Multiple spacecraft observations of energetic ions during a major geomagnetic storm Original Research Article

Orders of magnitudes increases of MeV ion intensities were measured by the POLAR spacecraft near the dayside magnetopause/magnetosheath boundary layers on May 4, 1998. From 6:00 to 12:30 UT, the Dst index reached negative values of -123 nT to -216 nT, which characterized a major geomagnetic storm period. During this period, the WIND spacecraft was about 213 RE upstream from the Earth, the GEOTAIL was in the post dusk magnetosheath, the INTERBALL-Tail was near the bow shock, and the POLAR traveled in its dayside outbound orbit from the high altitude cusp to the magnetosheath boundary layer. No comparable energetic particle flux was observed by WIND. The energetic ion fluxes measured by POLAR in upstream of the cusp were higher than that measured by both INTERBALL and GEOTAIL. The flux variations at Polar and Interball were similar, indicating a common ion source. Furthermore, INTERBALL responded to the variations later than POLAR, suggesting POLAR was closer to the source. Simultaneous multiple spacecraft observations thus suggest a likely source of substantial particle energization exists in the cusp region.