Comparison of energetic ions in cusp and outer radiation belt

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004ja010718, 2005 Comparison of energetic ions in cusp and outer radiation belt Jiasheng Chen and Theodore A. Fritz Center for Space Physics, Boston University, Boston, Massachusetts, USA Robert B. Sheldon NASA Marshall Space Flight Center, Huntsville, Alabama, USA Received 3 August 2004; revised 19 August 2005; accepted 27 September 2005; published 20 December [1] Recently, large diamagnetic cavities with a size of as large as 6 Re have been observed in the dayside high-altitude cusp regions. Associated with these cavities are charged particles with energies from 20 kev up to 10 MeV. Their seed population is a mixture of ionospheric and solar wind particles. The energetic ion intensity, charge composition, energy spectrum, and phase space density observed in the high-altitude dayside cusp have been compared with that measured in the outer radiation belt over an energy range of kev/e for protons and He ++ ions and of kev/e for O + ions. It is found that (1) the shape of ion energy spectra in the cusp is different from that in the outer radiation belt, (2) the ion phase space densities in the outer radiation belt are organized by magnetic moment, and (3) the ion phase space density of both He ++ and O + in the cusp are higher than that in the outer radiation belt at a given magnetic moment. These results suggest that (1) the measured cusp energetic ions cannot be explained by simple transport from an outer radiation belt (or ring current) source; rather, (2) a nonadiabatic energization process would be required to relate the cusp energetic ion population to the belt ion population, and (3) cusp energetic particles are potentially an additional independent source of the charged particles in the outer radiation belt. Citation: Chen, J., T. A. Fritz, and R. B. Sheldon (2005), Comparison of energetic ions in cusp and outer radiation belt, J. Geophys. Res., 110,, doi: /2004ja Introduction [2] In 1996, the POLAR spacecraft detected MeV charged particles in the dayside high-altitude cusp region [Chen et al., 1997, 1998; Sheldon et al., 1998]. These cusp energetic particles (CEP) with energies >20 kev were associated with diamagnetic cavities and large fluctuations in the local magnetic field. The CEP pitch angle distributions are different from an isotropic distribution and are peaked around 90 pitch angle, indicating a temporarily confined component [Chen et al., 1997; Chen and Fritz, 2000]. Owing to solar wind pressure, a geomagnetic field minimum at the equator moves to higher latitude (both North and South) and becomes two off-equatorial field minima in the high-altitude dayside cusps to trap charged particles [Mead, 1964; Shabansky and Antonova, 1968; Shabansky, 1971; Antonova and Shabansky, 1975]. Delcourt and Sauvaud [1999], Blake [1999], and Antonova et al. [2000] have suggested an outer radiation belt source of CEPs. This source has the particles being energized by processes in the geomagnetic tail associated with substorms. Ions energized in this manner can drift westward to form the ring current and outer radiation belt. Some of these charged particles may drift into the cusp by following the bifurcating field minima. On the other hand, since the cusp magnetic field lines are connected with the entire magnetopause Copyright 2005 by the American Geophysical Union /05/2004JA boundary layers, Fritz and Chen [1999] have suggested that the cusp energetic particles may enter into the boundary layers and contribute to the nightside ring current and the outer radiation belt. The present study will compare the energetic ion intensity, charge composition, energy spectrum and phase space density observed in the high-altitude dayside cusp with that measured in the outer radiation belt. [3] The ion data were obtained from the Magnetospheric Ion Composition Sensor (MICS) onboard POLAR. A feature of the POLAR spacecraft is the onboard interconnection of sensors for electronic communication. Data on the magnetic field can be communicated to ion sensors for use in data organization. The MICS sensor used an olive-shaped electrostatic analyzer, a secondary-electron generation/detection system, and a solid-state detector to measure the energy, time of flight, and energy per charge of the incident ions, which permit a unique determination of the ion s incident charge state, mass, and energy over 1 kev/e to 200 kev/e energy range and a determination of the phase space densities of various ion species and their relative abundances. The MICS data were sampled every 1/32 of a spin; that is, 32 sectors in each spin. Earlier versions of the MICS instruments have been described in detail by Wilken et al. [1992]. 2. CEP Event [4] Observationally, a CEP event is defined as follows: (1) a decrease in magnetic field magnitude in the dayside 1of13

2 Figure 1. The cusp energetic particle events observed by POLAR on 13 May The panels show (top) the variation of the kev/e He ++ flux, (middle) the 1 18 kev/e He ++ (solid line) and 1 10 kev/e O +6 (dotted line) fluxes, and (bottom) the magnetic field versus time, respectively. The distance of POLAR from the Earth (in R E ), the magnetic latitude (MLAT), and the magnetic local time (MLT), and the invariant latitude (ILAT) are shown at the bottom. cusp, (2) a more than one order of magnitude increase in intensity for the 1 10 kev ions, and (3) a more than three sigma increase above background for >40 kev ion intensity. One example is shown in Figure 1. On 13 May 1999, the POLAR spacecraft observed an extremely large diamagnetic cavity in the high-altitude cusp region in the morningside (Figure 1). From 1400 to 2320 UT on 13 May 1999, the local magnetic field strength showed a large decrease with strong field turbulence (bottom panel of Figure 1). The middle panel is the plot of the time profiles of the lower energy (1 10 kev/e) O +6 (dotted line) and (1 18 kev/e) He ++ (solid line), while the top panel is of the higher energy ( kev/e) He ++. These two panels exhibit orders of magnitude enhancement of the ion intensities. Figure 2, plotted by the OVT (Orbit Visualization Tool; see ovt.irfu.se), is the POLAR three-dimensional (3-D) orbit with respect to a model magnetosphere from 1300 UT on 13 May 1999 to 0100 UT on 14 May 1999, where the magnetospheric structure represents a shell of outermost field lines. The magnetic field model includes the internal field model IGRF and the external magnetospheric field model T96 [Tsyganenko and Stern, 1996]. It is noted that the POLAR spacecraft has an orbital period of about 18 hours. Figure 2 indicates that POLAR was well inside the magnetosphere on 13 May 1999, while Figure 1 shows that POLAR was in a cusp diamagnetic cavity (CDC) during half of its orbit period on that day, indicating that this CDC was extremely large with a size of about 6 R E in the latitudinal direction (see Figure 2), much larger than expected [e.g., Roederer, 1970]. [5] Another example is shown in Figure 3, which plots the energy spectrum of the 5/4/98 CEP event, where the MICS (open diamonds), IPS (open squares) and HIT (solid circles) are three different ion sensors onboard POLAR. On 4 May 1998, POLAR observed CEPs at and UT, and the measured cusp magnetic field strength decreased from a value of about 200 nt at 0643 UT to near 0 nt at 0656 UT [Chen and Fritz, 1999, 2001]. In Figure 3, the cusp energetic ions have an energy up to 10 MeV and can be represented by a power-law form 2of13

3 Figure 2. POLAR 3-D orbit with respect to a model magnetosphere from 1300 UT on 13 May 1999 to 0100 UT on 14 May See color version of this figure in the HTML. (straight line in Figure 3) over 7 kev to 10 MeV. For comparison, a Maxwellian distribution curve peaked at 1 kev is also plotted in Figure 3. This Maxwellian curve represents approximately the thermalized solar wind plasma energy distribution with a typical energy of 1 kev for ions (dominated by protons). The figure shows that the higher the ion energy, the larger the difference of the ion energy spectrum from the Maxwellian distribution. 3. Intensity Comparison Between CEPs and Outer Radiation Belt Ions [6] Figure 4 plots the ions and local magnetic field measurements by POLAR both in the high-altitude cusp and in the outer radiation belt regions on 20 April At UT, when the POLAR spacecraft was crossing through the dayside high-altitude cusp regions, it observed a more than one order of magnitude enhancement of the charged particle intensities and a large diamagnetic cavity (Figure 4). The diamagnetic cavity also features strong field turbulence (bottom two panels of Figure 4) that is associated with enhanced intensities of charged particles (top two panels). The second panel from top is the time-intensity profiles of the lower energy (1 10 kev/e) O +3 (solid line) and (1 18 kev/e) He ++ (dotted line), while the top panel is of the higher energy ( kev/e) O +2 (solid line) and ( kev/e) He ++ (dotted line), where the O +2 fluxes were dominated by singly ionized oxygen ions. Because the O +2 ions are of ionospheric origin and the O +3 and He ++ ions are of solar origin [Gloeckler et al., 1986], Figure 4 shows that the seed population of the energetic ions in the CEP event was a mixture of ionospheric and solar wind particles. This result is the same as that reported by Kremser et al. [1995] from low-altitude (<2.2 R E ) cusp observations. Figure 4 also shows that the flux of solar wind ions in the outer radiation belt (before 1142 UT) was much less than that in the cusp. In contrast, the top panel of Figure 4 indicates that the energetic ion fluxes were higher in the outer radiation belt than in the cusp. Compared with the cusp region the fluctuation power of the local magnetic field was weaker in the outer radiation belt. Figure 5 is a plot similar to Figure 2 but for POLAR orbit from 0700 to 1900 UT on 20 April Just like Figure 2, Figure 5 indicates that POLAR was well inside the magnetosphere during its cusp crossing at UT. [7] Figure 6 displays simultaneous observations of about kev proton fluxes by a polar-orbiting spacecraft (POLAR) and three geosynchronous satellites ( , LANL-97A, and ) from 0800 to 1830 UT on 20 April At UT POLAR was very close to the geostationary orbit, and its measured proton flux over kev was very close to the kev proton flux observed by geosynchronous satellite (top panel of Figure 6). According to the prediction of Delcourt and Sauvaud s [1999] simulation, the outer radiation belt protons may start to drift north to cusp at 16 hours local time. The vertical dashed line in each panel of Figure 6 marks 16 hours local time for each corresponding geosynchronous satellite. [8] For a more detailed comparison, data shown in Figure 4 were further divided into five sets, of which three sets from the outer radiation belt correspond to time intervals of , , and UT, and the other two sets from the cusp correspond to and UT, respectively. During these five time intervals, the kev proton fluxes measured by the geosynchronous satellites varied by a factor of less than 2 (Figure 6). The corresponding proton energy spectra are plotted in Figure 7 and exhibit three features: (1) the Figure 3. The measured CEP ion energy spectrum at and UT on 4 May 1998, where the MICS, IPS, and HIT are three sensors on board POLAR. The measured spectrum can be represented by a power-law form (straight line). The curve is the 1 kev Maxwellian energy distribution for shocked solar wind ions. 3of13

4 Figure 4. The outer radiation belt and the cusp regions observed by POLAR at UT on 20 April The panels show the variation of the kev/e O +2 (solid line) and the kev/e He ++ (dotted line) fluxes, the 1 10 kev/e O +3 (solid line) and 1 18 kev/e He ++ (dotted line) fluxes, the total magnetic field, and the magnetic field B y -component versus time, respectively. The distance of POLAR from the Earth (in R E ), the magnetic latitude (MLAT), the magnetic local time (MLT), and the invariant latitude (ILAT) shown at the bottom. cusp proton energy spectra in two different intervals ( UT, crosses; UT, open diamonds) are similar, (2) the shape of the cusp proton energy spectra are different than the outer radiation belt proton spectra, (3) at energies <10 kev, the cusp proton fluxes are much higher than the radiation belt one, while at >10 kev, the cusp proton fluxes are lower. 4. Ion Pitch Angle Distributions [9] Figure 8 shows the proton pitch angle distributions in counts per bin measured by POLAR during the five time intervals (regions), where each bin is 12 degrees wide and the error bar in each bin is the statistical error. As expected, the proton pitch angle distributions measured in the outer radiation belt (time intervals , , and UT) peaked at 90 and were symmetric with respect to it, which suggest a stably trapped proton population in the outer radiation belt. However, the proton pitch angle distributions observed in the cusp ( and UT) were asymmetric with respect to 90 pitch angle, which may be due to the asymmetric geometry of the CDCs. The asymmetry was more obvious at UT when POLAR was on open geomagnetic field lines (see Figure 5. POLAR 3-D orbit with respect to a model magnetosphere from 0700 to 1900 UT on 20 April See color version of this figure in the HTML. 4of13

5 Figure 6. Simultaneous observations of about kev proton fluxes by a polar-orbiting spacecraft (POLAR; dotted line in top panel) and three geosynchronous satellites ( , top; LANL-97A, middle; and , bottom) from 0800 to 1830 UT on 20 April The vertical dashed line in each panel marks 16 hours local time for each corresponding geosynchronous satellite. Figure 5), and most of the protons were from >90 pitch angle directions away from the Earth. Figures 9 and 10 are similar plots as Figure 8 but for He ++ and O + ions, respectively. The He ++ (Figure 9) and O + (Figure 10) pitch angle distributions were similar to the proton (Figure 8) distributions except for interval of UT when the He ++ ions showed an isotropic distribution (a fairly straight horizontal line in Figure 9) and the O + ions showed a butterfly distribution (Figure 10). Compared with the proton pitch angle distribution, Figures 9 and 10 suggest that at UT there were new ion sources from both parallel and antiparallel field directions. Such directions may be connected magnetically to the equatorward edges of the southern and the northern cusps. This observation suggests an additional ion source for the outer radiation belt He ++ and O + populations. 5. Ion Magnetic Moment Spectra [10] The bottom two panels of Figure 4 indicate that the local magnetic field in the outer radiation belt (before 1142 UT) was rather stable; one would expect that the three adiabatic invariants must be conserved for the belt ions. Figure 11 is a plot of the proton magnetic moment spectra for the same five regions shown in Figure 7 over kev energy ranges. The proton magnetic moment is determined by: E? /B = Esin 2 a/b with E being the proton kinetic energy; B, the magnitude of the local magnetic field; and a, the proton pitch angle. In Figure 11, the proton magnetic Figure 7. Proton energy spectra observed by the POLAR for five time intervals with three ( UT, open triangles; UT, solid circles; UT, open squares) in the outer radiation belt and two ( UT, crosses; UT, open diamonds) in the cusp regions on 20 April of13

6 Figure 8. The proton pitch angle distributions measured by POLAR during five time intervals with three ( , , and UT) in the outer radiation belt and two ( and UT) in the cusp regions. 6of13

7 Figure 9. The He ++ pitch angle distributions measured by POLAR during five time intervals as shown in Figure 8. 7of13

8 Figure 10. The similar plot as Figure 8 but for the O + pitch angle distributions. moment spectra is calculated by taking into account all of the proton pitch angle distributions as shown in Figure 8. The three different belt proton energy spectra shown in Figure 7 are now nearly overlapping in the magnetic moment spectra and form a S-shape curve shown in Figure 11 even if they were measured at different latitudes, different local times, and different altitudes in the outer radiation belt (see bottom of Figure 4). This demonstrates that the proton phase space density in the outer radiation belt is organized by magnetic moment. In contrast, compared to Figure 7, the two cusp spectra in Figure 11 do not change much. [11] Figures 12 and 13 are two plots similar to Figure 11, but for kev/e He ++ and kev/e O +, respectively. These two figures show three important results: (1) the He ++ and O + ion phase space densities in the outer radiation belt increased with increasing latitude and altitude at a given magnetic moment, (2) the magnetic moment spectra in the cusp regions are harder than that in the belt at high energy end, and (3) the phase space densities of both the He ++ and O + ion species in the cusp are higher than that in the outer radiation belt at a given magnetic moment. [12] Figures 14, 15, and 16 are similar plots as Figures 11, 12, and 13, respectively, but for taking into account ion pitch angles around 90 ( ) only. Most of the Figure 11. Proton magnetic moment spectra observed by the POLAR for five time intervals shown in Figure 7. All of the proton pitch angles shown in Figure 8 have been taken into account. 8of13

9 Figure 12. He ++ magnetic moment spectra observed by the POLAR for five time intervals shown in Figure 7. All of the He ++ pitch angles shown in Figure 9 have been taken into account. results shown in Figures 14, 15, and 16 are the same as that shown in Figures 11, 12, and Discussion [13] The solar wind conditions (in GSM coordinates) from UT on 13 May 1999 were displayed in Figure 17. During this period, the solar wind ion pressure (top panel) was about 3 (npa), and the solar wind V x component (panel 2 from top) was about 450 km/s. The interplanetary magnetic field (IMF) had a negative B y Figure 14. Similar plot as Figure 11 but only the proton pitch angles around 90 being taken into account. component (bottom panel), and that effects the location of the magnetic field merging which may explain why POLAR observed the CDC in the prenoon (about 9.6 hours MLT). Figure 17 shows that the IMF had a prolonged southward component at UT, and a northward component for most of time from 1540 to 2100 UT (dashed line in panel 4 from top); however, the particle intensity in the polar dayside cusps did not exhibit strong variations from 1410 to 2100 UT on that day (middle panel of Figure 1). Therefore the cusp particle intensity is not obviously dependent on the IMF direction. [14] Figure 18 plots the solar wind conditions (in GSM coordinates) during the time period shown in Figure 4. On 20 April 1999, the solar wind ion pressure was about 3 Figure 13. O + magnetic moment spectra observed by the POLAR for five time intervals shown in Figure 7. All of the O + pitch angles shown in Figure 10 have been taken into account. Figure 15. Similar plot as Figure 12 but only the He ++ pitch angles around 90 being taken into account. 9of13

10 Figure 16. Similar plot as Figure 13 but only the O + pitch angles around 90 being taken into account. 4 (npa) from 0800 to 1630 UT, and was about 4 7 (npa) from 1630 to 1830 UT; the solar wind V x component was about 500 km/s from 0800 to 1630 UT, and was larger in an absolute sense ( 600 km/s) at 1800 UT. At UT when POLAR was in the outer radiation belt, the solar wind V y changed between 20 and 70 km/s, the V z changed between 50 and 50 km/s, the three IMF components fluctuated between 7 and 7 nt; Figure 11 indicates that the energetic protons in the outer radiation belt were almost independent of the above solar wind condition changes. However, the energetic ions in the cusp regions were not completely independent of the solar wind conditions. Figures 4 and 18 show that lower CEP intensities were observed at UT and at UT when the IMF B z was positive and the IMF B x was negative. The proton energy spectra in the two cusp regions ( and UT) were very similar (Figure 7). However, a closer inspection indicates that the cusp proton intensities at energies <10 kev seem related to the solar wind ion pressure. At UT when the solar wind pressure was lower (3 4 npa), the <10 kev cusp protons showed a lower flux; while at UT when the solar wind pressure was higher (4 7 npa), the <10 kev cusp protons had a higher flux (Figure 7). This relation was not valid for >40 kev cusp protons that showed a higher flux at UT when the solar wind pressure was lower and a lower flux at UT when the solar wind pressure was higher (Figure 7). [15] Besides, from 1200 to 1800 UT the IMF B y was positive during most of the time and the cusp center should move into the afternoonside. Since the cusp was also observed in the morningside by POLAR during this time interval, it suggests that in addition to magnetic field (IMF and GMF) merging, the interaction between charged particles and the local field also play an important role. Any new model needs to include such an interaction in order to explain the location of these CDC observations. It further suggests a large CDC along the longitudinal direction. [16] From Figure 4, one can see that the cusp He ++ time profiles at higher energies ( kev/e) are different from that at lower energies (1 18 kev/e); just as ring current and radiation belts occupy distinct and overlapping regions of the dipole trap, so 1 kev/e and 55 kev/e ions occupy distinct and overlapping regions of the cusp. A geomagnetic field minimum at the equator may move to higher latitude because of the solar wind pressure and become two off-equatorial field minima in the high-altitude dayside cusps to trap charged particles [Mead, 1964; Shabansky and Antonova, 1968; Shabansky, 1971; Antonova and Shabansky, 1975]. This is important because it provides a connection between CEPs and the outer radiation belt (or ring current) populations. [17] Through numerical simulations, Delcourt and Sauvaud [1999], Blake [1999], and Antonova et al. [2000] suggested an outer radiation belt (or ring current) source of the CEPs. If the outer radiation belt (or ring current) is the dominant source for the CEPs, there is one necessary condition that the ion phase space density should be higher in the outer radiation belt than in the cusp at a given magnetic moment. [18] Even though the radiation belt protons have a much longer trapping time than the CEPs, Figures 11 and 14 show that only over the range kev/nt was the proton intensity in the outer radiation belt higher than that in the cusps. Over KeV/nT and >1 kev/nt, the proton phase space density in the outer radiation belt was lower than that in the high-altitude cusp at a given magnetic moment. Figures 12 and 13 (or 15 and 16) further show that the observed heavy ion phase space densities in the cusp are significantly higher than those measured in the outer radiation belt; this observational result is significant and very surprising because the radiation belt ions have much longer trapping time than the CEPs. Therefore the measured CEPs cannot be explained by the outer radiation belt (or ring current) source. In addition, Delcourt and Sauvaud [1999] predicted that the energy of 60 kev proton started at the tail source will decrease to about 5 10 kev after drifting into the high-altitude cusp, and one would expect that a 10 MeV ion in the cusp (Figure 3) should have an energy larger than 10 MeV in the tail source, which is too high for the substorm source. Generally, the CEPs do not show an obvious energy dispersion signature, which suggests a local effect rather than a transport effect. [19] The different shapes between the cusp and belt ion spectra (Figures 11, 12, 14, and 15) and the harder magnetic moment spectra in the CEPs at the higher energy end suggest that a nonadiabatic energization process is required to relate the cusp energetic ion population to the outer radiation belt ion population. The different shapes between the cusp and belt proton spectra (Figures 7, 11, and 14) also suggest that CEPs are not the direct source of the energetic protons in the outer radiation belt. However, a detailed inspection on Figures 12 (or 15) and 13 (or 16) indicate that at a given magnetic moment on the dayside the He ++ and O + ion phase space densities in the outer radiation belt increased with increasing latitude and altitude, and the phase space densities of the ion species in the high-altitude cusp are higher than that in the outer radiation belt, suggesting an energetic ion source in the high-altitude and high-latitude region. Therefore the CEPs are potentially an additional 10 of 13

11 Figure 17. The solar wind conditions observed by WIND in GSM coordinates at UT on 13 May The panels from top to bottom are the solar wind ion pressure, the solar wind velocity X component, the solar wind velocity Y component (solid line) and Z component (dashed line), the IMF B x component (solid line) and B z component (dashed line), and the IMF B y component, respectively. 11 of 13

12 Figure 18. The solar wind conditions observed by WIND in GSM coordinates at UT on 20 April The panels from top to bottom are the solar wind ion pressure, the solar wind velocity X component, the solar wind velocity Y component (solid line) and Z component (dashed line), the IMF B x component (solid line) and B z component (dashed line), and the IMF B y component, respectively. source of the charged particles in the outer radiation belt as suggested by Fritz and Chen [1999]. 7. Conclusions [20] Extremely large diamagnetic cavities with a size of as large as 6 R E have been observed in the dayside highaltitude cusp regions. These diamagnetic cavities are associated with strong magnetic field turbulence and with 20 kev up to 10 MeV charged particles. The seed population of the CEPs is a mixture of ionospheric and solar wind particles. Comparison of the energetic ions in the cusp and in the outer radiation belt over the energy range of kev/e has provided new insights about these two ion populations. The analysis of CEPs has shed light on the origins of the energetic particles in the outer radiation belt. Our principal conclusions are the following: [21] 1. The shapes of the proton energy spectra in the cusp are different from that in the outer radiation belt. [22] 2. The ion energy spectra in the outer radiation belt are organized by the magnetic moment and are independent of the solar wind conditions. [23] 3. The phase space density of the belt He ++ and O + ions, at a given magnetic moment, increased with increasing latitude and altitude. [24] 4. The magnetic moment spectra in the cusp are harder than that in the belt for magnetic moments >0.4 (kev/nt) for protons and >1 (kev/nt) for He ++ and O + ions. [25] 5. The phase space densities of both the He ++ and O + ion species in the cusp are higher than that in the outer radiation belt at a given magnetic moment. [26] 6. These observational facts suggest that the measured CEPs cannot be explained by simple transport from an outer radiation belt source, but rather a nonadiabatic energization process is required to relate the CEPs to the outer radiation belt ion population, making CEPs a potentially additional and independent source of the charged particles in the outer radiation belt. [27] Acknowledgments. We thank J. B. Blake, C. Z. Cheng, C.-I. Meng, M. Schultz, and G. L. Siscoe for useful discussions and E. Beiser for POLAR orbit plots. We want to acknowledge the contribution of B. Laubscher, R. Hedges, R. Vigil, and G. Lujan on the CAMMICE sensor system at the Los Alamos National Laboratory; S. Livi, H. Sommer, and H. Steinmetz at the Max Planck Institute for Aeronomy in Germany; M. Grande and colleagues at the Rutherford Appleton Laboratory in Great Britain; J. Fennell, R. Koga, P. Lew, N. Katz, J. Roeder, and B. Crain on the data processing units at the Aerospace Corporation; and the administrative support and interest provided by D. D. Cobb at the Los Alamos National Laboratory. We are grateful to C. T. Russell for providing the POLAR GMF data; D. DeLapp, M. Henderson, and G. Reeves for the LANL geostation- 12 of 13

13 ary energetic particle data; R. Lepping for the WIND IMF data; and K. Ogilvie for the WIND solar wind plasma data. This research was supported by NASA grants NAG5-2578, NAG5-7677, NAG5-7841, NAG5-9562, and NAG [28] Lou-Chuang Lee thanks both reviewers for their assistance in evaluating this paper. References Antonova, A. E., and V. P. Shabansky (1975), Particle and magnetic field in the outer dayside geomagnetosphere, Geomagn. Aeron., 15(2), Antonova, A. E., Y. I. Gubar, and A. P. Kropotlin (2000), Energetic particle population in the high-latitude geomagnetosphere, Phys. Chem. Earth C, 25(1 2), Blake, J. B. (1999), Comment on Cusp: A new acceleration region of the magnetosphere by Jiasheng Chen et al., Czech. J. Phys., 49(4a), Chen, J., and T. A. Fritz (1999), May 4, 1998 storm: Observations of energetic ion composition by POLAR, Geophys. Res. Lett., 26(19), Chen, J., and T. A. Fritz (2000), Origins of energetic ions in CEP events and their implications, Int. J. Geomagn. Aeron., 2, Chen, J., and T. A. Fritz (2001), Features of energetic ions near the compressed magnetosphere, J. Atmos. Sol. Terr. Phys., 63/5, Chen, J., T. A. Fritz, R. B. Sheldon, H. E. Spence, W. N. Spjeldvik, J. F. Fennell, and S. Livi (1997), A new, temporarily confined population in the polar cap during the August 27, 1996 geomagnetic field distortion period, Geophys. Res. Lett., 24(12), Chen, J., T. A. Fritz, R. B. Sheldon, H. E. Spence, W. N. Spjeldvik, J. F. Fennell, S. Livi, C. T. Russell, J. S. Pickett, and D. A. Gurnett (1998), Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere, J. Geophys. Res., 103(A1), Delcourt, D. C., and J.-A. Sauvaud (1999), Populating of the cusp and boundary layers by energetic (hundreds of kev) equatorial particles, J. Geophys. Res., 104, 22,635 22,648. Fritz, T. A., and J. Chen (1999), The cusp as a source of magnetospheric particles, Radiat. Meas., 30(5), Gloeckler, G., et al. (1986), Solar wind carbon, nitrogen and oxygen abundances measured in the Earth s magnetosheath with AMPTE/CCE, Geophys. Res. Lett., 13, Kremser, G., J. Woch, K. Mursula, P. Tanskanen, B. Wilken, and R. Lundin (1995), Origin of energetic ions in the polar cusp inferred from ion composition measurements by the Viking satellite, Ann. Geophys., 13, Mead, G. D. (1964), Deformation of geomagnetic field by the solar wind, J. Geophys. Res., 69, Roederer, J. G. (1970), Dynamics of Geomagnetically Trapped Radiation, Springer, New York. Shabansky, V. P. (1971), Some processes in the magnetosphere, Space Sci. Rev., 12, Shabansky, V. P., and A. E. Antonova (1968), Topology of particle drift shells in the Earth s magnetosphere, Geomagn. Aeron., Engl. Transl., 8, 844. Sheldon, R. B., H. E. Spence, J. D. Sullivan, T. A. Fritz, and J. Chen (1998), The discovery of trapped energetic electrons in the outer cusp, Geophys. Res. Lett., 25(11), Tsyganenko, N. A., and D. P. Stern (1996), Modeling the global magnetic field of the large-scale Birkeland current systems, J. Geophys. Res., 101, 27,187 27,198. Wilken, B., W. Weiss, D. Hall, M. Grande, F. Soraas, and J. F. Fennell (1992), Magnetospheric ion composition spectrometer onboard the CRRES spacecraft, J. Spacecraft Rockets, 29, 585. J. Chen and T. A. Fritz, Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA. ( jschen@bu.edu; fritz@bu.edu) R. B. Sheldon, NASA Marshall Space Flight Center, 320 Sparkman, Huntsville, AL 35805, USA. (rob.sheldon@msfc.nasa.gov) 13 of 13