t h i s model i s at a l l v a l i d , there can be only one "secondary focus" (at the southern end), and the 3 . 5 - 4 seconds represent a f u l l mirror period, i . e . protons with energies 0.25 to 0.20 Mev. 43. This provides the p o s s i b i l i t y of an experimental check on the v a l i d i t y of t h i s model. Observed signal amplitudes should be higher near the conjugate point than near the detonation point,. This does not appear to be the case, but the evidence i s base! on only one set of observations: Bomke et a l i a (1963) reported that the amplitude recorded at Hawaii was higher than that recorded at Samoa by a fa c t o r of about four. Another major objection to t h i s mechanism i s the sharpness of the commencement, and the c l e a r l y defined f r e -quency of the subsequent swings. In view of the broad energy d i s t r i b u t i o n spectrum of the protons, a continuous (seconds r i s e time) commencement and complex waveforms could be expected. Also t h i s model provides no explanation f o r the decreasing period. However, should the emission turn out to contain a major, reasonably monoenergetic source of low-energy (0.2-0.4 Mev) protons, the mechanism may have to be reconsidered i n more d e t a i l . It should be pointed out that the presented spectral d i s t r i b u t i o n data r e f e r to emitted neutrons i n general. The spectrum of the propor-t i o n which escapes the debris area would probably be s h i f t e d toward the low-energy end due to shielding by the debris. (The s h i f t could be considerable i f deliberate neutron shielding had been part of the test, but t h i s i s u n l i k e l y i n view of the bulk of the necessary shields.) However, the 44. general feature ( i . e . broad maximum, almost 2 Mev "wide") would presumably remain unchanged, even though i t may be centered around a lower energy, so that t h i s does not f i t the requirement f o r a monoenergetic f l u x . C O N C L U S I O N The preceding discussion indicates that hydro-magnetic standing waves along the f i e l d - l i n e through the detonation point provide the most l i k e l y explanation f o r the "phase B" signal. I t i s the only one of the mechanisms discussed i n t h i s report whieh f i t s a l l the observational data, and i s not dependent on a knowledge of the bomb type ( f i s s i o n and/or fusion). Whatever the tr i g g e r i n g mechanism, further theo-r e t i c a l work i s required on the energy conversion mechanism at the secondary focus, and on the speed-of-light propaga-ti o n of the secondary disturbance. No unique solution i s claimed - other models are possible, and with the li m i t e d amount of data a sensitive enough c r i t e r i o n f o r distinguishing between the d i f f e r e n t mechanisms i s hard to define. Some additional l i n e s of approach to t h i s problem are suggested i n Appendix I, which may y i e l d the necessary information i f additional observa-t i o n a l data become available. 45. APPENDIX I ADDITIONAL LINES OP APPROACH 1) More precise timing considerations. There are apparent time discrepancies of the order of 0.1-0.2 seconds between some of the basic stations. These are almost c e r t a i n l y not r e a l , being probably due to differences i n the recorded com-ponents or other f a c t o r s . If a network of highly accurate (better than 50 milliseconds), i d e n t i c a l l y instrumented observations become available, useful information could probably be extracted which would di s t i n g u i s h between the d i f f e r e n t models. Unfortunately, because of the r e l a t i v e l y slow r i s e time of natural "sudden" geomagnetic phenomena, most researchers i n t h i s f i e l d have not developed or applied the timing techniques and d i s c i p l i n e s which are required f o r t h i s type of work. Highly standardized instrumentation and recording of the same magnetic-coordinate component are also necessary f o r high-accuracy timing work, since the exact commencement of the disturbance i s otherwise hard to define. In view of the d i f f i c u l t y i n assembling even a few suitable ( i O . l sec) observations on a global scale, i t i s doubtful whether a set of such i o . 0 5 sec data ex i s t s , at least i n the u n c l a s s i f i e d domain. 4 6 . 2) Amplitude considerations. The present report has been based almost e n t i r e l y on timing considerations. An analysis of amplitude-location r e l a t i o n s could go a long way towards the removal of the ambiguity between the d i f f e r e n t mechan-isms, through the determinations of focus location, atten-uation c h a r a c t e r i s t i c s , and possible anisotroples i n propaga-t i o n . Unfortunately the data published so f a r are unsatis-factory from t h i s point of view, f o r two reasons: a) Most of the magnetic recordings obtained with the appro-priate bassband went of f - s c a l e during the f i r s t "phase B" fl u c t u a t i o n s . This means that only a lower amplitude l i m i t can be assigned, depending on the f u l l - s c a l e range of the p a r t i c u l a r instrumentation - which varies between 0.5 and 50r f o r d i f f e r e n t stations. Recorders f i t t e d with scale-l i m i t (bias) stepping mechanisms introduced an uncertainty of 1 or even 2 steps ( i . e . 1-2 f u l l scale ranges) because of the unusually rapid f u l l - s c a l e excursions - see f o r example Baker and Strome (1962). b) V a r i a b i l i t y i n recorded component and i n instrumentation. Quite apart from t e l l u r l c s (which cannot be included f o r amplitude comparisons) there are 6 normally acceptable ways of choosing a single-component recording (D, H, Z, X, Y, F), of which f i v e are usually recorded with eit h e r amplitude-l i n e a r or rate-of-change detectors. This means eleven d i f f e r e n t methods - quite apart from the widely varying f r e -quency response c h a r a c t e r i s t i c s . This makes i t almost 47. Impossible to extract amplitude comparisons to better than an order of magnitude, even i f the recordings had remained on scale. Also, v e r t i c a l components, p a r t i c u l a r l y at the frequencies involved i n the "phase B" signal, are f a r too heavily dependent on l o c a l geologic conditions to be useful f o r amplitude comparisons. In view of the a v a i l a b i l i t y of commercially-manufactured t o t a l force instruments, i t was to be hoped that at least a number of t o t a l - f i e l d ampli-tudes would have been available from i d e n t i c a l equipment. However, i n some cases the large and rapid variations of the "phase B" signal were beyond the range which could be e f f e c -t i v e l y recorded (because the counting period becomes com-parable with the time scale of the f l u c t u a t i o n s ) , and "phase B" recordings on some t o t a l - f i e l d instruments must be used with caution. For example, Unterberger and Byerly (I962) reported that two Rubidium Vapour magnetometers oper-ating at the same location (5 feet apart) gave coherent re-cordings up to H+2 sees and a f t e r H+30 sees, but not during the "phase B" fl u c t u a t i o n s . Judging from the published record reproductions, the same comment probably applies to the t o t a l - f i e l d recording obtained at Ottawa (Baker and Strome, 1962). To summarize, the normal observation routines and standard instrumentation were not designed to cope with the extraordinary features of t h i s a r t i f i c i a l geomagnetic disturbance. 48. 3) Geographic coverage. A l l the stations which reported major (large amplitude) and clearly-defined "phase B" a r r i v a l s l i e i n a geomagnetic longitude band about 205° wide (240° to 85°) . This may be suggestive of some sort of broad f i e l d - l i n e guiding e f f e c t , or at least of some hemi-spheric l i m i t a t i o n , although i t i s probably f o r t u i t o u s - i t may simply represent the geographic d i s t r i b u t i o n of stations having the appropriate sophisticated instrumentation and advance n o t i f i c a t i o n of the t e s t . Bomke et a l i a (1963) reported highly i s o t r o p i c propagation f o r the USAERDL stations (which cover a 105° band i n geomagnetic longitude to the east of Johnston Island). A d e f i n i t e answer to whether isotropy applies on a global scale as well w i l l have to await publication of records from other continents - i n p a r t i c u l a r from the Soviet t e l l u r i c network with i t s extended longitude coverage. Two i s o l a t e d reports which do not f i t into an i s o -tropic global coverage pattern remain to be explained, but both are of l i m i t e d significance u n t i l confirmed by other observations. a) A single channel (D) record obtained by the UK Signals Research and Development Establishment at Ascension (Geo-graphic coordinates: 6 = 8°S, 7v = l4 . 3°W; Geomagnetic l a t -itude V ~ 1°S) indicates a peak amplitude of under 0 .5 gamma. The frequency response of the system i s roughly 49. l i n e a r between 0.03 and 0.2 cps, with 3 db points at 0.007 and 2 cps. However, there i s some p o s s i b i l i t y that the eff e c t i v e response to rapid f l u c t u a t i o n s may be low because of abnormal pen response l i m i t a t i o n s (Stevens, private com-munication) . In comparison, H and P geomagnetic recordings of "phase B" obtained at two other equatorial stations, Huancayo and Jicamarca (both at AJ ro 1°S), indicate ampli-tudes comparable to those obtained at higher l a t i t u d e s (Casaverde et a l i a , 1963). However, the induction magneto-meters at these two stations were recording H (magnetic N-S), whereas D (magnetic E-W) was recorded at Ascension, b) A t e l l u r i c recording at Alert (6 = 82.5°N, A = 62.5°w~; V = 86°N) f a i l e d to give a measurable response (Caner and Whltham, 1962). Detection s e n s i t i v i t y of the instrument-ation i s a few mV/km, and response time about 1 second. In comparison, a t e l l u r i c "phase B" recording at Lincoln, New Zealand, exceeded hundreds of mV/km ( G i l l , 1962). To judge by the published record reproductions, the amplitude may have exceeded 1 V/km. Simil a r l y , on t e l l u r i c recordings obtained at Prince Albert, Sask. (6 = 53»2°N, X = 105.9°W; V = 62°N), the amplitude i s well i n excess of the 30 mV/km f u l l - s c a l e range (Graystone, 1963). At Meanook ( V = 6 l . 8°N) the amplitude probably exceeded 100 mV/km (Cook, private communication). I t should be pointed out that the Al e r t recording was obtained on a f i e l d survey (Law et a l i a , 1963), not as part of a regular observatory operation. I t i s 50. consequently of l i m i t e d r e l i a b i l i t y , because of the i n s t r u -mental uncertainties inherent i n temporary i n s t a l l a t i o n s . Normally, stations which do not record any s i g -n i f i c a n t e f f e c t s do not publish "negative" reports. However, the l o c a t i o n of other possible "blind spots" could help to resolve the ambiguities between the proposed mechanisms. For example, i f confirmed by other reports, the absence of E-W components i n the disturbance at equatorial stations would permit s i g n i f i c a n t deductions to be made about the horizon-t a l p o l a r i z a t i o n of the disturbance, and could provide an answer on the nature of t h i s disturbance. The need f o r more three-component recordings i s evident. A l t e r n a t i v e l y , confirmation of a gap at very high geomagnetic l a t i t u d e s (>80°) would be strong evidence f o r a broad f i e l d - l i n e guided mechanism rather than an is o t r o p i c e.m. wave. We would therefore be very interested to hear from stations with appropriate instrumentation (time resolution 1 second or better, detection s e n s i t i v i t y of a few gammas, frequency response to about 1 cps) which did not record any s i g n i f i c a n t pulsational a c t i v i t y at H+2 seconds ( i . e . at 0900:11s U.T. on July 9, 1962). I d e n t i f i c a t i o n of "bs" or "crochet" type events on standard magnetograms i s not d i r e c t l y relevant since the longer-period "main" phase i s probably due to e n t i r e l y d i f f e r e n t e f f e c t s , i n p a r t i c u l a r charged p a r t i c l e d r i f t s (see f o r example Pisharoty 1962, and Maeda et a l i a , 1964). 51. APPENDIX II X-RAY IONIZATION AND ALFVEN VELOCITIES BELOW THE DETONATION POINT In a discussion of the d i f f e r e n t mechanisms i t was pointed out that the propagation time of a hydromagnetic wave between the detonation point and an a l t i t u d e of 80 km could not be accurately computed because of the Ionization increase below the explosion. However, some l i m i t i n g values can be derived, since the main prompt i o n i z i n g e f f e c t s over t h i s path are due to x-ray energy deposition. There are two major uncertainties i n the para-meters used f o r computation of these e f f e c t s : a) the x-ray y i e l d , which could vary between 30$ and 70$ of the t o t a l y i e l d , depending on the type and construction of the bomb; b) the x-ray temperature of the radiating materials, which ranges between kT = 0.5 kev and 2 kev f o r an unshielded explosion. Since the energy from high temperature explosions Is deposited mainly at lower a l t i t u d e s where recovery i s very rapid (Latter and LeLevier, 1963)* lower temperature devices would have the maximum e f f e c t on the mean Alfven velocities over the entire path. Consequently an upper l i m i t f o r the propagation delay can be obtained by considering a 0.5 kev bomb with an x-ray y i e l d 75$ of the t o t a l y i e l d 52. ( i . e . 1050 k i l o t o n s f o r " S t a r f i s h " ) , and a lower l i m i t f o r 2 kev and 350 k i l o t o n s . The energy d e p o s i t i o n as a f u n c t i o n of a l t i t u d e has been computed u s i n g the s a d d l e - p o i n t method o u t l i n e d by L a t t e r and L e L e v i e r (1963): E < Z ) " $ ( f f * ^tsfr r ( z ) . [ c . h ( Z ) f ? exp[- i [ c . H ( z ) ] ^ where E ( z ) : the energy d e p o s i t i o n (ergs/cm3) per u n i t x-ray f l u x kT: the x-ray temperature i n kev r ( z ) : the d e n s i t y at a l t i t u d e z (gr/cm3) h ( z ) : the t o t a l mass per u n i t area of the atmosphere between the e x p l o s i o n and a l t i t u d e z and C = 10.2 x 1 0 3/(kT ) 3 The v a l u e s of the parameters r ( z ) and h(z) have been d e r i v e d from the ARDC model atmosphere (Minzner, Champion and Pond, 1959). The x-ray f l u x at any a l t i t u d e i s g i v e n by P(z) = 3.2 x 108y/R2, where Y i s the x-ray y i e l d i n k i l o t o n s , and R the d i s t a n c e i n km, i . e . R = 400-z f o r " S t a r f i s h " . M u l t i p l y i n g E ( z ) by p ( z ) f o r each a l t i t u d e g i v e s the magni-tude o f the i o n i z i n g p u l s e , and si n c e 2 . 1 x 1 0 1 0 i o n p a i r s are produced f o r each e r g of d e p o s i t e d energy, the r e s u l t a n t i o n i z a t i o n AN can be computed. F i g u r e 8 shows the value s of AN at a l t i t u d e s from 50 to 350 km, f o r 0 .5 and 2 kev temper-a t u r e s and an x-ray y i e l d of 1050 k i l o t o n s . 53. The c o r r e s p o n d i n g A l f v e n v e l o c i t i e s as a f u n c t i o n of a l t i t u d e have been d e r i v e d and are p l o t t e d i n P i g u r e 9. The p r o p a g a t i o n time d e l a y between a l t i t u d e s of 350 and 80 km based on these v e l o c i t i e s i s T m a x = 46 seconds (0.5 kev, 1050 k i l o t o n s ) T m i n - 10.6 seconds (2 kev, 350 k i l o t o n s ) . Some q u a l i f y i n g comments should be made: 1) The ambient i o n i z a t i o n has been n e g l e c t e d , s i n c e n i g h t -time i o n d e n s i t i e s are s e v e r a l o r d e r s of magnitude below the x-ray i o n i z a t i o n d e n s i t i e s . 2) I o n i z a t i o n by other prompt energy emissions has been ig n o r e d . In p a r t i c u l a r , neutron p r o d u c t s and the low energy t a i l of the y-ray p u l s e c o n t r i b u t e t o i o n i z a t i o n i n t h i s r e g i o n . However, energy c o n s i d e r a t i o n s and a few check-computations showed t h a t t h e i r c o n t r i b u t i o n would amount to about 1-2$ of the x-ray e f f e c t s , which can be n e g l e c t e d as f a r as the A l f v e n v e l o c i t i e s are concerned. 3) The s a d d l e - p o i n t method g i v e s a good approximation to the v a l u e s o b t a i n e d by exact n u m e r i c a l i n t e g r a t i o n methods f o r a l t i t u d e s up to about 120 km ( L a t t e r and L e L e v i e r , 1963). Above t h i s h e i g h t the divergence becomes p r o g r e s s i v e l y worse, the s a d d l e - p o i n t d e r i v e d values b e i n g c o n s i s t e n t l y lower than the exact v a l u e s . Above 200 km i t i s l i t t l e b e t t e r than a rough e s t i m a t e . N e v e r t h e l e s s , f o r computations of A l f v e n v e l o c i t i e s t h i s i s q u i t e adequate, s i n c e about 60$ of the t o t a l time d e l a y i s i n t r o d u c e d between a l t i t u d e s of 120 and FIG. 8 X - R A Y IONIZATION BELOW T H E DETONATION POINT TEMPERATURE: 0.5 Kev X-RAY YIELD: |050 (KILOTONS) 0.5 Kev 350 350 300 250 UJ o H 200 20 30 40 50 60 70 FIG. 9 A L F V E N VELOCITIES BELOW T H E DETONATION POINT U l 56. 80 km. A fa c t o r - o f - 2 error i n i o n i z a t i o n densities over | the rest of the path would a f f e c t the t o t a l delay time by a maximum of 20$. 4) No attempt has been made to even estimate conditions just below the explosion point ( i . e . 350-400 km a l t i t u d e ) . The d i f f i c u l t y has been avoided by postulating that the piston i t s e l f was s t i l l d r i v i n g through most of t h i s layer ( i . e . n e g l i g i b l e time delay - expansion v e l o c i t i e s are about 100-1000 km/sec). The triggered hydromagnetic wave was then assumed to have started at an al t i t u d e of 350 km. Most of the bubble expansion i s upwards into less r e s i s t a n t medium (Colgate, 1963), and 50 km appears to be a reasonable e s t i -mate of the maximum downward range of the expansion. 5) The values of the parameters r(z) and h(z) are suitable mean values, but Johnson (1961) has emphasized that actual values at any one time could be s i g n i f i c a n t l y d i f f e r e n t . Some check ca l c u l a t i o n s have been c a r r i e d out, using extreme values- f o r these parameters, and the o v e r a l l e f f e c t on the delay time i s of the order of ±2 to 5$. A further possible ±1 to 2$ error i s introduced by the variations i n the mean molecular weight. To summarize, as f a r as the "minor" sources of error are concerned, the delay times should be considered +25$, -5$, i . e . Tmax = 4 4 - 5 8 seconds Tmin = 10 -.13 seconds. 57. A further major uncertainty i s introduced by the shock-wave nature of the disturbance. The piston v e l o c i t y i s at least an order of magnitude greater than the Alfven v e l o c i t y i n the medium below. The hydromagnetic Mach number M = l/\f2 where a i s the density behind the shock front and a0 i s the undisturbed density (Lundquist, 1952). In the absence of detailed information about the explosion c h a r a c t e r i s t i c s , we can arrive at l i m i t i n g (maximum) values of M by considering a strong shock. For t h i s case a/aQ = (7 + l ) / ( 7 - 1) where 7 i s the r a t i o of the s p e c i f i c heats. The correct value of 7 i s not known - an acceptable range f o r t h i s region i s 7 = 1.4 to 7 = 2. The low values of 7 used by Caner and Whitham (1962) to explain the p o s s i b i l i t y of very high Mach numbers are not j u s t i f i e d , since the medium i s by no means f u l l y ionized (1-2$ only), and since the shock d i r e c t i o n i s normal to the d i r e c t i o n of the f i e l d . For shocks normal to the f i e l d d i r e c t i o n i n a medium where the magnetic pressure i s much higher than the hydrodynamic pressure, 7 = 2 can be used (Montgomery, 1959) and M = 2.45. At the other extreme, the r a t i o of the s p e c i f i c heats up to an a l t i t u d e of 90 km i s normally defined to be 1.4 (Minzner, Champion and Pond, 1959). For 7 - 1.4, M = 4 .6 . Applying t h i s maximum value an of M to T m j _ n , we arrive at/extreme value f o r T m j _ n = 2.2-2.8 seconds. a/a0 + (a/« 0) 2 58. I t would appear therefore that f o r bomb tempera-tures up to 2 kev a 2 second time delay i s not impossible, although not very l i k e l y . 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