Features of energetic ions near the compressed magnetosphere

A possible cusp acceleration mechanism has been investigated during a major geomagnetic storm on May 4, 1998 when the magnetosphere was compressed and eroded. At 5:00-12:05 UT, the WIND spacecraft was about 213R(E) upstream f rom the Earth, the GEOTAIL was in the post dusk magnetosheath, and the POLA R traveled in its outbound orbit from the equatorial radiation belt to the high-altitude dayside cusp and crossed the magnetopause into the magnetoshe ath. Many instances of in situ ion energization (to > MeV) were observed by POLAR during this period. Simultaneous observations indicated that no comp arable flux was observed by WIND. Ion fluxes measured by POLAR were higher than that measured by GEOTAIL, indicating ion source regions near or within the magnetosphere. The measured ions show some interesting features: (I)In the radiation belt, in the magnetopause boundary layer, and in the dayside cusp, most 1-200 keV/e fluxes were around pitch angles of 90 degrees; whil e in the magnetosheath, the fluxes came from a broad hemispherical directio n between similar to sunward and earthward. (2) At around 6:00 UT, the MeV ion flux was lower in the magnetosheath than in the adjacent magnetosphere. (3) There were two CEP (cusp energetic particle) events with 2-3 orders of magnitudes enhancements of MeV ion fluxes. (4) The MeV ions in one CEP eve nt had a peak flux higher than that of the intense outer radiation belt in the equatorial plane. (S)The 100 keV/e O<+3 ions which originated from the ionosphere were observed in both the outer cusp and the magnetopause bounda ry layer. (6) There were no large magnetic field fluctuations upstream befo re the bow shock. These observations suggest a likely source of substantial particle energization exists in the cusp region. (C) 2001 Elsevier Science Ltd. All rights reserved.