J. Chen and T. A. Fritz
Center for Space Physics, Boston University, Boston
In 1996, a new magnetospheric phenomenon called a cusp energetic particle (CEP) event was discovered by the POLAR spacecraft [Chen et al., 1997a]. The measured ions in the CEP events had energies up to 8 MeV [Chen et al., 1998]. A key question is where do the cusp energetic ions come from?
The cusps, by definition, are near zero magnetic field magnitude and funnel-shaped areas between field lines that map to the dayside and nightside of the magnetopause surface. Theoretically, for perfect shielding, the cusps are focal points for the shielding currents confining the magnetosphere [Chapman and Ferraro, 1931]; for less than perfect shielding, the cusps become open funnels for direct entry of magnetosheath plasma into the magnetosphere [e.g., Crooker, 1979; Crooker et al., 1991; Marklund et al., 1990; Meng, 1982; Newell and Meng, 1987, 1991; Reiff, 1979; Reiff et al., 1977]. In practice, the cusp regions are identified either by minimum local magnetic fields [Farrell and Van Allen, 1990] or by a combination of low magnetic field strength, high plasma intensity, and the presence of broadband plasma wave noise [Chen et al., 1998; Fung et al., 1997].
The POLAR spacecraft was launched, on February 24, 1996, into a 1.8 9 RE polar orbit with an inclination of 86 o and a period of 18 hours. Over the first year, the spacecraft sampled high-altitude regions in the north and low altitudes in the south and had a spin period of about 6 seconds. The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) onboard Polar consists of two sensors, the Heavy Ion Telescope (HIT) and the Magnetospheric Ion Composition Sensor (MICS), designed to measure the charge and mass composition within the geomagnetosphere over the energy range of 1 keV/e to 60 MeV, to determine the fluxes of various ion species, the ion's incident charge state, and their relative abundances. In making these measurements CAMMICE provides important information about the origins of the energetic particles and the identity of mechanisms by which these ions are energized and transported from their source populations within geospace [Chen et al., 1997a, 1998]. The Imaging Proton Sensor (IPS) and the Imaging Electron Sensor (IES) onboard POLAR were part of the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) Experiment [Blake et al., 1995] and were designed to measure 3-dimensional proton and electron angular distributions. For the IPS the energy range covered was 20 keV to 10 MeV and for IES it was 20 keV to 400 keV. Both the CAMMICE and CEPPAD sensors provide excellent spectral and temporal resolution with data being sampled every 1/32 of a spin. The polar orbits of the POLAR spacecraft thus provide an excellent opportunity to investigate the energetic particles in the cusp regions. The GEOTAIL spacecraft was launched on July 24, 1992, with a 8 200 RE orbit and starting from February 1995, its apogee was reduced to 30 RE. The Energetic Particle and Ion Composition (EPIC) instrument onboard GEOTAIL was designed to measure ions over the energy range of 8 keV/e to 6 MeV [Williams et al., 1994]. These ion data provide important information about the origins of the cusp energetic particles.
Figure 1 shows an example of a series of CEP events. On August 27, 1996, at about 0840 UT when the Polar spacecraft was 9 RE (Earth's radius) from the Earth at 67o geomagnetic latitude (MLAT) and 14.7 hours local time (MLT), the POLAR/CAMMICE detected a large increase of the intensity of 1-200 keV/e He++ ions (panel 2 from top of Figure 1) that corresponded to large fluctuations and a decrease in the magnitude of the local geomagnetic field (GMF) measured by the Magnetic Field Experiment (MFE) [Russell et al., 1995] on POLAR (panel 3 from top of Figure 1). In other words, Figure 1 suggests that the 1-200 keV/e helium intensity was anticorrelated with the field magnitude. The period lasted more than two hours (about 0800-1036 UT). In panel 2, the four helium peaks, that were associated with four local minima in the field magnitude and corresponded to four different regions: 0800-0830 UT, 0830-0912 UT, 0912-1006 UT and 1006-1036 UT, were designated as four individual CEP events. The numbers of 1, 2, 3, and 4 within the vertical dashed lines in panel 2 mark the four different regions in this period. The most remarkable feature is that there is an unexpected increase of the 0.52-1.15 MeV and 1.15-1.8 MeV helium fluxes detected by the HIT sensor within the event period (bottom two panels of Figure 1).
The four-peak feature in the high latitude dayside cusp was also found both in the 20-200 keV electron fluxes (Figure 2a) measured by the Imaging Electron Sensor (IES) and in the 20-500 keV proton fluxes (Figure 2b) by the Imaging Proton Sensor (IPS) [Chen et al., 1998]. It is noticed that the energetic electron intensities were enhanced only by a factor of 10, while the energetic proton intensities increased by a factor of 200, indicating a process that distinguished ions from electrons if these particles were involved with the same process that was responsible for producing the CEP events. A total of 75 CEP events were detected during 1996 [Chen et al., 1998].
Figure 3 exhibits the event-averaged positions of the CEP events in the magnetosphere with plots of MLT versus MLAT (panel a) and MLAT versus R/RE (panel b) in polar coordinates [Chen et al., 1998]. In panel (a), the four dashed circles from inside to outside indicate the MLAT positions from 80o to 50o, respectively. In panel (b), the dashed circles represent the distance of POLAR from the Earth (in RE ). Panel (a) reveals that the CEP events were observed in the dayside. Both panels in Figure 3 also reveal that the CEP events spanned more than 20 degrees in geomagnetic latitude, which is different from the expectation of the low altitude cusp where it is observed to be only about four degrees wide, or less, in latitude [e.g., Marklund et al., 1990; Menietti and Burch, 1988; Yamauchi et al., 1996], but is comparable with the high altitude cusp results [Fung et al., 1997; Haerendel et al., 1978; Lundin, 1985; Sandahl et al., 1997; Zhou et al., 1997]. Two other interesting features are that the CEP events extended more degrees in latitude in the afternoonside than that in the morningside, and that there was a 14-MLT peak of the CEP events. All CEP events were observed at radial distances greater than 7 RE.
The helium ion charge state in Figure 1 reveals that compared to He ++ (panel 2), the He + flux (top panel) was negligible. Since solar particles have higher ion charge states than that of ionospheric particles [Gloeckler et al., 1986], the top two panels of Figure 1 suggest a solar source for the helium fluxes in the CEP events. One possible solar source is the solar energetic particle events. Large helium fluxes have been observed in the solar energetic particle events [e.g., Chen et al., 1994, 1995; Hovestadt et al., 1984; Ma Sung et al., 1981; Mason et al., 1983; Webber et al., 1975]. Chen et al. [1997a] have discussed the possibility of the solar energetic particle events as a direct and transient source of the particles in the CEP events, and have ruled it out due to a lack of simultaneous comparable fluxes being observed by the WIND spacecraft. Another possible solar source is the solar wind plasma. Chen and Fritz [1999a, 1999b] have determined the He ++ /H + ratios simultaneously both in the interplanetary medium and near the cusp during the May 4, 1998 geomagnetic storm period and have found that the ratio near the cusp at energy of about 1 keV/e was close to that of solar wind plasma in the interplanetary medium, which suggest that the solar wind plasma is the seed population of the cusp energetic particles.
Figure 4 displays the measured CEP helium energy spectrum (solid circles) at 0830-0848 UT on August 27, 1996. For comparison, a Maxwellian distribution curve peaked at 1 keV is also plotted in the figure. This Maxwellian curve represents approximately the thermalized solar wind plasma energy distribution with a typical energy of 1 keV. The figure shows three features: (1) The helium ions in this CEP event have energy up to 4 MeV/e (or 8 MeV); (2) there is a spectral break at about 20 keV/e; and (3) the higher the helium energy, the larger the difference of the helium energy spectrum from the Maxwellian distribution. Therefore, if the thermalized solar wind plasma is the seed population of the particles observed in the CEP events, they should be energized somewhere near or within the magnetosphere by possible sources and source regions which will now be discussed.
One possibility is the bow shock that accelerates some low energy solar wind particles to a higher energy. Previous observations indicated that at energies larger than 60 keV the ion fluxes were almost the same either before the bow shock in the upstream or after the bow shock in the magnetosheath [Ellison et al., 1990; Krimigis et al., 1978; West and Buck, 1976]. Such observations imply either that the lower energy solar wind ions have been accelerated by the bow shock to energies larger than 60 keV and then have been isotropically scattered into upstream or downstream or that the > 60 keV ions were accelerated somewhere other than the bow shock and then passed through it. Lee  indicated that the > 60 keV ion flux was independent of distance upstream from the bow shock. On August 27, 1996 at 0833-0903 UT, POLAR was in the cusp, and GEOTAIL was in the upstream region from the bow shock and on the dawn side of the Earth. Figure 5 [Fritz and Chen, 1999a] compares the > 60 keV ion flux observed by the EPIC instrument onboard GEOTAIL to that observed by the IPS onboard POLAR. This comparison reveals that the ion flux in the CEP event is higher than that in the upstream region. On September 18, 1996 at 0915-0945 UT, GEOTAIL was again in the dawn side upstream region from the bow shock, and nine days later on September 27, 1996 at 0400-0430 UT, GEOTAIL was in the downstream region from the bow shock in the magnetosheath. Figure 6 compares the ion fluxes observed by the GEOTAIL in the upstream (panel a) and downstream (panel b) to that observed by the POLAR in the cusp at these time periods [Chen and Fritz, 1998]. Again, the ion fluxes in the CEP event is higher than that in the downstream and upstream. Another feature in both Figures 5 and 6 is that the shape of the ion energy spectra in the CEP events is different from that in either the upstream or the downstream.
Figure 7 shows a comparison of the qBn (top panel) [Trattner et al., 1999] and the ion fluxes (bottom panel) [Fritz and Chen, 1999b] measured by GEOTAIL, where the qBn is the angle between the IMF and the shock normal and is determined by tracing the IMF at GEOTAIL back to the bow shock. While the qBn shows large variations from 20 o to 90 o, the ion fluxes at energies larger than 0.47 MeV are independent of both the quasi-parallel ( qBn < 45 o ) and quasi-perpendicular ( qBn > 45 o ) bow shocks. Sarris et al.  has shown that energetic electrons are not produced during the interaction of the solar wind plasma with the bow shock. The enhancements of the energetic electron intensities in the CEP events, shown in Figure 2a, are not expected by bow shock acceleration models. One key prediction of the bow shock acceleration model was that the ion energy spectra in energy per charge were species independent [Lee, 1982; Lee et al., 1981]. However, POLAR observations indicated that near the cusp the energy spectra in energy per charge were ion species dependent [Chen and Fritz, 1999a, 1999b]. Therefore, acceleration at the bow shock alone cannot explain the observational results of the energetic particles in the CEP events.
The other possibility for producing the enhanced fluxes in the dayside cusp is the dipolarization process during substorms. The substorm can accelerate low energy geomagnetic tail particles to higher energy as the particles are transported inward on the nightside magnetosphere [e.g., Aggson et al., 1983; Delcourt et al., 1990; Hesse and Birn, 1991; Lezniak and Winckler, 1970; Lopez et al., 1990; Quinn and Southwood, 1982]. Those particles may reach drift paths that are connected to the polar cusp through the Shabansky orbit [Antonova and Shabansky, 1975; Mead, 1964; Shabansky, 1971]. Recently, Delcourt and Sauvaud  showed that such a orbit exists only for a narrow range of radial distances when the charged particles are started from the equatorial plane in the geomagnetic tail around 7.4 RE. At closer distances the particles execute normal drift motion confined to the equatorial plane; at larger distances these particles reach the dusk flank and are lost to the interplanetary medium. The dipolarization process during substorms is difficult to account for the observed cusp ion intensities that were increased by orders of magnitude.
One feature of the Shabansky orbit is that the charged particles cross the dawn-dusk plane. There is another kind of orbit, called a Sheldon orbit, in which the particles drift on a closed path in the dayside cusp without crossing the dawn-dusk plane. Figure 8 shows examples of the Sheldon orbits for 1 MeV electrons at three different locations in a static cusp geometry [Sheldon et al., 1998]. The charged particles are shown mirroring around the minimum magnetic field near the cusp center, and drifting in closed drift shells around the cusp field line. It is noticed that the cusp has a locally outward magnetic gradient in contrast to the typical inward gradient in the radiation belt. In fact, trajectory calculations indicate that, in a static GMF model, the cusp geometry can trap the tens of keV to MeV charged particles with bounce periods of about 0.1-1.1 s and drift periods of about 2-80 s [Chen et al., 1997b; Sheldon et al., 1998].
From observations, POLAR measurements reveal that the GMF in the CEP events is not static, but very turbulent. Figure 9 is another example of a the CEP event, which was observed at 7.0-10.1 UT on September 18, 1996, where the numbers of 1, 2, 3, and 4 within the vertical dashed lines mark four different time periods: 7.0-7.8 UT, 7.8-8.6 UT, 8.6-9.1 UT, and 9.1-10.1 UT. Each period corresponds to a helium peak and a local minimum in the GMF magnitude. The large increase of the 0.52-1.15 MeV helium flux (top panel) corresponded to a large decrease in the magnitude of the local turbulent GMF (bottom panel). Figure 10 plots the pitch angle distributions of the helium ions in counts per sample number. The three different regions (1, 2, and 4 shown in Figure 9) are showed in the three panels of Figure 10. For a stable trapping distribution, a peak at 90 o pitch angle is expected; in contrast, for an isotropic distribution, a fairly straight horizontal line is expected. Figure 10 shows that the pitch angle distributions of the MeV helium ions in the CEP events were different from an isotropic distribution and also different from a stable trapping distribution; most helium ions were detected within 45 o -135 o pitch angles. In the real geomagnetic field, the solar wind pressure changes the dipole into quadrupolar, and the solar plasma injection from the magnetosheath can create a diamagnetic cavity to form a trapping geometry with two local maxima of GMF intensities and a local minimum field intensity in the cusp [Chen et al., 1997a, 1998]. Such a cusp geometry may explain why the MeV charged particles can stay in cusp for hours and why the ion pitch angle distribution was different from an isotropic distribution. Figure 10 indicates that the pitch angle distribution in the CEP events was different from a nominal "peaked at 90 o '' pitch angle distribution due probably to the large fluctuations of the local field in the cusp, where the dB/B was very large. Therefore, the high-altitude cusp is probably not a stable trapping region but rather a dynamic trapping region that can only temporarily confine charged particles [Chen et al., 1997a].
Figure 9 suggests that the local turbulent magnetic fields play an important role in organizing the measured energetic helium intensities. For further understanding of the quantitative relationship between the MeV helium flux and the turbulent field, the two time periods of 8.6-9.1 UT and 9.1-10.1 UT on September 18, 1996 were chosen as two extreme cases for turbulent magnetic energy spectral analysis. The spectral analysis method developed by Chen  is used to determine the power spectra of the local GMF turbulence with two components (parallel and perpendicular) to the mean field in each time period. The results are plotted in Figure 11 for fluctuations in the ultra low frequency (ULF) range of about 0.002 to 3 Hz, corresponding to periods of about 0.33-500 seconds that cover the drift-, bounce-, and gyro-periods of the cusp energetic ions when they are temporarily confined in the high-altitude dayside cusp. The solid line is the parallel component of the spectra, and the dashed line is the perpendicular component. The top two lines (solid and dash) are for 9.1-10.1 UT (period 4 in Figure 9) when the largest CEP event on September 18, 1996 was detected, the bottom two lines are for 8.6-9.1 UT (period 3 in Figure 9) when the smallest CEP event on September 18, 1998 was detected. Figure 11 shows that the parallel component of the power spectrum is larger than the perpendicular component for the 9.1-10.1 UT period but is almost the same as the perpendicular component for the 8.6-9.1 UT period. Because the parallel component of the turbulent magnetic power will generate a perpendicular inductive electric field, the fact that the parallel component of the turbulent magnetic power is larger than the perpendicular one implies that the acceleration change of the charged particles will be larger in the perpendicular direction than in the parallel one.
Integrating the spectral component parallel to the mean field over the ULF ranges, one obtains a term that is proportional to the ULF parallel turbulent magnetic energy. The mirror parameter was defined as the ratio of the square root of the integration of the parallel turbulent spectral component over the ULF ranges to the local mean field [Bieber et al., 1993]. Figure 12 plots the mirror parameter versus the MeV helium fluxes for the four time periods shown in Figure 9. A remarkable feature of Figure 12 is that there is a clear correlation between the cusp MeV helium flux and the mirror parameter. Since the mirror parameter provides a measure of the influence of mirroring interactions, the correlation shown in Figure 12 indicates there is a local effect in the cusp.
Similarly, integrating the spectra over the ULF frequency ranges, summing up two perpendicular and one parallel components, and multiplying the resulting power by a factor of 2, one obtains a term that is proportional to the ULF turbulent magnetic energy. The factor of 2 is needed to include the power at negative frequencies. Figure 13 displays the integrated turbulent power versus the MeV helium fluxes for the four time periods shown in Figure 9. Again, a clear correlation of the cusp MeV helium flux with the ULF turbulent magnetic energy was found. The ULF turbulent magnetic energy will produce the inductive electric field to resonately interact with the cusp charged particles, which is probably the physical mechanism that converts electromagnetic energy into ion energy. Further calculations indicate that about 67% of total magnetic energy is the ULF turbulent energy for the 9.1-10.1 UT period but only about 0.17% of total magnetic energy is the ULF turbulent energy for the 8.6-9.1 UT period.
Figure 14 associates the event-averaged counting rate of the 1-200 keV/e helium with two different local GMF parameters: dB2 (left panel) and B (right panel), where represents event average and dB = Bi+1 - Bi from 6 seconds resolution field data [Chen et al., 1998; Fritz et al., 1999]. The four open squares are the four events on August 27, 1996. The event-averaged method was used to reduce the irregular and random fluctuations and to analyze the statistical properties. Figure 14 reveals that the 1-200 keV/e helium intensities were best organized by dB2 (left panel), and that there was an anti-correlation between 1-200 keV/e helium intensities and the event-averaged (mean) field (right panel). The least squares fits give a correlation coefficient of 0.692 for the left panel and -0.596 for the right panel in Figure 14.
The magnetic field turbulence, in association with the decreases in magnetic field and increases in helium count rates, may be an indication of plasma injection from the magnetosheath through high latitude reconnection. The bottom panel of Figure 9 suggests that a strong diamagnetic cavity can be produced at high latitude in the polar cusp. The anti-correlation between 1-200 keV/e helium intensities and the event-averaged (mean) field in the right panel of Figure 14 is consistent with the existence of a diamagnetic cavity. In Figure 14, since the dB2 term is proportional to the turbulent magnetic field energy density, the correlation between 1-200 keV/e helium counting rates and the dB2 in the left panel may be interpreted to mean that the turbulent magnetic energy density is converted into the helium ion's kinetic energy. This points to a resonant acceleration mechanism by the inductive electric field for the cusp helium ions as a strong possibility.
In brief, Figures 11, 12, 13 and 14 present direct observational evidence that the high-altitude dayside cusp is an acceleration region of the magnetosphere.
In addition to the CEP events, POLAR observed velocity-dispersed ions in the dayside high- L regions [Spence and Blake, 1997]. These energetic ions (up to 1 MeV) of unexplained origin were frequently found in the vicinity of the cusp and the dayside quasi-trapping region with time-energy-dispersion (TED) signatures [Fritz et al., 2000]. Figure 15 is an example, which shows three TED signatures at 1800-2040 UT (about 13.7-13.8 MLT and 75 o invariant latitude) [Fritz et al., 2000]. When POLAR passed the above region, the highest energy ions (approaching or exceeding one MeV in many cases) are seen first regardless of the direction of the satellite. The TED signature can last from a few minutes to hours. From March 1996 to May 1997, 802 TEDs have been observed [Karra and Fritz, 1999]. Figure 16 is a histogram showing the frequency of occurrence of 802 TED signatures as a function of MLT [Karra and Fritz, 1999]. This figure reveals that most of the TEDs were found at 1200-1500 MLT. Such a local time distribution is similar to that of the CEP events (see Panel a of Figure 3). The other two similarities of the TEDs to the CEPs are the ion energies (up to MeV) and the event time intervals (minutes to hours) (see Figure 9). These three similarities seems to suggest a relationship between the TEDs and the CEPs. The TEDs most likely represent the leakage of the energetic particles from the cusp trap due to the decay of the diamagnetic cavity associated with individual CEP events; the largest energy ions come out first giving rise to the TED signatures.
As mentioned before, the cusp is connected to the nightside equatorial plane through the Shabansky orbits. While such orbits exist only for a narrow range of radial distances when the charged particle starts from the nightside equatorial plane, the converse is not true. If the charged particles start in the cusp they have almost complete access to the equatorial plasma sheet and outer magnetosphere. Figure 17, as a preliminary attempt, compares the proton fluxes measured in the two different regions over energy range of 20 eV to 2 MeV. In Figure 17, the solid circles represent the proton flux observed by the Applications Technology Satellite (ATS 6) [Fritz and Cessna, 1975] at L = 6.6 near the midnight meridian for equatorially mirroring particles during a substorm period on July 20, 1974 [Fritz et al., 1977], and the open symbols (diamonds, triangles and squares) represent the proton flux from a combination of POLAR sensors during the CEP event period. This is the same ATS 6 spectrum that was used by Spjeldvik and Fritz  as their source spectrum in their radial diffusion modeling. The interesting point is that the proton fluxes measured in the cusp were comparable to those observed at the nightside geostationary orbit in the inner magnetosphere.
These two examples are very suggestive and reveal a direction for investigating the implications of the CEP events to the charged particles within the magnetosphere. Of course, to reach a correct conclusion more studies and analyses are needed.
The CAMMICE and CEPPAD sensors on board POLAR spacecraft have observed a new magnetospheric phenomenon called a CEP event. The events were detected in the dayside cusp for hours with an individual event lasting for tens of minutes, in which the measured ions had energies up to 8 MeV. All of these events were associated with a dramatic decrease and large fluctuations in the local magnetic field strength. Our principal conclusions are the following:
(1) The energetic electrons have been observed during the CEP event periods. However, the enhancement of the energetic electron fluxes are much less than that of energetic ion fluxes.
(2) Simultaneous observations have indicated that the ion fluxes in the CEP events were higher than that in both upstream and downstream from the bow shock, and the shape of the ion energy spectra in the CEP events is different from that in either the upstream or the downstream. Therefore, bow shock acceleration alone cannot explain the measured ion flux in the CEP events.
(3) The turbulent power of the local magnetic field in the ultra-low frequency (ULF) ranges is correlated with the intensity of the energetic ions. Such ULF ranges correspond to periods of about 0.33-500 seconds that cover the gyroperiods, the bounce periods, and the drift periods of tens of keV to MeV charged particles when they were temporarily confined in the high-altitude dayside cusp.
(4) These observations represent a discovery that the high-altitude dayside cusp is a new acceleration and dynamic trapping region of the magnetosphere.
(5) Energetic ions were also measured in the vicinity of the cusp and the dayside quasi-trapping region with a time-energy-dispersion (TED) signature. Both the TEDs and the CEPs were found to peak around 1400 MLT. Such a similarity seems to suggest a relation between the TEDs and CEPs.
(6) Trajectory calculations indicate that the cusp geometry is connected via gradient and curvature drift of these energized ions to the equatorial plasma sheet as close as the geostationary orbit at local midnight.
(7) The dayside cusp is potentially an important source of magnetospheric particles.
Aggson, T. L., J. P. Heppner, and N. C. Maynard, Observations of large magnetospheric electric fields during the onset phase of a substorm, J. Geophys. Res., 88, 3981, 1983.
Antonova, A. E., and V. P. Shabansky, Particle and magnetic field in the outer dayside geomagnetosphere, Geomagn. Aeron., 15, 2, 297, 1975.
Bieber, J. W., J. Chen, W. H. Matthaeus, C. W. Smith, and M. A. Pomerantz, Long-term variations of interplanetary magnetic field spectra with implications for Cosmic Ray Modulation, J. Geophys. Res., 98, 3585, 1993.
Blake, J. B., et al., CEPPAD experiment on POLAR, Space Sci. Rev., 71, 531, 1995.
Chapman, S., and V. C. A. Ferraro, A new theory of magnetic storms, J. Geophys. Res., 36, 171, 1931.
Chen, J., Long-term modulation of cosmic rays in interplanetary magnetic turbulence, Ph.D. thesis, Univ. of Del., Newark, 1989.
Chen, J., and T. A. Fritz, Correlation of cusp MeV helium with turbulent ULF power spectra and its implications, Geophys. Res. Lett., 25, 4113, 1998.
Chen, J., and T. A. Fritz, May 4, 1998 storm: observations of energetic ion composition by POLAR, Geophys. Res. Lett., 26, 2921, 1999a.
Chen, J., and T. A. Fritz, Origins of energetic ions near the compressed magnetosphere, EOS Trans. AGU, 80, No. 46, F862, 1999b.
Chen, J., D. Chenette, T. G. Guzik, M. Garcia-Munoz, K. R. Pyle, Y. Sang, and J. P. Wefel, A model of solar energetic particles for use in calculating LET spectra developed from ONR-604 Data, Adv. Space Res., 14 (10), 675, 1994.
Chen, J., T. A. Fritz, R. B. Sheldon, H. E. Spence, W. N. Spjeldvik, J. F. Fennell, and S. Livi, A new, temporarily confined population in the polar cap during the August 27, 1996 geomagnetic field distortion period. Geophys. Res. Lett., 24, 1447, 1997a.
Chen, J., T. A. Fritz, R. B. Sheldon, and H. E. Spence, The high altitude dayside cusp: A new trapping and acceleration region of the magnetosphere, EOS Trans. AGU, 78, No. 46, F592, 1997b.
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, Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere, J. Geophys. Res., 103, 69, 1998.
Chen, J., T. G. Guzik, and J. P. Wefel, The 3 He/ 4 He ratios for solar energetic particle events during the CRRES mission, Astrophys. J., 442, 875, 1995.
Crooker, N. U., Dayside merging and cusp geometry, J. Geophys. Res., 84, 951, 1979.
Crooker, N. U., F. R. Toffoletto, and M. S. Gussenhoven, Opening the cusp, J. Geophys. Res., 96, 3497, 1991.
Delcourt, D. C., and J.-A. Sauvaud, Populating of the cusp and boundary layers by energetic (hundreds of keV) equatorial particles, J. Geophys. Res., 104, 22,635, 1999.
Delcourt, D. C., J.-A. Sauvaud, and A. Pedersen., Dynamics of single-particle orbits during substorm expansion phase, J. Geophys. Res., 95, 20,853, 1990.
Ellison, D. C., E. Mobius, and G. Paschmann, Particle injection and acceleration at Earth's bow shock: Comparison of upstream and downstream events, Astrophys. J., 352, 376, 1990.
Fairfield, D. H., Average and unusual locations of the Earth's magnetopause and bow shock, J. Geophys. Res., 76, 6700, 1971.
Farrell, W. M., and J. A. Van Allen, Observations of the Earth's polar cleft at large radial distances with the Hawkeye 1 magnetometer, J. Geophys. Res., 95, 20,945, 1990.
Fritz, T. A., and J. R. Cessna, ATS 6 NOAA low energy proton experiment, IEEE Trans. Aerospace Electron. Sys., AES 11 (6), 1145, 1975.
Fritz, T. A., and J. Chen, The cusp as a source of magnetospheric particles, Radiation Measurements, 30, 5, 599, 1999a.
Fritz, T. A., and J. Chen, Reply, Geophys. Res. Lett., 26, 1363, 1999b.
Fritz, T. A., J. Chen, R. B. Sheldon, H. E. Spence, J. F. Fennell, and S. Livi, C. T. Russell, and J. S. Pickett, Cusp energetic particle events measured by POLAR spacecraft, Phys. Chem. Earth (C), 24 (1-3), 135, 1999.
Fritz, T. A., J. Chen, and R. B. Sheldon, The role of the cusp as a source for magnetospheric particles: A new paradigm? Adv. Space Res., 25 (7-8), 1445, 2000.
Fritz, T. A., et al., Significant initial results from the environmental measurements experiment on ATS 6, NASA Technical Paper 1101, 34 pp., NASA Scientific and Technical Information Office, Washington, DC, 1977.
Fung, S. F., T. E. Eastman, S. A. Boardsen, and S.-H. Chen, High-altitude cusp positions sampled by the Hawkeye satellite, Phys. Chem. Earth, 22, 653, 1997.
Gloeckler, G., et al., Solar wind carbon, nitrogen and oxygen abundances measured in the Earth's magnetosheath with AMPTE/CCE, Geophys. Res. Lett., 13, 793,
Haerendel, G., G. Paschmann, N. Sckopke, H. Rosenbauer, and P. C. Hedgecock, The frontside boundary layer of the magnetosphere and the problem of reconnection, J. Geophys. Res., 83, 3195, 1978.
Hesse, M., and J. Birn, On dipolarization and its relation to the substorm current wedge, J. Geophys. Res., 96, 19,417, 1991.
Hovestadt, D., B. Klecker, G. Gloeckler, F. M. Ipavich, and M. Scholer, Survey of He + /He 2+ abundance ratios in energetic particle events, Astrophys. J., 282, L39, 1984.
Karra, M., and T. A. Fritz, Energy dispersion features in the vicinity of the cusp, Geophys. Res. Lett., 26, 3553, 1999.
Krimigis, S. M., D. Venkatesan, J. C. Barichello, and E. T. Sarris, Simultaneous measurements of energetic protons and electrons in the distant magnetosheath, magnetotail, and upstream in the solar wind, Geophys. Res. Lett., 5, 961, 1978.
Lee, M. A., Coupled hydromagnetic wave excitation and ion acceleration upstream of the Earth's bow shock, J. Geophys. Res., 87, 5063, 1982.
Lee, M. A., G. Skadron, and L. A. Fisk, Acceleration of energetic ions at the Earth's bow shock, Geophys. Res. Lett., 8, 401, 1981.
Lezniak, T. W., and J. R. Winckler, Experimental study of magnetospheric motion and the acceleration of energetic electrons during substorms, J. Geophys. Res., 75, 7075, 1970.
Lopez, R. E., D. G. Sibeck, R. W. McEntire, and S. M. Krimigis, The energetic ion substorm injection boundary, J. Geophys. Res., 95, 109, 1990.
Lundin, R., Plasma composition and flow characteristics in the magnetospheric boundary layers connected to the polar cusp, in The Polar Cusp, edited by J. A. Holtet and A. Egeland, pp. 9-32, D. Reidel, Hingham, Mass., 1985.
Ma Sung, L. S., G. Gloeckler, D. Hovestadt, Ionization states of heavy elements observed in the 1974 May 14-15 anomalous solar particle event, Astrophys. J., 245, L45, 1981.
Marklund, G. T., L. G. Blomberg, C.-G. F a lthammar, R. E. Erlandson, and T. A. Potemra, Signatures of the high-altitude polar cusp and dayside auroral regions as seen by the Viking electric field experiment, J. Geophys. Res., 95, 5767, 1990.
Mason, G. M., G. Gloeckler, D. Hovestadt, Temporal variations of nucleonic abundances in solar flare energetic particle events. I. Well-connected events, Astrophys. J., 267, 844, 1983.
Mead, G. D., Deformation of geomagnetic field by the solar wind, J. Geophys. Res., 69, 1181, 1964.
Meng, C.-I., Latitudinal variation of the polar cusp during a geomagnetic storm, Geophys. Res. Lett., 9, 60, 1982.
Menietti, J. D., and J. L. Burch, Spatial extent of the plasma injection region in the cusp-magnetosheath interface, J. Geophys. Res., 93, 105, 1988.
Newell, P. T., and C.-I. Meng, Cusp width and Bz : Observations and conceptual model, J. Geophys. Res., 92, 13,673, 1987.
Newell, P. T., and C.-I. Meng, Ion acceleration at the equatorward edge of the cusp: Low altitude observations of patchy merging, Geophys. Res. Lett., 18, 1829, 1991.
Quinn, J. M., and D. J. Southwood, Observation of parallel ion energization in the equatorial region, J. Geophys. Res., 87, 10,536, 1982.
Reiff, P. H., Low-altitude signatures of the boundary layers, in Magnetospheric Boundary Layers, Eur. Space Agency Spec. Publ., ESA SP-148, 167, 1979.
Reiff, P. H., T. W. Hill, and J. L. Burch, Solar wind plasma injection at the dayside magnetospheric cusp, J. Geophys. Res., 82, 479, 1977.
Russell, C. T., R. C. Snare, J. D. Means, D. Pierce, D. Dearborn, M. Larson, G. Barr and G. Le, The GGS/POLAR magnetic fields investigation, Space Sci. Rev., 71, 563, 1995.
Sandahl, I., R. Lundin, M. Yamauchi, U. Eklund, J. Safrankova, Z. Nemecek, K. Kudela, R. P. Lepping, R. P. Lin, V. N. Lutsenko, and J.-A. Sauvaud, Cusp and boundary layer observation by INTERBALL, Adv. Space Res., 20, 823, 1997.
Sarris, E. T., S. M. Krimigis, C. O. Bostrom, and T. P. Armstrong, Simultaneous multispacecraft observations of energetic proton and electron bursts inside and outside the magnetosphere, J. Geophys. Res., 83, 4289, 1978.
Shabansky, V. P., Some processes in the magnetosphere, Space Sci. Rev., 12, 299, 1971.
Sheldon, R. B., H. E. Spence, J. D. Sullivan, T. A. Fritz, and J. Chen, The discovery of trapped energetic electrons in the outer cusp, Geophys. Res. Lett., 25, 1825, 1998.
Spence, H. E., J. B. Blake, First observations by the CEPPAD imaging proton spectrometer aboard POLAR, Adv. Space Res., 20 (4-5), 933, 1997.
Spjeldvik, W. N., and T. A. Fritz, Composition of hot plasmas in the inner magnetosphere: Observations and theoretical analysis of protons, helium ions, and oxygen ions, In Space Research VIII, edited by M. J. Rycroft and A. C. Strickland, 317 pp., Oxford, Eng., Pergamon Press, 1978.
Trattner, K. J., S. A. Fuselier, W. K. Peterson, and S.-W. Chang, Comment on: "Correlation of cusp MeV helium with turbulent ULF power spectra and its implications'', Geophys. Res. Lett., 26, 1361, 1999.
Webber, W. R., E. C. Roelof, F. B. McDonald, B. J. Teegarden, and J. Trainor, PIONEER 10 measurements of the charge and energy spectrum of solar cosmic rays during 1972 August, Astrophys. J., 199, 482, 1975.
West, Jr., H. I., and R. M. Buck, Observations of > 100-keV protons in the Earth's magnetosheath, J. Geophys. Res., 81, 569, 1976.
Williams, D. J., R. W. McEntire, C. Schlemm II, A. T. Y. Lui, G. Gloeckler, S. P. Christon, and F. Gliem, GEOTAIL energetic particles and ion composition instrument, J. Geomagn. Geoelectr., 46, 39, 1994.
Yamauchi, M., H. Nilsson, L. Eliasson, O. Norberg, M. Boehm, J. H. Clemmons, R. P. Lepping, L. Blomberg, S.-I. Ohtani, T. Yamamoto, T. Mukai, T. Terasawa, and S. Kokubun, Dynamic response of the cusp morphology to the solar wind: A case study during passage of the solar wind plasma cloud on February 21, 1994, J. Geophys. Res., 101, 24,675, 1996.
Zhou, X.-W. and Russell, C. T., The location of the high-latitude polar cusp and the shape of the surrounding magnetopause, J. Geophys. Res., 102, 105, 1997.