1Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, United States
From 05 to 13 UT on May 4, 1998, the Dst index reached negative values of -123 nT to -216 nT, which characterized a major geomagnetic storm period. During this period, the magnetosphere was compressed; the POLAR spacecraft traveled in its outbound orbit from the radiation belts (crossing through an intense ring current in the equatorial plane at 05 UT) to the high altitude dayside cusp and crossed the magnetopause into the magnetosheath. The GEOTAIL spacecraft was located downstream from the bow shock in the magnetosheath near the equator and the WIND spacecraft was about 214 Re from the Earth. Many instances of in situ ion energization (to > 1 MeV) similar to the CEP (cusp energetic paxticle) structure in the normal cusp (Chen et al., GRL, 24, 1447, 19971 were observed during this magnetic storm. Simultaneous observations indicated that in the energy range of 110-400 keV the ion fluxes in the CEP-like events measured by POLAR were much higher than those downstream from the bow shock near the equator, and that no comparable ion fluxes were measured by WIND. Due to high solar wind density, POLAR was in the magnetosheath during the interval 09-12 UT; the MeV ions observed by POLAR in this time period are interpreted as leakage from the CEP events. These observations are consistent with previous results (Chen et al., JGR, 103, p.69, 1998) showing that the high-altitude dayside cusp is a new acceleration region of the magnetosphere.