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Rocket measurements of current distribution in a normal and an intense equatorial electrojet
Journal
of Atraosph~rlcand Terrestrial Physica, Vol. 38, pp. 307 to 311. Pergamon Press, 1976. Printed in Northern Ireland
Rocket measurements of current distribution in a normal and an intense equatorial electrojet K. B U R R O W S Appleton Laboratory, Ditton Park, Slough, Berks., SL3 9JX, England and T. S. G. SASTRY Physical Research Laboratory, Ahmedabad, India (Received 31 July 1975) A b s t r a c t - - T w o Petrel rockets, each carrying a rubidium magnetometer, were launched from Thumba, at times close to the diurnal electrojet peaks, on different days. The peak electro jet intensity during one of the days was unusually large, approaching twice that of the other day. A comparison is made of the ionospheric current distribution during these two events.
1. INTRODUCTION
magnetograms (BURROWS, 1970, a n d references
Since in situ measurements of the equatorial electrojet were reported (SINGER e t a l . , 1951; CAH~L, 1959; MAYNARDetal., 1965) a n u m b e r of investigations have been carried out of its detailed structure and of its variability, in time and space, with both rocket a n d ground magnetometers. I t is now well established t h a t the electrojet consists of a n intense current layer centered at a height of 106 ± 2 k m and t h a t below this peak the current density decreases rapidly to a negligible value at about 90 kin. Above the peak, however, the current decreases more slowly in a diffuse 'tail' which extends up to 130-140 kin. I n addition to this main peak a second, weaker, layer has been reported, near the latitude centre of the electrojet above Southern India, at a height of approximately 140 k m (MAYI~ARD and Ci~ILL, 1965) and, in
cited therein). More recently SASTR¥ (1970) launched three rockets, during a single day, to investigate the height variation of the electrojet currents during the build-up a n d decay of a single electrojet event. He reported t h a t during the afternoon, when the ground-level magnetic field associated with the jet currents had reduced to approximately 50 ~ of its value at the peak time, the current profile was less intense and broader, compared with the peak time, b u t t h a t the height of m a x i m u m current density did not change appreciably. We report here an a t t e m p t to investigate the variation in vertical current density profile as between two very different electrojet events; one on a 'normal' day a n d one one on a day when the peak diurnal magnetic field variation, as recorded at ground level, was approximately twice the normal amplitude.
another flight from Thumba, the possibility of two weak subsidiary layers at 132 km and 162 km was reported (SAsTRY, 1968). A cross-section of the jet current distribution (i. e. variation with both height and latitude) over South America, was obtained in a series of shipboard launchings off the coast of Peru (DAws etal., 1967). These flights were, however, made on different days and, although the results were normalized for variations in peak jet intensity, they still included effects of day-to-day variability in the jet current distribution, in both height and latitude. Accordingly the day-to-day variability of the latitude profile (and intensity) of the electrojet was investigated, using ground observatory
2. GROUND D A T A Since the detailed vertical structure of the electrojet is known to vary significantly with distance from the latitude centre (DAws el al., 1967) and since the centre of the jet is known to wander slightly in latitude from day to day (BURROWS, 1970) it was important to monitor the latitude profile of the electrojet in order t h a t a n y significant change could be identified and, if necessary, taken into account in the interpretation of f i g h t data. To supplement existing standard magnetic observatories a c h a i n o f r e c o r d i n g r u b i d i u m m a g n e t o m e t e r s
307
308
K. BUal~OWS and T. S. G. SASTaY days of the two rocket flights and no appreciable
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Petrel P l l 0 was launched from the T h u m b s Equatorial Rocket Launching Site (TERLS) (8.52°iN~, 76-87°E, dip let. 0.47°S) at If00 hr IST on ~ - " ~ . / L.] ' , , ' ~ - ~ ~ Tr~......, 2 F e b r u a r y 1972 and P59 was 1aun ched from the stone site at 1130 hr IST on 5 F e b r u a r y 1972. Each Ko~Wo.¢: rocket carried a (scalar) dual-cell rubidium-vapour magnetometer, axially mounted under the glass To'P.o r ~ :'::=~ fibre nose cone, and a single-channel FM telemetry oo o5 ,o ,5 zo 14 transmitter. To remove the sensor as far as HoursIST practicable from the magnetic field perturbations Fig. 1. H Magnetograms from Southern Indian Ob- associated with the rocket motor tube, a forebody servatories, 2 February 1972. Dip Latitudes: Tri- release mechanism, consisting of a piston operated vandrum --0.50 °, Kovilpatti +0.167 °, Kodaikanal b y compressed gas, was included (ROGERS and +1.502 °, Triehinapalli +2.40 °, Annamalainagar P~WO~T~, 1971). Magnetic fields associated with T2.756, IKyderabad W10.13°. Launch time, TQ, of P110 is indicated by an arrow, the payload itself were minimized b y balancing the external fields of the two essential latching relays and b y laying all current carrying cables 'nonwas, therefore, established for the period of the inductively'. With these precautions the 'heading rocket launch campaign. The H magnetograms error' (variation of measured field with attitude) of from the various p e r m a n e n t a n d temporary the complete forebody was insignificantly different magnetic observatories for the days of the two from the intrinsic heading error of the sensor. rocket flights in question are reproduced in Figs. 1 Unfortunately this payload lay-out entailed flying a n d 2. 2 F e b r u a r y was a disturbed day ( E K p = 22) a nose-tip ballast of approx. 10 lb of lead, to though the m a i n disturbance did not occur until achieve aerodynamic stability; this significantly after the flight of P l l 0 (Fig. 1). 5 F e b r u a r y was reduced apogee on both flights. I n an a t t e m p t to quiet ( E K p = 12) and, on this occasion the regain some of the lost height, 'low-drag' a n t e n n a s amplitude of the diurnal excursion in H, associated were used a n d the leading edges of the tail fins were with the eloctrojet, was exceptionally large, sharpened. These were the first two flights from approaching twice t h a t of 2 F e b r u a r y (Fig. 2). T E R L S of rockets carrying rubidium vapour F r o m an inspection of the relative amplitude of magnetometers. the daily ranges at the various equatorial stations The rocket trajectories were determined b y a it is evident t h a t the latitude distribution of combination o f r a d a r a n d t h o T E R L S 'Tone-range' current, within the electrojet, was similar on the facility. The latter equipment consists of two m u t u a l l y perpendicular R F interferometers which record continuously the variation in relative phase of the telemetry carrier. From this record the time ~H ~ variation of rocket azimuth and elevation was Hy~,~ computed. The position and velocity vector of the rocket at 80 kin, deduced from radar, were used to ~04 determine a free-fall trajectory from which azimuth and elevation, relative to the tone-range site, were A~,~,o~,o~o, computed. The computations were compared with /// \ \ ~ - - - - ~ \ \ ~ ~ T r ' c h ......., the above measured values and, in an iterative ~o~0,~o°o, technique, small adjustments were made, to the ......... six parameters specifying the rocket position and Ps9 velocity at 80 kin, until the agreement between , , , , , Y~°~" measured and computed values of azimuth and O0 05 IO 15 20 24 Hours' 5T elevation was as close as possible, throughout the entire free-fall section of the flight. Since a change Fig. 2. H 1Yiagnetograms for 5 February 1972 (SeeFig. 1 caption and note change in scale compared with Fig. 1). of ~: 0"5 km would produce a measurable difference Launch time, To, of P59 is indicated by an arrow, between the measured and computed values of
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Rocket measurements of current distribution in a normal and an intense equatorial electrojet
a z i m u t h s a n d e l e v a t i o n t h e a c c u r a c y of t h e final trajectories is of this order.
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a m i n i m u m n e a r 105 k m a n d followed b y a m o r e g r a d u a l r e t u r n t o w a r d s a v e r t i c a l line at g r e a t e r heights, The p l o t of residuals against h e i g h t p r o v i d e s a useful e o n f i r m a t i o n o f t h e a e e u r a c y o f t r a j e c t o r y d e t e r m i n a t i o n . The m a g n e t i c field below a n idealized h o r i z o n t a l 'sheet' c u r r e n t is of identical a m p l i t u d e , b u t opposite in sign, to t h e field a b o v e t h e current, H e n c e a plot of residuals against h e i g h t should be s y m m e t r i c a l a b o u t t h e zero field v a l u e and, outside t h e c u r r e n t layer, t h e profile should be a v e r t i c a l s t r a i g h t line. T h e ascent a n d descent d a t a should also agree. B y inspection of Fig. 3 it is e v i d e n t t h a t t h e results f r o m b o t h r o c k e t flights, w h i l s t n o t perfect, c o m e close t o m e e t i n g all t h r e e of these criteria. P59, h o w e v e r , did n o t fully p e n e t r a t e t h e m a i n electrojet c u r r e n t layer, P59 flew at a s l i g h t l y lower e l e v a t i o n t h a n P l l 0
a n d hence r e a c h e d a lower apogee, b u t this resulted in t h e g e o m a g n e t i c v e c t o r lying in a sensor 'live' zone t h r o u g h o u t t h e e n t i r e flight. P l l 0 , on t h e o t h e r h a n d , flew in an a t t i t u d e closer to t h e v e r t i c a l a n d this caused t h e g e o m a g n e t i c v e c t o r t o w a n d e r in a n d o u t of t h e sensor e q u a t o r i a l d e a d zone as t h e r o c k e t precessed. F o r t u i t o u s l y t h e periods of g o o d d a t a on t h e descent flight coincided in h e i g h t w i t h t h e gaps in d a t a during t h e ascent flight; hence a c o m p l e t e v e r t i c a l profile was o b t a i n e d b y a c o m b i n a t i o n of t h e t w o sections of t h e flight (Fig. 3). The curves shown are based on a 7-point r u n n i n g m e a n of t h e data. Unfortunately the forebody separation mechan i s m failed in b o t h rockets a n d this has resulted in small quasi-periodic v a r i a t i o n s in t h e m a g n e t o m e t e r d a t a arising f r o m m a g n e t i c m a t e r i a l in t h e m o t o r case. I t is this feature w h i c h sets t h e
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CAIN, 1968), a n d t h e s e c o m p u t e d v a l u e s were s u b t r a c t e d f r o m t h e v a l u e s as m e a s u r e d b y t h e r o c k e t m a g n e t o m e t e r s . F i g u r e 3 shows, superimposed, a v e r t i c a l profile o f t h e s e 'residuals' for b o t h r o c k e t flights. The influence of t h e electrojet is e v i d e n c e d as a r a p i d decrease in t h e slope of t h e profiles at approx. 100 fun, r e d u c i n g to
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4. RESULTS AND DISCUSSION
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p r a c t i c a l limit to t h e a c c u r a c y of t h e d a t a interp r e t a t i o n . I t p r o v e d i m p r a c t i c a l to eliminate this error b y a p p l y i n g a sinusoidal correction, as has b e e n d o n e p r e v i o u s l y (DAviS et al., 1967), since t h e p e r i o d of t h e v a r i a t i o n s was f o u n d to change slightly d u r i n g flight. F i g u r e 4 shows a d e t a i l e d c o m p a r i s o n of the small features of P59 m a g n e t i c profile o b s e r v e d during both ascent a n d descent flights. E a c h point plotted is a m e a s u r e m e n t of the slope of the
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Fig. 3. Magnetometer data from flights P l l 0 (2 February 1972) and 1)59 (5 February 1972).
m a g n e t i c field profile o b t a i n e d d u r i n g one spin period of t h e vehicle (0.15 sec for P l l 0 , 0.13 sec for P59). A p l o t of this n a t u r e is t h e m o s t sensitive m e t h o d of i d e n t i f y i n g a small s t r u c t u r a l feature in t h e ionospheric current profile and if a f e a t u r e of t h e ascent d a t a is r e p r o d u c e d on t h e descent record, it p r o v i d e s persuasive e v i d e n c e t h a t t h e feature is real. E v i d e n c e like this has led to the d i s c o v e r y o f a subsidiary layer a b o v e t h e main electrojet p e a k (MAYNARD a n d CAH~T,, 1965). I n Fig. 4 t h e r e is a n obvious feature at approxim a t e l y 116 k m which correlates almost e x a c t l y in
310
K. BURROWS and T. S. G. SASTRY
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height, as between ascent and descent data, whereas the similar features close to 1 3 0 k m exhibit a pronounced anti-correlation. Initially it was assumed t h a t the magnetic field changes at 116 km resulted from a subsidiary current layer. However a closer examination of both height and time variations led to the conclusion t h a t only perturbations associated with the rocket motor tube were present. I f the rate of precession of the rocket had remained constant the perturbation at 116 k m on the ascent would have occurred at a different height on descent, b u t there was evidence of a change in precession rate, possibly resulting from gas escaping past the piston in the forebedy release. This change, apparently caused the perturbation, coincidentally, to occur also at 116 k m on the descent, and the coincidence cannot therefore be accepted as evidence of a n a t u r a l feature. Since the magnetic field produced b y the electrojet is directed S-l~, parallel to the total geomagnetie force, F , the current density at a n y height can readily be inferred from the gradient of the magnetic field profiles shown in Fig. 3. Figure 5 shows a plot of current density as a function of height. P l l 0 data are derived from a m a n u a l best-fit curve drawn through the ascent a n d descent data of Fig. 3. I n the case of P59 the computer 7-point r u n n i n g mean readings for ascent a n d descent were averaged at corresponding heights and the current density profile was deduced
from the slope of this average data profile. Hence the precession period oscillations of Fig. 3 appear as an apparent modulation in current density in Fig. 5. Similarly a quasi-periodic modulation is evident in the lower portion of P l l 0 profile; this results from the difficulty of drawing a compromise bestfit curve through both ascent and descent data. I n spite of the limitations on the accuracy of the current density determinations a n u m b e r of interesting features emerge from a comparison of the 1)59 and P l l 0 profiles of Fig. 5 as follows: (a) I t is immediately evident that during the exceptionally intense electrojet of 5 F e b r u a r y 1972, the general form of the current density distribution with height was similar to that found in the normal diurnal electrojet. The profile shows the charaeteristic sharp lower border followed by a gradual fall-off with height above the peak at 104 kin. Indeed if the P59 profile is 'normalized', b y the ratio of the two peak amplitudes, the curve of P59 closely follows that of P l l 0 , within the accuracy limitation set b y the oscillations discussed above. (b) The heights of the peaks in the two current distribution profiles are, within experimental error, identical on these two, very different, days. (c) From the area under the current profile curves of Fig. 5 it is possible to estimate the total height-integrated current flowing between 80 and 140 lun. I n this computation it is assumed that no current flowed at heights below 90 km a n d this was taken as the zero for the 1)59 profile. This assumption can be justified on the grounds that the ascent magnetic profile of Fig. 3 showed a constant slope at all heights below 90 kin; the small deviation from the vertical in this height range can, therefore, be attributed to either a small trajectory error or a small error in the computed reference field. (The descent profile exhibits large modulations as the rocket re-enters the atmosphere.) On this basis it can be shown that the total current recorded by P l l 0 was 93 A/kin a n d a total of 169 A/kin were detected by 1)59, a ratio of 1:1.82. The displacement of the H magnetograms from the mean night-time level on the T r i v a n d r u m magnetograms (the closest station to the launch site) was 75 y during flight P l l 0 , (1 ? = lnT) and 135 ~, during flight P59, a ratio of 1:1.80. Thus the ratio of electrojet magnetic fields recorded at ground level (1.80) agrees very closely with the ratio of total currents measured b y the two rockets (1.82). I t can therefore be concluded that the exceptionally large magnetic field recorded at ground level on 5
Rocket measurements of current distribution in a normal a n d a n intense equatorial electrojet February was the result of an enhancement of the normal electrojet and did not result from the appearance of a large additional current at a different height or from variations in the extraterrestrial ring current, 5. CONCLUSION The results presented in this paper suggest that the form of the vertical distribution of current n e a r t h e c e n t r e o f t h e e q u a t o r i a l e l e c t r o j e t does n o t change significantly, from day to day, even when t h e t o t a l e l e c t r o j e t i n t e n s i t y i n c r e a s e s b y as m u c h as a f a c t o r o f t w o . T h e t o t a l j e t c u r r e n t , h o w e v e r , i n c r e a s e s i n p r o p o r t i o n t o t h e m a g n e t i c effects o b s e r v e d a t g r o u n d level,
311
Acknowledgements--This work formed p a r t of a collaborative programme in which the Appleton Laboratory carried out the experiment with considerable support from the Mullard Space Science Laboratory, in providing parts of the payload, a n d the launch facilities were provided b y the Physical Research Laboratory, Ahmedabad. The launchings were carried out during a Commonwealth Collaborative Programme from the T h u m b a Equatorial Rocket Launching Site, Southern India. The authors are indebted to Dr. M. J . USHER Of Reading University who prepared the rocket magnetometers, Mr. J. D. S T O G I E a n d Mrs. MARY MILLER of NASA for processing the flight tapes on the specialpurpose rubidium line a t the Goddard Space Flight Center a n d to the Director, I n d i a n I n s t i t u t e of Geomagnetism for supplying data from p e r m a n e n t I n d i a n Magnetic Observatories. The work is published b y permission of the Director of the Appleton Laboratory.
REFERENCES
BURROWS K. CAHILL L . J . DAVIS T. N., B u R r o w s K. a n d STOY.4.R~ J . D . MAYNAI%D N. C., CAHILL L. J. a n d SAS~RY T. S. G. M ~ A l ~ D N. C. a n d C ~ I L L L . J . NL~Y~--RD N . C . ROGERS N. a n d PAPWOl%TH C . D . SASTRY T. S . G . SASTRY T. S . G .
1970 1959 1967 1965
J. J. J. J.
1965 1967 1971 1968 1970
SrNGER S. F., MAPLE E. a n d BOWEN W . A .
1951
J. geophys. Res. 70, 5923. J. geophys. Res. 72, 1863. J . B . I . S . 24, 161. J. geophys. Res. 78, 1789. Space Research X , p. 778. Berlin. J. geophys. Res. 56, 265.
Reference is also made to the following unpublished mate¢~al : CAIN J . C. a n d CAr~ S~I~LEY J. 1968
geophys. geophys. geophys. geophys.
Res. Res. Res. Res.
75, 64, 72, 70,
1319. 489. 1845. 1241.
Akademie Verlag,
Rept. to IAGA Commission I I , Working Group 4, Goddard Space Flight Center P r e p r i n t X-61268-501.
of Atraosph~rlcand Terrestrial Physica, Vol. 38, pp. 307 to 311. Pergamon Press, 1976. Printed in Northern Ireland
Rocket measurements of current distribution in a normal and an intense equatorial electrojet K. B U R R O W S Appleton Laboratory, Ditton Park, Slough, Berks., SL3 9JX, England and T. S. G. SASTRY Physical Research Laboratory, Ahmedabad, India (Received 31 July 1975) A b s t r a c t - - T w o Petrel rockets, each carrying a rubidium magnetometer, were launched from Thumba, at times close to the diurnal electrojet peaks, on different days. The peak electro jet intensity during one of the days was unusually large, approaching twice that of the other day. A comparison is made of the ionospheric current distribution during these two events.
1. INTRODUCTION
magnetograms (BURROWS, 1970, a n d references
Since in situ measurements of the equatorial electrojet were reported (SINGER e t a l . , 1951; CAH~L, 1959; MAYNARDetal., 1965) a n u m b e r of investigations have been carried out of its detailed structure and of its variability, in time and space, with both rocket a n d ground magnetometers. I t is now well established t h a t the electrojet consists of a n intense current layer centered at a height of 106 ± 2 k m and t h a t below this peak the current density decreases rapidly to a negligible value at about 90 kin. Above the peak, however, the current decreases more slowly in a diffuse 'tail' which extends up to 130-140 kin. I n addition to this main peak a second, weaker, layer has been reported, near the latitude centre of the electrojet above Southern India, at a height of approximately 140 k m (MAYI~ARD and Ci~ILL, 1965) and, in
cited therein). More recently SASTR¥ (1970) launched three rockets, during a single day, to investigate the height variation of the electrojet currents during the build-up a n d decay of a single electrojet event. He reported t h a t during the afternoon, when the ground-level magnetic field associated with the jet currents had reduced to approximately 50 ~ of its value at the peak time, the current profile was less intense and broader, compared with the peak time, b u t t h a t the height of m a x i m u m current density did not change appreciably. We report here an a t t e m p t to investigate the variation in vertical current density profile as between two very different electrojet events; one on a 'normal' day a n d one one on a day when the peak diurnal magnetic field variation, as recorded at ground level, was approximately twice the normal amplitude.
another flight from Thumba, the possibility of two weak subsidiary layers at 132 km and 162 km was reported (SAsTRY, 1968). A cross-section of the jet current distribution (i. e. variation with both height and latitude) over South America, was obtained in a series of shipboard launchings off the coast of Peru (DAws etal., 1967). These flights were, however, made on different days and, although the results were normalized for variations in peak jet intensity, they still included effects of day-to-day variability in the jet current distribution, in both height and latitude. Accordingly the day-to-day variability of the latitude profile (and intensity) of the electrojet was investigated, using ground observatory
2. GROUND D A T A Since the detailed vertical structure of the electrojet is known to vary significantly with distance from the latitude centre (DAws el al., 1967) and since the centre of the jet is known to wander slightly in latitude from day to day (BURROWS, 1970) it was important to monitor the latitude profile of the electrojet in order t h a t a n y significant change could be identified and, if necessary, taken into account in the interpretation of f i g h t data. To supplement existing standard magnetic observatories a c h a i n o f r e c o r d i n g r u b i d i u m m a g n e t o m e t e r s
307
308
K. BUal~OWS and T. S. G. SASTaY days of the two rocket flights and no appreciable
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latitude movements. FLIGHT°f the jetDATAcentreoccurred. '
Petrel P l l 0 was launched from the T h u m b s Equatorial Rocket Launching Site (TERLS) (8.52°iN~, 76-87°E, dip let. 0.47°S) at If00 hr IST on ~ - " ~ . / L.] ' , , ' ~ - ~ ~ Tr~......, 2 F e b r u a r y 1972 and P59 was 1aun ched from the stone site at 1130 hr IST on 5 F e b r u a r y 1972. Each Ko~Wo.¢: rocket carried a (scalar) dual-cell rubidium-vapour magnetometer, axially mounted under the glass To'P.o r ~ :'::=~ fibre nose cone, and a single-channel FM telemetry oo o5 ,o ,5 zo 14 transmitter. To remove the sensor as far as HoursIST practicable from the magnetic field perturbations Fig. 1. H Magnetograms from Southern Indian Ob- associated with the rocket motor tube, a forebody servatories, 2 February 1972. Dip Latitudes: Tri- release mechanism, consisting of a piston operated vandrum --0.50 °, Kovilpatti +0.167 °, Kodaikanal b y compressed gas, was included (ROGERS and +1.502 °, Triehinapalli +2.40 °, Annamalainagar P~WO~T~, 1971). Magnetic fields associated with T2.756, IKyderabad W10.13°. Launch time, TQ, of P110 is indicated by an arrow, the payload itself were minimized b y balancing the external fields of the two essential latching relays and b y laying all current carrying cables 'nonwas, therefore, established for the period of the inductively'. With these precautions the 'heading rocket launch campaign. The H magnetograms error' (variation of measured field with attitude) of from the various p e r m a n e n t a n d temporary the complete forebody was insignificantly different magnetic observatories for the days of the two from the intrinsic heading error of the sensor. rocket flights in question are reproduced in Figs. 1 Unfortunately this payload lay-out entailed flying a n d 2. 2 F e b r u a r y was a disturbed day ( E K p = 22) a nose-tip ballast of approx. 10 lb of lead, to though the m a i n disturbance did not occur until achieve aerodynamic stability; this significantly after the flight of P l l 0 (Fig. 1). 5 F e b r u a r y was reduced apogee on both flights. I n an a t t e m p t to quiet ( E K p = 12) and, on this occasion the regain some of the lost height, 'low-drag' a n t e n n a s amplitude of the diurnal excursion in H, associated were used a n d the leading edges of the tail fins were with the eloctrojet, was exceptionally large, sharpened. These were the first two flights from approaching twice t h a t of 2 F e b r u a r y (Fig. 2). T E R L S of rockets carrying rubidium vapour F r o m an inspection of the relative amplitude of magnetometers. the daily ranges at the various equatorial stations The rocket trajectories were determined b y a it is evident t h a t the latitude distribution of combination o f r a d a r a n d t h o T E R L S 'Tone-range' current, within the electrojet, was similar on the facility. The latter equipment consists of two m u t u a l l y perpendicular R F interferometers which record continuously the variation in relative phase of the telemetry carrier. From this record the time ~H ~ variation of rocket azimuth and elevation was Hy~,~ computed. The position and velocity vector of the rocket at 80 kin, deduced from radar, were used to ~04 determine a free-fall trajectory from which azimuth and elevation, relative to the tone-range site, were A~,~,o~,o~o, computed. The computations were compared with /// \ \ ~ - - - - ~ \ \ ~ ~ T r ' c h ......., the above measured values and, in an iterative ~o~0,~o°o, technique, small adjustments were made, to the ......... six parameters specifying the rocket position and Ps9 velocity at 80 kin, until the agreement between , , , , , Y~°~" measured and computed values of azimuth and O0 05 IO 15 20 24 Hours' 5T elevation was as close as possible, throughout the entire free-fall section of the flight. Since a change Fig. 2. H 1Yiagnetograms for 5 February 1972 (SeeFig. 1 caption and note change in scale compared with Fig. 1). of ~: 0"5 km would produce a measurable difference Launch time, To, of P59 is indicated by an arrow, between the measured and computed values of
,
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Rocket measurements of current distribution in a normal and an intense equatorial electrojet
a z i m u t h s a n d e l e v a t i o n t h e a c c u r a c y of t h e final trajectories is of this order.
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a m i n i m u m n e a r 105 k m a n d followed b y a m o r e g r a d u a l r e t u r n t o w a r d s a v e r t i c a l line at g r e a t e r heights, The p l o t of residuals against h e i g h t p r o v i d e s a useful e o n f i r m a t i o n o f t h e a e e u r a c y o f t r a j e c t o r y d e t e r m i n a t i o n . The m a g n e t i c field below a n idealized h o r i z o n t a l 'sheet' c u r r e n t is of identical a m p l i t u d e , b u t opposite in sign, to t h e field a b o v e t h e current, H e n c e a plot of residuals against h e i g h t should be s y m m e t r i c a l a b o u t t h e zero field v a l u e and, outside t h e c u r r e n t layer, t h e profile should be a v e r t i c a l s t r a i g h t line. T h e ascent a n d descent d a t a should also agree. B y inspection of Fig. 3 it is e v i d e n t t h a t t h e results f r o m b o t h r o c k e t flights, w h i l s t n o t perfect, c o m e close t o m e e t i n g all t h r e e of these criteria. P59, h o w e v e r , did n o t fully p e n e t r a t e t h e m a i n electrojet c u r r e n t layer, P59 flew at a s l i g h t l y lower e l e v a t i o n t h a n P l l 0
a n d hence r e a c h e d a lower apogee, b u t this resulted in t h e g e o m a g n e t i c v e c t o r lying in a sensor 'live' zone t h r o u g h o u t t h e e n t i r e flight. P l l 0 , on t h e o t h e r h a n d , flew in an a t t i t u d e closer to t h e v e r t i c a l a n d this caused t h e g e o m a g n e t i c v e c t o r t o w a n d e r in a n d o u t of t h e sensor e q u a t o r i a l d e a d zone as t h e r o c k e t precessed. F o r t u i t o u s l y t h e periods of g o o d d a t a on t h e descent flight coincided in h e i g h t w i t h t h e gaps in d a t a during t h e ascent flight; hence a c o m p l e t e v e r t i c a l profile was o b t a i n e d b y a c o m b i n a t i o n of t h e t w o sections of t h e flight (Fig. 3). The curves shown are based on a 7-point r u n n i n g m e a n of t h e data. Unfortunately the forebody separation mechan i s m failed in b o t h rockets a n d this has resulted in small quasi-periodic v a r i a t i o n s in t h e m a g n e t o m e t e r d a t a arising f r o m m a g n e t i c m a t e r i a l in t h e m o t o r case. I t is this feature w h i c h sets t h e
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CAIN, 1968), a n d t h e s e c o m p u t e d v a l u e s were s u b t r a c t e d f r o m t h e v a l u e s as m e a s u r e d b y t h e r o c k e t m a g n e t o m e t e r s . F i g u r e 3 shows, superimposed, a v e r t i c a l profile o f t h e s e 'residuals' for b o t h r o c k e t flights. The influence of t h e electrojet is e v i d e n c e d as a r a p i d decrease in t h e slope of t h e profiles at approx. 100 fun, r e d u c i n g to
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I n order to resolve t h e a n t i c i p a t e d m a g n e t i c field associated w i t h t h e e l e c t r o j e t c u r r e n t s a reference
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4. RESULTS AND DISCUSSION
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p r a c t i c a l limit to t h e a c c u r a c y of t h e d a t a interp r e t a t i o n . I t p r o v e d i m p r a c t i c a l to eliminate this error b y a p p l y i n g a sinusoidal correction, as has b e e n d o n e p r e v i o u s l y (DAviS et al., 1967), since t h e p e r i o d of t h e v a r i a t i o n s was f o u n d to change slightly d u r i n g flight. F i g u r e 4 shows a d e t a i l e d c o m p a r i s o n of the small features of P59 m a g n e t i c profile o b s e r v e d during both ascent a n d descent flights. E a c h point plotted is a m e a s u r e m e n t of the slope of the
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Fig. 3. Magnetometer data from flights P l l 0 (2 February 1972) and 1)59 (5 February 1972).
m a g n e t i c field profile o b t a i n e d d u r i n g one spin period of t h e vehicle (0.15 sec for P l l 0 , 0.13 sec for P59). A p l o t of this n a t u r e is t h e m o s t sensitive m e t h o d of i d e n t i f y i n g a small s t r u c t u r a l feature in t h e ionospheric current profile and if a f e a t u r e of t h e ascent d a t a is r e p r o d u c e d on t h e descent record, it p r o v i d e s persuasive e v i d e n c e t h a t t h e feature is real. E v i d e n c e like this has led to the d i s c o v e r y o f a subsidiary layer a b o v e t h e main electrojet p e a k (MAYNARD a n d CAH~T,, 1965). I n Fig. 4 t h e r e is a n obvious feature at approxim a t e l y 116 k m which correlates almost e x a c t l y in
310
K. BURROWS and T. S. G. SASTRY
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height, as between ascent and descent data, whereas the similar features close to 1 3 0 k m exhibit a pronounced anti-correlation. Initially it was assumed t h a t the magnetic field changes at 116 km resulted from a subsidiary current layer. However a closer examination of both height and time variations led to the conclusion t h a t only perturbations associated with the rocket motor tube were present. I f the rate of precession of the rocket had remained constant the perturbation at 116 k m on the ascent would have occurred at a different height on descent, b u t there was evidence of a change in precession rate, possibly resulting from gas escaping past the piston in the forebedy release. This change, apparently caused the perturbation, coincidentally, to occur also at 116 k m on the descent, and the coincidence cannot therefore be accepted as evidence of a n a t u r a l feature. Since the magnetic field produced b y the electrojet is directed S-l~, parallel to the total geomagnetie force, F , the current density at a n y height can readily be inferred from the gradient of the magnetic field profiles shown in Fig. 3. Figure 5 shows a plot of current density as a function of height. P l l 0 data are derived from a m a n u a l best-fit curve drawn through the ascent a n d descent data of Fig. 3. I n the case of P59 the computer 7-point r u n n i n g mean readings for ascent a n d descent were averaged at corresponding heights and the current density profile was deduced
from the slope of this average data profile. Hence the precession period oscillations of Fig. 3 appear as an apparent modulation in current density in Fig. 5. Similarly a quasi-periodic modulation is evident in the lower portion of P l l 0 profile; this results from the difficulty of drawing a compromise bestfit curve through both ascent and descent data. I n spite of the limitations on the accuracy of the current density determinations a n u m b e r of interesting features emerge from a comparison of the 1)59 and P l l 0 profiles of Fig. 5 as follows: (a) I t is immediately evident that during the exceptionally intense electrojet of 5 F e b r u a r y 1972, the general form of the current density distribution with height was similar to that found in the normal diurnal electrojet. The profile shows the charaeteristic sharp lower border followed by a gradual fall-off with height above the peak at 104 kin. Indeed if the P59 profile is 'normalized', b y the ratio of the two peak amplitudes, the curve of P59 closely follows that of P l l 0 , within the accuracy limitation set b y the oscillations discussed above. (b) The heights of the peaks in the two current distribution profiles are, within experimental error, identical on these two, very different, days. (c) From the area under the current profile curves of Fig. 5 it is possible to estimate the total height-integrated current flowing between 80 and 140 lun. I n this computation it is assumed that no current flowed at heights below 90 km a n d this was taken as the zero for the 1)59 profile. This assumption can be justified on the grounds that the ascent magnetic profile of Fig. 3 showed a constant slope at all heights below 90 kin; the small deviation from the vertical in this height range can, therefore, be attributed to either a small trajectory error or a small error in the computed reference field. (The descent profile exhibits large modulations as the rocket re-enters the atmosphere.) On this basis it can be shown that the total current recorded by P l l 0 was 93 A/kin a n d a total of 169 A/kin were detected by 1)59, a ratio of 1:1.82. The displacement of the H magnetograms from the mean night-time level on the T r i v a n d r u m magnetograms (the closest station to the launch site) was 75 y during flight P l l 0 , (1 ? = lnT) and 135 ~, during flight P59, a ratio of 1:1.80. Thus the ratio of electrojet magnetic fields recorded at ground level (1.80) agrees very closely with the ratio of total currents measured b y the two rockets (1.82). I t can therefore be concluded that the exceptionally large magnetic field recorded at ground level on 5
Rocket measurements of current distribution in a normal a n d a n intense equatorial electrojet February was the result of an enhancement of the normal electrojet and did not result from the appearance of a large additional current at a different height or from variations in the extraterrestrial ring current, 5. CONCLUSION The results presented in this paper suggest that the form of the vertical distribution of current n e a r t h e c e n t r e o f t h e e q u a t o r i a l e l e c t r o j e t does n o t change significantly, from day to day, even when t h e t o t a l e l e c t r o j e t i n t e n s i t y i n c r e a s e s b y as m u c h as a f a c t o r o f t w o . T h e t o t a l j e t c u r r e n t , h o w e v e r , i n c r e a s e s i n p r o p o r t i o n t o t h e m a g n e t i c effects o b s e r v e d a t g r o u n d level,
311
Acknowledgements--This work formed p a r t of a collaborative programme in which the Appleton Laboratory carried out the experiment with considerable support from the Mullard Space Science Laboratory, in providing parts of the payload, a n d the launch facilities were provided b y the Physical Research Laboratory, Ahmedabad. The launchings were carried out during a Commonwealth Collaborative Programme from the T h u m b a Equatorial Rocket Launching Site, Southern India. The authors are indebted to Dr. M. J . USHER Of Reading University who prepared the rocket magnetometers, Mr. J. D. S T O G I E a n d Mrs. MARY MILLER of NASA for processing the flight tapes on the specialpurpose rubidium line a t the Goddard Space Flight Center a n d to the Director, I n d i a n I n s t i t u t e of Geomagnetism for supplying data from p e r m a n e n t I n d i a n Magnetic Observatories. The work is published b y permission of the Director of the Appleton Laboratory.
REFERENCES
BURROWS K. CAHILL L . J . DAVIS T. N., B u R r o w s K. a n d STOY.4.R~ J . D . MAYNAI%D N. C., CAHILL L. J. a n d SAS~RY T. S. G. M ~ A l ~ D N. C. a n d C ~ I L L L . J . NL~Y~--RD N . C . ROGERS N. a n d PAPWOl%TH C . D . SASTRY T. S . G . SASTRY T. S . G .
1970 1959 1967 1965
J. J. J. J.
1965 1967 1971 1968 1970
SrNGER S. F., MAPLE E. a n d BOWEN W . A .
1951
J. geophys. Res. 70, 5923. J. geophys. Res. 72, 1863. J . B . I . S . 24, 161. J. geophys. Res. 78, 1789. Space Research X , p. 778. Berlin. J. geophys. Res. 56, 265.
Reference is also made to the following unpublished mate¢~al : CAIN J . C. a n d CAr~ S~I~LEY J. 1968
geophys. geophys. geophys. geophys.
Res. Res. Res. Res.
75, 64, 72, 70,
1319. 489. 1845. 1241.
Akademie Verlag,
Rept. to IAGA Commission I I , Working Group 4, Goddard Space Flight Center P r e p r i n t X-61268-501.