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 213RE upstream from the Earth, the GEOTAIL was in the post dusk magnetosheath, and the POLAR traveled in its outbound orbit from the equatorial radiation belt to the high-altitude dayside cusp and crossed the magnetopause into the magnetosheath. Many instances of in situ ion energization (to ΒΏMeV) were observed by POLAR during this period. Simultaneous observations indicated that no comparable ux was observed by WIND. Ion uxes 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: (1) In the radiation belt, in the magnetopause boundary layer, and in the dayside cusp, most 1-200 keV=e uxes were around pitch angles of 90 β’ ; while in the magnetosheath, the uxes came from a broad hemispherical direction between βΌsunward and earthward. (2) At around 6:00 UT, the MeV ion ux 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 uxes. (4) The MeV ions in one CEP event had a peak ux higher than that of the intense outer radiation belt in the equatorial plane.
(5) The 100 keV=e O Β‘+3 ions which originated from the ionosphere were observed in both the outer cusp and the magnetopause boundary layer. (6) There were no large magnetic ΓΏeld uctuations upstream before the bow shock. These observations suggest a likely source of substantial particle energization exists in the cusp region.