Low altitude observations of the energetic electrons in the outer radiation belt during isolated substorms

Phwf. Spwr SC., Vol. 33, No. 11. pp. 1259-1266. Prmted I” Great Rntain.

1985

00324633385 $3.00 + 0.00 CJ 1985 Pergamon Press Ltd

LOW ALTITUDE OBSERVATIONS OF THE ENERGETIC ELECTRONS IN THE OUTER RADIATION BELT DURING ISOLATED SUBSTORMS L. VARGA,*

D. VENKATESAN*t

and C.-I. MENGt

* Department

of Physics, The University of Calgary, Calgary, t The Johns Hopkins University, Applied Physics Laboratory, (Received in final firm

Alberta, Canada T2N IN4 Laurel, MD 20707, U.S.A.

2 May 1985)

Abstract-The low energy (t-20 keV) detector registering particles onboard the polar-orbiting low altitude (- 850 km)DMSP-F2 and -F3 satellitesalso records high energy electrons penetrating the detector walls. Thus we can study the dynamics of this electron population at L = 3.5, during isolated periods of magnetospheric substorms identified by the indices of aurora] electrojet (AE), geomagnetic (K,) and ring current (D,,). Temporal changes in the electron flux during the substorms are observed to be an additional contribution riding over the top of the pre-storm (or geomagnetically quiet-time) electron population ; the duration of the intervalofintensityvariationisobserved to beabout thesameas that oftheenhancement oftheAEindex.This indicates the temporal response of the outer rsdiation belt to the substorm activity, since the observation was madein the “horns” ofthe outer radiation belt. The observed enhanced radiation at low altitude may associate with the instantaneous increase and/or dumping of the outer radiation belt energetic electrons during each isolated substorm activity.

INTRODUCTION

A number of studies of temporal variations of the electron population in the terrestrial magnetosphere at various altitudes and covering a wide range of energies have established their association with geomagnetic storms including a positive correlation between the peaks of the daily K, sums and the radiation belt electron (E > 40 keV) flux enhancements and the correlation between variations of the trapped electron intensity and the 27-day recurrent geomagneticactivity for L > 3.5 (e.g.Forbush et al., 1961 ; Frank el al., 1964; Frank, 1965 ; Williams and Smith, 1965 ; Craven, 1966 ; Lanzerotti et al., 1967; Williams et al., 1968; Vampola et al., 1971; and Lyons and Williams, 197.5), where L refers to the McIlwain (1961) magnetospheric parameter. The geomagnetic perturbation revealed a radial dependence; in general, a decrease with decreasing values of L (Forbush et ~11.; 1962; Frank et ul., 1964; Williams ef ul., 1968; and Williams and Smith, 1965). Also, Forbush e/ ul. (1962) and Williams L’/al. (1968) have shown electron population enhancements at lower I, (e.g. = 2.5 or 2) have occurred only during large magnetic storms whereas at higher L values (e.g. =4.9 or 5.5), they have occurred even for smaller magnetic disturbances. The plasmapause location seems important to the radial location of the changes of the trapped particle population (Vampola el al., 1971 ; and Lyons and Williams, 1975). During magnetically quiet periods (Kp 5 l), the plasmapausc may bc situated at L, > 5

and, consequently, only the region outside the plasmapause has shown some geomagnetic activityrelated variation of trapped ilux (Lyons and Williams, 1975). From whistler data analysis for locating the plasmapause, Carpenter et al. (1971) have reported the increase of substorm-related electron flux just outside of the retreating plasmasphere. The electron population inside the plasmapause (e.g. L z 3.5) has been observed to decay slowly in about 10 or more days via pitch angle and radial diffusion processes (Lyons and Williams, 1975). As the geomagnetic storm strength increases, the plasmapause contracts to lower L values and even regions situated at L zz 3.5 can then experience a direct injection of fresh electrons. Mere we study the high resolution temporal variations in the energetic electron population at L = 3.5 during isolated periods ofmyrzetosphrric suh.storms, emphasizing the relatively short-term (or immediate) changes that occur during the “switch on and OK” periods of the substorm. The experiment was not originally designed for studying the cncrgeticelzctrons ; thus note energetic particles are no1 measured directly but we only infer the changes in them. based on the penetration effect seen by the DMSP aurora1 eicctron (520 keV) detector.

We use the data simultant:~)t~~l! obtained during the year 1978 by two circular ;mlar-orbiting low

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altitude (N 850+20 km) satellites DMSP-F2 and DMSP-F3 with orbital periods of 101 min. The orbits are inclined 7” to the geographic axis, and tilted towards the nightside over the northern polar region. The resulting sun-synchronous orbits enable the highlatitude particle observations near the dawn-dusk meridional plane. In geomagnetic latitude and localtime coordinate system, we observe a systematic diurnal variation of the orbits caused by a precession of the geomagnetic pole about the axis of rotation. The set of two electrostatic analyzers onboard each satellite register respectively electrons, 50 eV-1 keV (low energy) and l-20 keV (high energy). Occasionally very intense fluxes of energetic electrons > 20 keV are seen ; however, in this data collection mode, intense flux of energetic electrons can penetrate through the instrument package shielding directly into the spiratron and not through the curved plates of the energy discriminator. (See Hardy et ul., 1979 for details about the particle detector.) Figure 1 shows a plot of the electron count-rates of three selected energy channels obtained during the southern polar pass of the satellites ; Regions A and B marked therein refer to the polar cap and the aurora1 oval, respectively. Using the particle data from the DMSP satellites, Hardy et al. (1982) have studied the polar cap region. The aurora1 oval electron precipitation during various magnetically disturbed conditions have also been examined by Makita et al. (1983). We study here region C (see Fig. 1) below the aurora1 oval location, near - 60” geomagnetic latitude. The enhanced counts seen between the latitudes, MLAT = 5O-61” and centered on L x 3.5 (see Fig. 1, two lower traces) are recorded during the satellite traversal of the low altitude extension of the outer radiation zone; these arise from the penetrating energetic electrons referred to earlier. Note similar observations by the low altitude polar-orbiting satellite Injun 5 at latitude A = 62” (Frank and Ackerson, 1971) have been interpreted as the background response of the analyzers to energetic electrons trapped in the outer radiation zone. The precise threshold of theenergy ofthe penetrating radiation is somewhat uncertain; however, since the count-rate profiles due to the radiation belts are the same for all the high energy channels, we can reasonably conclude that the energy threshold of the radiation is at least 20 keV, since it is insensitive to energy discrimination by the detector. The absence of these peaks in the top plot (from low energy detector covering 50 eV-1 keV) of Fig. 1 is attributed to better shielding from its physical location in the satellite, and also the smaller cross-section of the spiratron used in the low energy detector.

et ul.

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FE. 1. EXAMPLE OF THE DMSP-F2 ELECTRON COUNT-RATE PKOFILEINTHEENERGY BINSOF 267,235O AND 20200eV DURING ITS PASS OVER THE POLAR REGION OF THE SOUTHERN HEMISPHERE. The regions marked A, B and C refer to the polar cap, aurora1 oval and the radiation belt respectively. The satellite location in the magnetic local time (M.L.T.) and the corrected magnetic latitude are shown at the top.

With data from both DMSP satellites we observe the high energy electron flux in the low-altitude “horns” or protrusions of the outer radiation belt in the evening and morning sectors of both hemispheres. The multiple observations for each orbit of - 101 min provides high temporal resolution and thus are particularly suitable for study of the temporal response of the high energy electron flux in the radiation belt during isolated periods ofmagnetospheric storms. Since both satellites are at low altitudes and their orbits have a westward drift in the geographic coordinate system, we must distinguish and separate the truly temporal electron flux variations from the geographic longitudinal variations which arise from the geomagnetic anomalies, and the consequent effect on the mirror points of the trapped electrons. Similar observations have been reported by Imhof and Smith (1965). Such a separation can be made by suitable selection criteria (listed below) in the data analysis. (a)Theisolatedsubstormperiodmust bepreceded by a sufficient interval of geomagnetically quiet conditions, so that the two satellites could map out the typical quiet-time (i.e. the background) electron Rux as

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Figure 3 gives typical examples of geographic longitudinal profiles of the radiation flux at I_.= 3.5 observed by the DMSP-FZ and DMSP-F3 satellites during geomagnetically quiet periods. A similar dependence of the inner radiation belt electron flux on geographic longitude, at 1, = 1.2-l .7 has been made by lmhofand Smith(1965j,andits theoretical treatmentat 1. = 1.25 by Roederer and Welch (1966) should be mentioned. Our observations here refer to L = 3.5 + 0.2 from both hemispheres. The variations of the radiation due to the high energy electron flux in the outer radiation belt detected in the Northern (Panels E and F) and the Southern (Panels A, 13and C) Hemispheres are shown in Fig. 3 as a function of the geographic longitude. These proliles (barring those on Panels A and E) are also used as the standard quiet-time curves [or alternatively called, calibration curves] to obtain A@(r), the net change ofelectron flux at L = 3.5kO.2 during the selected cases of isolated substorms. Each point represents the high energy electron flux in the satellite’s orbital plane at the altitude of 850 & 20 km averaged over a time interval of 20 s, centered on the count-rate peak (in Zone C. Fig. 1) during each polar region crossing.

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a function of geographic longitude. In practice. about h 16consecutive hours of quiescence (average value of AE over entire period. 5 50 nT) was necessary for the two satellites to accomplish this. (b) Again, a geomagnetically quiet period of a few hours after the geomagnetic perturbation was required as a criterion, so that the relaxation time could be estimated. During 1978, we found only five instances satisfying the above criteria (see Fig. 2), three of which are discussed here. The net high energy llux response A@(t) during the geomagnetically disturbed period for each event was obtained from the following :

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The background electron flux measured on two occasions (September and October 1978) at L = 3.5f 0.2 in the Northern Hemisphere is given in Panels E and F of Fig. 3 ; the latter is also used for calibration. The points represent count-rates averaged over an interval of k 10 s centered on the time of count-rate peaks ofthe radiation belt registered by the satellite (see Fig. 1). Proceeding eastward from 180” E to 300” E, we observe the similarity between the longitudinal magnetic field strength profile at 900 km altitude (Panel G) and the longitudinal count-rate profile. Note the sharp drop in the count-rate to the background level beyond 300” longitude, as seen in Panels E and F although the scalar magnetic field (B) still increases. This disagreement is commented upon later.

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LONGITUDE (degrees) FIG. 3b. PLOTOF HIGHENERGYELECTRONCOUNT-RATE(PANT:LS A, B AND C) vs GEOQRAYHIC LONGITUDE.IN THE SOUTHERN HEMISPHERE. Panel D shows a plot of the scalar magnetic field, 5, as a function of geographic longitude in the Southern Hemisphere at an altitude of 900 km.

Note the systematic variation in the measured flux profiles in Panels A, B and C, of Fig. 3 ; the count-rates between 80” E and 220’ E have all dropped to a low “background” level; subsequently, East of the 220” E longitude, they gradually increase to values comparable to those seen at O-20” E longitude in all the three panels. Thus, the maximum values are observed in the vicinity of the South Atlantic anomaly: the magnitudes of these maxima vary from one panel to another: for example. in Panel C count-rate peak at - 360” is larger by a factor of 6 than the corresponding one in Panel A. Our observations indicate an empirical dependency between these peak values and the ring current index, D,,, averaged over the interval of the particular observation; for Panels A, B, and C these are - 5, -- 15, and - 35 nT respectively. The longitudinal variation of the scalar magnetic field, B, at L = 3.5 to.2 in the Southern Hemisphere is shown in Panel D in Fig. 3. From the satellite’s geographical position (altitude, latitude and longitude) the scalar H values can be obtained from available standard sources of isocontours (e.g., Venkatesan, 1965a,b ; Stassinopoulos, 1970). The count-rate profiles ofcount-ratesduringthegcom:~gneticallyquiet periods (Panels A, B and C) and that of the scalar magnetrc field (Panel D) in Fig. 3 arc similar.

Intense

event of 6, 7,8 September 1978 For this event, the quiet-time profile was obtained during 18 : 00 U.T., 6 September to 18 : 00 U.T., 7 September. Both AE and D,, indices show the onset of the intense magnetic disturbance at -23:00 U.T., 7 September (Fig. 4). The response of A@(t) to this disturbance in the morning and the evening sectors in both the Northern and Southern Hemispheres is shown in Fig. 4. After 23 : 00 U.T., note specifically the increase in the evening sector in both hemispheres; this enhancement occurs within 1 h and reaches a peak in both hemispheres simultaneously at 04: 00 U.T., 8 September, coinciding with the time of maximum in both AE and II,, profiles. During the recovery phase 04:OO lJ.T.-09:OO U.T., the A@(t) profile reveals decrease following in general the AE profile. The morning sector count-rate profiles shown in Fig. 4 reveal a different behavior. Fluctuations with countrates frequently dropping to the quiet-time or calibration level are seen in the Southern Hemisphere; even lesser response is seen in the Northern Hemisphere with count-rate hardly rising above the background.

The quiet-time period is selected between two moderate substorm periods discussed in this section. The main substorm period (see AE and D,, in Fig. 5) commenced at 04 : 00 U.T., 17 September; the duration of this isolated substorm being 6 h. During period U.T., 17 to -03:OO 15:00 U.T., i6 September September the count-rate fluctuates occasionally dropping below the quiet- time level. At - 04 : 00 U.T. enhancement of the there is R sudden modcrate radiation flux in both sectors and hemispheres (not contparable to the previous event). After this increase.

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middle. Plots obtained from the Southern Hemisphere from both the morning and evening sectors are presented at the bottom.

intensity fluctuations are seen ; however, the count-rate is always above the calibration level over -6-7 h, comparable to the duration of AE enhancement. The count-rate Inrofiles dron -r to the calibration level in both

The format ofpresentationls thesameasin Fig.J.Thetop two curves refer to the Northern Hemisphere, while the bottom two refer to the Southern Hemisphere.

sectors of both hemispheres between 12 : 00 U.T. and 14:OO U.T. In addition, after 13: 00 U.T. in the Southern Hemisphere evening sector the electron flux continues to drop well below the quiet-time value. Weah

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Both satellites detect radiation flux enhancements (seeFig.6); thefluxfalls back to thecalibrationleveljust before the second substorm begins. Both AE and D,, indicate that the second and largest of the three geomagnetic perturbations occurs at 03:OO U.T., 21 September; this lasts for 5 h. The onset of the A@(t) enhancement is observed at or shortly after 03 : 00 U.T. in both sectors of the two hemispheres, and reaches a maximum within 2-3 h; the count-rate enhancement spans 179 h. The onset of the third and the smallest of the magnetic perturbations occurs at 16: 00 U.T., 21 September and lasts - 2 h ; AE reaches a maximum value of 200 nT. The dotted lines in Fig. 6 indicate the absence of data between the maximum value of A@(t),,, and the recovery time. The count-rate returns to the calibration level just before 24: 00 U.T. giving a net duration of enhancement A@(t) of - 8-9 h.

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The format of presentation is the same as in Fig. 4. The top two curves refer to the Northern Hemisphere, while the bottom two refer to the Southern Hemisphere.

long interval of low geomagnetic activity. The calibration interval for this event is from 20 : 00 U.T., 18 September to 02: 00 U.T., 20 September. The first perturbation starts at 20:00 U.T., 20 September (see sudden increase in AE). This substorm lasts - 3 h with peak AE value of only 320 nT.

AND CONCLUSIONS

The dynamics of the energetic electron flux during isolated periods of magnetospheric substorms have been examined here by monitoring the low altitude horns ofthe radiation belts at L z 3.5 using DMSP-F2 and DMSP-F3 satellites at - 850 km. We have studied the immediate changes occurring in the outer radiation belts during isolated substorms; such a study needs data with relatively high temporal resolution. These observations have been made near the dawn-dusk meridian in both hemispheres. We deal with the variation of the high energy electron flux during isolated substorm activity. During the magnetically quiet-time (calibration) intervals, the electron flux at L = 3.5 at the altitude of the satellite’s low polar orbit is observed to be strongly dependant on the local magnetic field anomalies. Previous observations have reported similar flux modulation at such L values, for example, Forbush et al. (1962) and at lower L-values, Imhof and Smith (1965), Vernov et al. (1966). It is well known that the count-rate increase measured by the low altitude satellites in the vicinity of the South Atlantic anomaly is due to the lowering of the mirror points ofthe trapped particles which then results in an increase of the particle flux at the altitude of the satellite orbit. Such a possibility has also been pointed out by Forbush et al. (1962) from an analysis of data from Explorer VII. We observe this effect in all the three Panels A, B and C of Fig. 3 between -220” E and 100” E. In the region between - 100” E and 220” E, the satellite’s count-rates are only at the background level. This shows that during the calibration periods the satellite does not register any electrons corresponding

Low altitude observations of the energetic electrons to the scalar geomagnetic field at the mirror point, B, being 2 0.40 Gauss. The onset of the enhancement of the electron flux which we associate with the substorm activity occurs at about the same time as the onset of the geomagnetic perturbation, as indicated by the indices of AE and D,,. The enhanced radiation flux also shows, in most of the cases studied, a one to one correlation with the AE index. The timing and duration of the Q increase in the electron population coincide with those of the AE index. Panels E and Fin Fig. 3 reveal that the modulation of the electron flux as a function of longitude is not as prominent in the Northern Hemisphere as in the Southern, since the magnitude of the magnetic anomalies is not just as pronounced. For example, the scalar magnetic field B varies from 0.26 to 0.42 Gauss in the Southern, and only from 0.36 to 0.40 Gauss in the Northern Hemisphere. Due to this asymmetric magnetic field strength variation, at - 320” E longitude in theNorthern Hemisphere, we are able to observe the so-called “wind-shield viper” effect (Roederer, 1966) seen as a sharp drop in the count-rate. As previously mentioned, during the quiet-time (calibration) periods, the satellites are observing only electrons mirroring at scalar magnetic field, B 5 0.40 Gauss and therefore as the magnetic field at 100 km in the Southern Hemisphere reaches values of B - 0.40 Gauss (- 320”) the electrons will begin to precipitate (see Fig. 3, Panel H). This will appear as adecrease in the count-rate of the profile of the Northern Hemisphere’s longitudinal electron flux. Note our adoptions of 100 km as a reasonable altitude for the particle precipitation. A few examples of the changes in the longitudinal electron flux profiles (the squares) as observed by the two DMSP satellites during some of the isolated events are shown in Fig. 7 for both hemisphere crossings during the two substorm periods of 6,7, 8 September and 19,20,21 September 1978. The solid dots are the calibration curves representing the quiet time electron flux vs longitude profiles. It is evident that during some of the crossings of the low altitude extension of the radiation belts, the satellites encountered dramatic changes with respect to the quiet time count-rate profiles. The important deviation from the quiet-time electron flux profile is the appearance of counts in the longitudinal region between 100” E and 220” E in the Southern Hemisphere. Both DMSP satellites under the magnetically quiet conditions were measuring only the background in all cases studied. This indicates that the satellite observations between longitudes 100” E and 220” E are now detecting an electron population which is either in the loss cone or in

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the drift loss cone. The stably-trapped electron population will have their mirror points above the satellite’s orbital plane. These observed changes in the distribution of mirror points and therefore also in pitch angle distribution during the geomagnetically disturbed periods seem to relate to the equatorial observations made by Lyons and Williams (1975) viz., the occurrence of a distortion of the pitch angle distribution with respect to pre-storm conditions during the injection interval. Temporal changes in the electron flux during isolated substorms are shown in Figs. 4, 5 and 6 and can be regarded as the additional electron population riding on top of the pre-storm one. This electron population represented by A@(t), fills the loss and drift loss cone during the expansive phase of the substorm and seems to be modulated only by the intensity of the substorm.

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On the other hand, the quiet-time electron flux as measured by the DMSP spacecraft represents stably trapped electron population and seems to be modulated only by the magnetic field. The density of this quiet-time population is related to previous geomagnetic activity and as mentioned earlier, quantitatively related with a value of D,, averaged over the period of observation. The decay time for this population ofelectrons is much longer than that for the former one. Studies have shown (e.g. Lyons and Williams, 197.5; Withams and Smith, 1965 ; Williams et nl., 1968) that electron density reaches the pre-storm value in the order of a few days. For example, Williams and Smith (1965) have reported a maximum lifetime for these electrons in theenergy intervals E > 280 keV and E > 1.2 meV of 4-5 days and 6-7 days at 3 < L G 5 respectively. In addition, since this longitudinal modulation by the magnetic field is the same for all cases studied, it appears that there exist a quiet time equilibrium pitch angle distribution (see also Lyons and Williams, 1975) for these stably-trapped particles. It is relevant to point out that Fillius and McIlwain (1967) have pointed out the adiabatic betatron acceleration of a trapped particle by a geomagnetic storm. In summary, this study shows that monitoring of energetic electrons during isolated substorms can be carried out in this manner. It is appropriate to note that we have made use of observations with a detector not originally designed for such a study. Acknowledgetnenrs-This research was supported by the U.S. Air Force Office of Scientific Research Grant 84-0049 and the Atmospheric Science Division of the National Science Foundation Grant ATM-8315041 to The Johns Hookins University Applied Physics Laboratory and Grant No. 691565 to D. Venkatesan from the National Science and Engineering Research Council, Ottawa, Canada. REFERENCES Carpenter, D. L., Park, C. G., Arms, J. F. and Williams, D. J. (i971) Position of the plasmapause during a stormtime increase in traooed energetic (E > 280 keV) electrons. J. ~eop~~ls. Res. 7x 4669. Craven,J. D.(1966)Temporal variationsofeiectronintensities at low altitudes in the outer radiation zone as observed with satellite Injun 3. J. grophys. Rex 71, 5643. Fillius. W. R. and McIlwain. C. E. (1967) Adiabatic betatron acceleration by a geomagnetic storm. J. geophys. Res. 72,40 11. Forbush, S. E., Venkatesan, D. and McIlwain, C. E. (1961) Intensity variations in outer Van Allen radiation belt. J. geophys. Res. 66,227s. Forbush, S. E., Pizzella, G. and Venkatesan, D. (1962) The

morphology and temporal variations of the Van Allen radiation belt, October 1959 to December 1960. J. geophys. Res. 67, 365 1. Frank, L. A., Van Allen, J. A. and Hills, H. K. (1964) A study of charged particles in the Earth’s outer radiation zone with Explorer 14. J. yeophys. Res. 69,2171. Frank, L. A. (1965) Inward radial diffusion of electrons of greater than 1.6 million electron volts in the outer radiation zone. J. geophys. Res. 70,3533. Frank, L. A. and Ackerson, K. L. (1971) Observations of charged particle precipitation into the aurora1 zone. J. geophys. Res. 76,36 12. Hardy, D. A,. Gussenhoven, M. S. and Huber, A. (1979) The precipitationelectrondetector(SSJ/3)forbiock SDjHight a5 DMSP satellites: Calibration and data uresentation. Ren. AFGL-RF-79-0210, Hanscom Air Force Base, Bedford, Mass. Hardy, D. A., Burke, W. J. and Gussenhoven, M. S. (1982) DMSPoptical andelectron measurements in the vicinity of polar cap arcs. J. ~eo~~ys. Res. 87,2413. Imhof, W. L. and Smith. R. V. (1965) Loneitudinal variations ofhigh energy electrons at low altitudes~J. geophys. Rex 70, 569. Lanzerotti, L. J., Roberts, C. S. and Brown, W. L. (1967) Temporal variations in the electron flux at synchronous altitudes. J. geophys. Res. 72, 5893. Lyons, L. R. and Williams, D. J. (1975) The storm and poststorm evolution of energetic (35-560 keV) radiation belt electron distribution. J. ~eophys. Res. 80, 3985. Makita. K.. Menu. C-I. and Akasofu. S-I. (1983) The shift of the aurora1 electron precipitation boundaries in the dawndusk sector in association with geomagnetic activity and interplanetary magnetic field. J. geophys. Rex 88, 7967. McIlwain, C. E. (1961) Coordinates for mapping the distribution of magneiicaliy trapped particles. J. geophys. Res. 66, 368 1. Roederer, J. G. (1966) Southern hemisphere anomalies. Space Res. 6, 117. Roederer, J. G. and Welch, J. A. (1966) Theoretical description of trapped electron diffusion in the South American Anomaly, Space Res. 6, 148. Stassinopoulos,E.G.( 1970) WoridMapsofContoursofBand L and Flux Contours. NASA, SP-3054. Vampola, A. L., Kroons, H. C. and McPherson, D. A. (197f) Outer-zoneelectron precipitation. J. geophys. Res. 76,7609. Venkatesan, Bharathi (1965a) Iso-contours of magnetic shell parameters B and L. J. geophys. Res. 70, 3771. Venkatesan, Bharathi (1965b) Graphic charts and altitude plots for selected values of geomagnetic shell parameters B and L. University of Iowa Research Report 65-8. Vernov, S. N.,Nesterov, V. E., Savenko, 1. A., Shavin, P. I, and Sharvina, K. N. (1966) Discovery and investigation of the Brazil anomaly by spaceships and the Cosmos series of satellites. Space Rex 6, 165. Williams, D. J. and Smith, A. M. (1965) Daytime trapped electron intensities at high latitudes at 1100 kilometers. .J. geophys. Res. 70, 54 1. Williams, D. J., Arens, J. F. and Lanzerotti, I,. J. (1968) Observations of trapped electrons at low and high altitudes. J. yeophys. Res. 73,5673. _