Review of CRRES ring current observations

Adv. Space Res. Vol. 24 No. 3, pp. 3 1l-320, 1997 8 1997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/97 $17.00 + 0.00 PII: SO273-1177(97)00673-X

REVIEW OF CRRES RING CURRENT OBSERVATIONS R. H. W. Friedel and A. Korth Mux-Plunck-lnstitutfiir Aeronomie. Mux-Planck-Str. I, Kutlmburg-Linduu, Germmy

D-37191

Abstract

MeV proton bursts associated with substorms and Abel et al. [1994] examined the inner radiation belt electrons The distribution of electrons and ions with L-value (L 5 using six satellites from 1966 to 1991. Friedel and Korth 8) as measured by two instruments on board CRRES [1995] reported on the outer zone variablity as observed over the whole mission lifetime (07.1990-10.1991) is preby the whole CRRES mission: it is these findings which sented. CRRRS is well suited for examining the particle are extended in this paper. budget (electrons 21 - 1633 keV, ions 37 - 3200 keV) over long periods. It shows the outer radiation belt to Here we present electron and ion intensity variations be very dynamic, both in position and in strength. Sub- determined with the energetic particle spectrometer onstantial enhancements of electron and ion flux on short board the Combined Release and Radiation Effects Sateltimescales are seen, and are strongly related to Kp and lite (CRRES) over the whole mission lifetime and also the ring current as measured by DST. The position of the for some selected periods of interest. To understand the inner boundary of the outer radiation belt is variable but state of the magnetosphere at any one given time one has fixed for a given period and level of activity, while the to take into account its past history. The outer radiation strength of this belt decays over a period of a month days belt is a region in constant transition, either being injected after initiation. The inner radiation belt is very stable and with new populations or in a state of decay, with provirtually unaffected by outside disturbances. Very intense longed periods of extremely low population levels. Poperiods of activity (such as the March 1991 storm) can sition and width of the slot region varies dynamically lead to the creation of an additional, high-energy belt in from being extremely wide (up to 2 Earth radii) to being the slot region, which persists over a period of years, and temporarily filled - a phenomenon also observed by Baker also a new type of high energy ion gap near L = 2.5 et al. [1994] using SAMPEX high-energy electron data. which lasts for two months. Detailed examinations of These authors also found a connection between strong sostorm-time and substorm-time ring current behavior show lar wind flows and flux enhancements, a result which is that only for the former can a good agreement with DST corroborated by this study for the adjoining lower energy be found. Further, the use and limitations of the order- range. ing parameter L is discussed, which is shown to be very The previously reported formation of an extra radiation sensitive to the model used to calculate it. belt by the March 1991 storm [Korth and KmpoZu, 19941 0 1997 COSPAR. Publishedby Elsevier Science Ltd. is shown clearly in this data. We also report on a previously undetected effect which was only extracted from the Introduction data after considerable re-processing: the formation of a Most studies of energetic particles in the inner magne- high energy ion slot after the March 1991 storm, tosphere have tended to examine phenomena at ever increasing time resolution. Studies of long-term phenom- Iwo disturbed periods are examined in detail to show ena have been rare. William [ 19661found a 27-day peri- the different behavior of the ring current region during a odicity in electron fluxes over 280 keV in the outer xone “classic” storm period and a period of enhanced substorm (L 2 3.5). using measurements from the 1963 38c satel- activity. Here we can show that the classic interpretation lite, as did Gussenhoven et al. [ 19871 using electrons of D,ql- as a ring current indicator does not hold for this above 2.5 MeV measured on-board the Defense Mete- case of a small variation in DST. orological Satellite Program (DMSP) spacecraft ( 1984, 1985). Baker et al. [1979] concentrated on E > 0.:1

R. H. W. Frwlel and A. Korth

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Instrumentation Data for this study is from the EPAS (Electron-Proton Angle Spectrometer) instrument, which is identical to the instrument flown on GEOS-2 (a European geostationary satellite), and was thus designed for the geospace environment at 6.6 RE. EPAS measures full electron and ion pitch-angle distributions in 14 channels for electrons in 12 channels for ions as shown in Table 1.

FIELDCLANP

Table 1: EPAS energy channels Channel

Electrons (keV)

Ions (keV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

21.0-31.5 3 1s-40.0 40.0-49.5 49.5-59.0 59.0-69.0 69.0-81.0 8 1.O-94.5 94.5-l 12.5 112.0-129.5 129.5-151.0 151.0-177.5 177.5-208.0 208.0-242.5 242.5-285.0

31-54 54-69 69-85 85-l 13 113-147 147-193 193-254 254-335 335-447 447-602 602-805 805-3200 ... ...

Figure 1: Schematic cross section through the EPAS instrument. front detector of the telescope directly through the aperture. The front detectors are 100~ silicon detectors with a 300~ thick anti-coincidence or back detector. Particles penetrating the back detector will be disregarded by the electronics. Thus for ions clean spectra are almost always obtained above L> 3.5, and even below L-3.5 the effective geometric factor for background particles which enter the front detectors but miss the anticoincidence detectors is small, so that a contamination of the spectrum can only be expected during periods of very high MeV electron fluxes. To extend data coverage to higher electron energy data of the MEA instrument [Kzmpdu et al., 1992) was also used.

A schematic of the instrument is given in Figure 1. A full description of the instrument can be found in K&J et al. Mission and data description [ 19921. CRRES was launched on 25 July, 1990 into an elliptical orbit at an inclination of 18.1°. CRRES had a period of Electron detectors are housed in aluminum (shielding 9h5Om and covered the L-space from 1.2 5 L 5 8. The electrons up to 1 MeV) and have no anti-coincidence deorbit apogee precessed from the pre-noon sector at launch tectors. Background effects from penetrating particles to the early dusk sector at the end of the mission. Data show up in the channels around 100 keV, which correcoverage is shown in Figure 2. sponds to the energy deposited by minimum ionising particles traversing the 300~ silicon electron detector with vertical incidence. Edge effects and oblique traverses lead to a spread of the background peak in the energy spectmm around 100 keV. This background peak is only seen during very disturbed times, while for the large March 24, 1991 storm all channels are saturated. In quiet times (periods with low fluxes for EK > 1 MeV) these detectors deliver clean spectra. For L-values below 3.5 background correction becomes necessary, as the background geometric factor approaches 47r and the plots shown here are no longer quantitative in this region. This does not affect the conclusions in this paper, which are of a qualitative nature. The ion detectors consist of two-element telescopes. Ions above 32 keV and electrons above 800 keV can reach the

The data presented here are L-sorted plots versus time for the whole lifetime of the CRRES mission (for example, Figure 3). The color code gives the intensity of the particle flux which is automatically adjusted to give the best dynamic range for the data. For each half-orbit (4h55m) the flux was averaged over bins 0.2 L wide and presents one vertical stripe at the time of apogee/perigee on the plots. Even though each half orbit covers a range of different MLT, and consists of both outbound and inbound passes, no dependence on MLT or pass direction was found. The 50-orbit data (Dec.9O/Jan.91) gap is due to power conservation on the spacecraft while in the Earth’s shadow.

CXRES Ring C‘urrent Obsewations

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model). 2. Flux values where transformed to their equivalent values at the equator. All data was processed for au equatorial pitch angle value of 45O, since CRRES is almost always above the minoring point for these particles. After this process the data is c~sidembly smoothed and orbit to orbit comparisons become possible, Further, weaker long-term features which are otberwise lost in the data can be seen. However, since the L-values used ate from the Olson-pfitzer quiet time model of 1977, this leads to distortions in the plots at times when the magnetosphere is strongly compressed, leading to an overestimation of the L-value. Panels l-4 of Figure 3 give a comprehensive view of ion and electron flux (for peak energies around 60 and 300 keV) sorted according to L-value and day of the year (DGY). Each vertical stripe starts below the stable trapping region, traverses the inner zone (m~imum flux LN 1.5), the slot region, and then the outer zone (L> 3) until apogee. The L-value at apogee changes slightly, depending on the magnetic latitude, and can reach up to L-8 at times. In the lower three panels of Figure 3 the occurrence of SSC’s, hourly &T and daily Kp values are shown.

Overall Dynamics

Figure 2: Orbital coverege of CRRFS. The top panel shows actual orbit occupancy in hours, the bottom actual data hours for the EPAS instrument. Post-processing the data There is, however, a dependence on magnetic latitude. CRRES covered magnetic latitudes up to f25O, so for any given pitch angle at the satellite the actual equatorial pitch angle may be different from orbit to orbit for the same Lvalue range. This leads to a smearing of the data. Further, the originaI L-value for the data is from the Olson-Pfitzer quiet time model of 1977, which is not a pitch-angle dependent model - and does not take account of the so-called L-shell splitting in a non-dipole magnetic field. At L = 6 the difference between 15’ and 90° pitch angle particles can be up to 0.5 L! This further smears out the data. Great care was taken to filter out these effects. This included the following steps: 1. Use of the comprehensive magnetic iield model package “UNIRAD” [Lemaire ef al., 19!35] to calculate pitch angle dependant L-values for the Olson-Pfitzer (quiet)

Severaloutstanding features can be read off Figure 3 directly: Numerous sharp flux increases are observed in the electron and ions in the L-range between 3 and 6 throughout the whole period. The enh~cements occur during one or two half-orbits and are correlated with a strong decrease of DST. The decay period of electrons energized in the outer zone is less than 30 days, decaying first at high L, presumably by loss-scattering mechanisms and adiabatic diffusion (panels 1 and 2). This effect is also seen by SAMPEX for MeV electrons [Baker et al., 19941. Electrons show similar variability over L for the whole energy range sampled with EPAS. For ions, however, the lifetime of particles in the outer zone increases markedly with energy and at 300 keV an almost continuous belt is formed with all the shorter term (less than 30 days) variablity filtered out (panel 3). The width of the electron slot region depends on the level of disturbance and tends to be broader during quiet times. In very quiet periods such as from Dec.90 - Mar.91 the outer radiation belt all but disappears, with particles near the loss cone almost totally being last. The inner border of the outer radiation belt shows clear steep cut-offs, with a different sharp cut-off in L for different events: The first few events in panel 2 reach down to L-3.2, 2.7, 3.6, 3.4 respectively. When large events occur on shorter time scales than one

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H.W. Fnedel and A. Kortb

B % I.4

0 70 F E

Figure 3: L-sorted data for all 1067 CRRES orbits from July 90 to August 91. From top to bottom: 45” equatorial pitch angle electrons and ions near 60 keV and near 300 keV (notebelow L-3.5 flux is dominated by omni-directional background flux), SSC’s, Kp and DST values. Energies given are peak energies of the selected channel. month, remnants of the previous injection or acceleration (at a different L-value) can still exist, leading to a multiple outer radiation belt structure (panels I and 2 around DOY 260, 1990). The inner radiation belt is very stable, shows a sharp outer boundary and little dynamics.

Extra radiation belts or slots

The large March 24, 1991 storm has a very clear signature in this plot: A new radiation belt is created after the onset of this storm in the slot region (between the inner and outer radiation belts, panels l-4). Large fluxes of relativistic electrons (up to 25 MeV) were accelerated (or injected) by the shock event flrampda and Korrh, 19921,

CRRES Ring C’urrent Observations

Figure 4: The top two ion energy channels for the period of the intense 1991 storm. Note the appearance of a new radation belt slot after the storm around L = 2.6 in the top channel only. which are seen by EPAS as background particles. This extra belt exhibits cross-L motion of the flux peak to smaller L from L-2.4 to L-2.05 over a period of half a year (up to the end of the CRRES lifetime) [Korth and Vumpola, 19941. This extra radiation belt continues to be observed by other spacecraft for more than one year after the termination of the CRRES mission (e.g. on AKEBONO, AS. Yukimato, personal communication, 1994; SAMPEX, Looper et al. [ 19941) A new effect is show in Figure 4. At the onset of the March 24, 1991 storm particles are lost from the outer edge of the radiation belt down to L = 2.5. While most particles recover after a period of a few days a region around L = 2.5 for the highest ion channel of EPAS (see Table 1) remains depressed for much longer: This region re-fills only after a period of around two months. This effect only became detectable after re-processing the data as described above.

Intense Storm An isolated intense (-140 nT) magnetic storm is represented in Figure 5 over a time period of 13.5 days. The storm is caused by an SSC (triangle in panel 9) which happened 15 hours before the main storm onset. At storm onset the solar wind velocity increased further to 530

kmls and the interplanetary magnetic field component B, turned southward to a minimum value of -16 nT (panel 8). After the B, component turned northward the magnetosphere recovered. The two lower energy channels in Figure 5 represent that part of the ring current which is mainly carried by ions and electrons in the energy range 30-200 keV. We identify here the maximum of the outer radiation belt in this energy range as being roughly the ‘position’ of the ring current. The pre-storm maximum is at L = 4.5 to 5 for these energies. During the main phase of the intense storm the ring current intensifies and the maximum of the ring current is displaced earthward to L = 3.5, however, the outer radiation belt regions (L > 5) are kept populated. The slot region narrows and widens again later. The recovery time of the slot increases with increasing energy. After about 40 hours the maximum of the ring current has moved outward from L = 3.5 to L = 4.6 for 77-keV ions (close to pre-storm conditions) to L = 4.0 for 99-keV, and to L = 3.8 for 170-keV ions. For 170 keV the recovery to pre-storm level can take up to 30 days (not shown here). Electrons above a few IO0 keV are lost at all L-values during the onset of the storm (panels 2 and 3) and return after about I2 hours. This is clearly seen in the 602 keV channel (panel3) which shows a large increase after about half a day and has a behavior similar

K. H. W. Fnedel and A.

:

Keith

1285

Ih valuer

3.1

Figure 5: L-sorted data for the 27/28 of November 1990 intense magnetic storm. Total time period is about 2 weeks. The panets show from top to bottom: Electron fluxes for three peak energies (45,264, and 602 keV), ion fluxes for three peak energies (77,99, and 703 keV), AE-index, inte~l~e~ magnetic field B,, L)sT. to the ring current ions. The ring current decay mirrors to 11, 1990 (between the dashed blue vertical lines). No the “storm decay” as measured by DST. This storm is SSCs occur during the above period. This small magnetic storm is triggered by substorms (LIST--25 nT). In reported on in more detail in Km& and Fried4 [ 19961. this event subsets are continously occurring for more Small Storm than four days due to fluctuations in the interphutetary Figure 6 shows the same ~~gement of data as Figure ma~eti~ field com~nent Bz. The DST slowiy decreases 5. We wiii concentrate on the event from November 6

CXRES

4.0

4.3

Ring

317

Current Observatioaa

3.8

3.9

3.6

4.0

MLT

(apogee)

Figure 6: L-sorted data for a small storm triggered by continuous substorm activity over about 80 hours. The total period amounts to ten days. The arrangement of data is the same as in Figure 3. over this period and increases abruptly after the substorm activity stops. The AE-index fluctuations are reflected by the injections of low-energy electrons (first panel, 45 keV) at L-values between 5 and 7. The higher energy channels (panels 2 and 3) show a flux decay and the outer belt shrinks during the period of substorm activity. After northward turning of the &-component the electron flux

intensifies abruptly over a large L-region. The ring current (paneis 4 and 5) decreases during the period of the substorm activity and increases after the substorm activity has ceased and the &-component has turned northward. The energetic ions (panel 6, 703 keV) are not effected at all during this event.

31X

R. H. W. Friedel and A. Korth

Discussion and Comparison

event.

There is clear evidence for an outer zone acceleration mechanism operating in the L-range from 3 to 6, that this is not an injection process from the outer magnetosphere can be seen from Figure 3 which shows a clear upper limit to the flux enhancements for all but the enhancement of the March 24,199 1 storm. This upper limit is also clearly shown by Baker et al. I19941 for MeV electrons which were sampled by SAMPEX out to L-l 1 and also show this cut-off between L-6 and 7. It Seems that the hypothesis of the Earth’s magnetosphere being an effective accelerator [Baker et al., 19941 also applies to electrons and ions in the energy range examined measured by EPAS. The flux increases are not correlated with substorms which occur on much shorter (1 to 5 hours) timescales.

The loss mechanisms and their lifetimes for ring current particles can be explained by charge exchange [Cornwall, 19721. However, charge exchange is not the only dissipation mechanism which operates on the ring current. Cornwall ef al. [1971] predicted resonant interaction of ring current ions with plasma waves. Electrons below energies of 200 keV intensify at storm onset and are injected to low L-values into the slot region. After a few days, during the recovery phase, the slot region develops again and broadens to pre-storm size in the same time frame as the ions. First indications of the slot region are realized after about one day. Electrons above 200 keV disappear during the onset of magnetic storms in the outer magnetosphere and return during the recovery phase. This was also seen by SAMPEX [Baker et al., 19941. Some possible mechanisms are listed in Li et al. [1996]. The reason for the initial dropout of energetic electrons is possibly understood better: Model calculations by Bourdarie et al. [ 19961 show that during the onsets of magnetic storms electrons drift further out on the dayside and get lost at the magnetopause. A first indication of a flux decrease is already obsewed at the occurrence of the SSC well before the onset of the magnetic storm (Figure 5). The subsequent recovery takes about a day and can be caused by substorm injections, and/or radial diffusion, or radial transport due to sudden impulses [Kztnpola and Korfh, 19921. Baker et al. [ 19941 spoke of the magnetosphere as a “cosmic electron accelerator of substantial strength and efficiency”. However, the exact mechanism by which this occurs is still a matter of debate [Li et al., 19961.

These data show that the term “outer radiation belt” is a gross over-simplification of a region of the magnetosphere which is inherently dynamic for much of the energy range of both electrons and ions examined here (only for ion energies approaching 300 keV is a more stable, continuous belt formed). This region exhibits flux-variations of up to three orders of magnitude within a few half-orbits, but also shows prolonged periods of very high or low flux levels. These flux enhancements also show a large variation in L-value, both in initial position and lifetime. It becomes clear that to understand the state of the energetic particle population in the outer zone at any one given moment in time requires detailed knowledge of the past history of this region, normally for periods of up to 30 days but also for periods in excess of one year for extreme events such as the March 24,199 1 storm. The mechanism responsible for the radiation belt gap at high ion energies The Nov 6-11 storm presented here would normally be as reported here is not understood as yet. classed as a small storm according to DST, Here, howWe have presented data from two very different storm ever, the decrease of DST to -30 nT is gradual and the time dynamics of particles in the inner magnetosphere. mechanism responsible for that is quite different. The The first storm presented in Figure 5 is an intense storm main ring current ions actually show a decrease in intenof more or less classical behavior. There is a SSC which sity and a narrowing of width. This is also seen, to a lesser compresses the magnetosphere and causes some particles extent, in the electron data. So the decrease in DST canto be lost before a southward turning of the IMF trignot be linked to a increase in the global ring current. Degers the main phase. The ring current is well represented tailed examinations of the panels showing the DST and by ions of 77 and 99 keV (the same behavior is shown AE indices show that each step-wise decrease in Dsr is for the whole range from 30-206 keV, which according correlated to a peak in AE, which in turn is well correto Williams 119871accounts for the bulk of the ring curlated to the substorm injections seen in the top panel elecrent density) and it is thus no surprise that DST follows tron data. The recovery after the southward fluctuating the time variation of the overall density of these particles IMF turns north is quite abrupt and too fast for normal quite closely. Apart from the time just around onset there ring current decay. is little substorm activity as monitored by AE, moreover the top panels of Figure 5 shows only a few isolated sub- This suggests that here the decrease in DST is entirely storm injections well into the recovery phase. Substorm due to substorm effects. This effect must be large enough effects on the DST index are likely to be small for this locally (as substorm are events localized over a few night-

CRRES Ring Current Observations

side MLT) to offset the global decrease of ring current particles which would normally lead to an increase in Dm. One suggestion is that the substorm current systems that are set up, such as the substorm current wedge, are responsible for this behavior: Formation of the substorm current wedge leads to the ionospheric elechojets which are reflected in AE, closure of the current in the magnetosphere leads to a partial enhancement of the ring current and it is this enh~~ment that is regected in a decrease in DST. Continuing substorms also disrupt the closed drift paths of “background” ring current particles which are then either lost at the magnetopause or switch over to region two currents in the ionosphere through the substorm current system. Gonzalez d al. [ 19941had suggested that a series of substorms could lead to a minor ring current intensification by looking only at the DST index, These were termed HILDCAA events (high intensity long duration continuous AE activity). In fact many authors had used substorms as a source for the re-filling of the the radiation belts after large storms. Our data here shows that such HILDCAA events can occur quite apart from main storms, and that the effect on tbe ring current particle population is quite the opposite of what would be expected: A decrease. With the turning northward of the IMF in Figure 6 the AE activity stops and the ring current particle fluxes recover to what they were before: There is no enhancement. The effect on DST is thus not due to an enhancement of the ring current by injected particles from substorms, but must rather be diictly related to the localized, enhances Sutton currents such as the substorm current wedge. We also examined data from the plasma instrument LEPA on CRRES (not shown here). It was possible that, even though MEB and MEA see flux decreases during this active substo~ period, the peak of the ring~u~ent distribution had moved to lower energies where there are more particles, making up for the loss at higher energies. However, electrons and ions down to 150 eV showed exactly the same behavior - a decrease during high AE activity time, with an abrupt recovery at the northward turning of the IMF when AE activity ceases.

319

3. The width of the slot region depends on the level of disturbance and tends to be broader during quiet times. 4. In~nsifi~ti~s of the particle flux are correlated with Dn and Kp. 5. The inner radiation belt is very stable, showing a sharp outer boundary and no dynamics. The large March 24,199 1 storm has a very clear signature in these plots: 1. A new radiation belt is created after the onset of this storm (between the inner and outer radiation belts). Further, a new radiation slot for high energy ions is observed. 2. Intense magnetic activity affected the energetic particle population throughout the sampled part of the magnetosphere. 3. Large fluxes of relativistic electrons (up to 25 MeV) were accelerated (or injected) by the shock event [I&mpola and Korth, 19921. 4. This extra belt exhibits cross-L motion to smaller L from L-2.4 to L-2.05 over a period of half a year (up to the end of the CRRES lifetime) [Kurd and Vamp&, 19941 5. The extra slot decays over a period of two months. For intense storms, the energetic particle storm-time dynamics can be summarized as follows: 1. The ring current intensifies after storm onset. 2. The ring current is displaced with respect to the prestorm outer radiation belt. The displacement is dependent on the strength of the storm and the past history of the magnetosphere, and can go as far as the slot region. 3. The source for the storm time ring current requites the acceleration of the previously existing trapped ion population via inward displacement under conservation of the first two adiabatic invariants [Lyons and Wliams, 19761. 4. The ring current decay mirrors the “storm decay” as measured by DST . For small, s&storm-related storms we summarize the behavior thus:

1. The ring current as measured by DST and as measured by the main current-carrying particles &have differently. 2. D,g decreases which each su~uent substo~ (AE increase) and is well correlated with substorm injections Conclusion seen with 45 keV electrons. For the overall dynamics the behavior can be summarized 3. Ring current particles (especially ions) are anticorrelated to AE showing a decrease at each subsequent thus: substorm and an abrupt recovery after substorm activity 1, Decay period of particles injected from sun is ~1 is turned off by a northward turning of the IME month. 4. HILDCAA events decrease DST by a different mecha2. Electrons and low energy ions decay first at high L, nism: The substorm current wedge. presumably by loss-scattering mechanisms and cross-L These results show the danger of using DST alone to ininward motion. High energy ions are more stable.

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terpret ring current behavior. Since DST is a global pa- Gussenhoven, M. S., E. G. Mullen, R. C. Filz, D. H. rameter but made out of data from only four equatorial Brautigam. and F. A. Hanser, New low altitude dose ground stations a large local effect (such as the substorm measurements, IEEE Transactions on Nuclear Science, N&34,676-683. 1987. current wedge) can override the global behavior as seen in the particles here. The ideas of substorm being responsiKorth, A., and R. H. W. Friedel, Dynamics of energetic ble for the refilling of the radiation belts on a global scale ions and electrons between L = 2.5 and L = 7 durwill have to be re-examined on the basis of the data preing magnetic storms, J. Geophys. Res., 101, submitted, sented here. 1996. Acknowledgments Korth, A., and A. L. Vampola, Cross-L motion of a relaWe thank D. Baker (at LASP, Boulder), J. Fennell and B. tivistic electron belt formed in the slot region, J. GeoBlake (both at The Aerospace Corporation, Los Angeles) phys. Res., 99, 13529-l 3535, 1994. for many uselful discussions. Korth, A., G. Kremser, B. Wilken, W. GUttIer, S. L. Ullaland, and R. Koga, Electron and proton wideangle spectrometer (EPAS) on the CRRES spacecraft, J. Spacecr. Rockets, 29.609-614, 1992. References Abel, B., R. M. Thorne, and A. L. Vampola, Solar cyclic behavior of trapped energetic electrons in Earth’s inner radiation belt, .I. Geophys. Res., 99,19427-19431, 1994.

Lemaire, J., A. D. Johnstone, D. Heynderickx, D. Rodgers, S. Szita, and V. Pierrard, TREND-2 (Trapped Radiation Environment Model Development) Final Report, European Space Agency Contract Report, ESTECIContract No. 9828/9UNIfFM’, 1995.

Baker, D. N., R. D. Belian, P. R. Higbie, and E. W. Li, X., D. N. Baker, M. Temerin, T. E. Clayton, E. Reeves, Hones (Jr.), High-energy magnetospheric protons and R.A.Christensen, J.B.Blake, R. Nakamura, and S. G. their dependence on geomagnetic and interplanetary Kanekal, Multi-satellite observations of the outer zone conditions, J. Geophys. Res., 84.7138-7154, 1979. electron variation during the 3-4 november 1993 magnetic storm, J. Geophys. Res., 101, submitted, 1996. Baker, D. N., J. B. Blake, L. B. Callis, J. R. Cummings, D. Hovestadt, S. Kanekal, B. Klecker, R. Mewaldt, and Looper, M. D., J. B. Blake, R. A. Mewaldt, J. R. CumR. D. Zwickl, Relativistic electron acceleration and demings, and D. N. Baker, Observations of the remnants cay time scales in the inner and outer radiation belts: of the ultrarelativistic electrons injected by the strong SAMPEX, Geophys. Res. Lett., 21,409-412, 1994. SSC of 24 march 1991, Geophys. Res. Lett., 21,20792082,1994. Bourdarie, S., D. Boscher, and T. Beutier, Modelling the high energy perticle leakage in the outer part of the Lyons, L., and D. Williams, Storm-associated variations of equatorially mirroring ring current protons, l-800 Earth’s belts, J. Geophys. Res., 102, submitted, 1996. kev, at constant adiabatic invariant, J. Geophys. Res., Cornwall, J. M., Radial diffusion of ionized helium and 81,216220, 1976. protons: A probe for magnetospheric dynamics, J. Vampola, A. L., and A. Korth, Electron drift echoes in Geophys. Res., 77,1756-1770, 1972. the inner magnetosphere, Geophys. Res. Lett., 19,625628, 1992. Cornwall, J. M., F. V. Coronti, and R. M. Thome, Unified theory of SAR arc formation at the plasmapause, Vampola, A. L., J. V. Osborne, and B. M. Johnson, J. Geophys. Res., 76.4428-4445, 1971. CRRES magnetic electron spectrometer AFGL-7015A (MEA), J. Spacecr. Rockets, 29.592-594, 1992. Friedel, R. H. W., and A. Korth, Long-term observations of kev ion and electron variability in the outer radiation Williams, D. J., A 27-day periodicity in outer zone belt from CRRES, Geophys. Res. L&t., 22.1853-1856, trapped electron intensities, J. Geophys. Res., 71, 1995. 1815-1826, 1966. Gonzalez, W., J. Joselyn, Y. Kamide, H. Kmehl, G. Ros- Williams, D. J., The Earth’s ring current: Present situation and future thrusts, Physica Scripta, T18, 140-l 5 1, taker, B. Tsurutani, and V. Vasyliunas, What is a ge1987. omagnetic storm?, J. Geophys. Res., 99, 5771-5792, 1994.