Ion composition of the near-Earth plasma sheet in storm and quiet intervals: Geotail/EPIC measurements

We investigate the ion composition of the near-Earth plasma sheet in storm and quiet intervals, using energetic (9–210 keV) particle flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. In 1998 four magnetic storms (minimum Dst −20 nT. The energy density of the H+, He+, and O+ ions was computed from the EPIC/STICS data for these storm and quiet-time events. We obtained the following results: (1) The energy density is higher during storms than during quiet times for all ion species (H+, He+, and O+); (2) the He+/H+ energy density ratio during storms is 0.01–0.02, while that during quiet times is ∼0.01; and (3) the O+/H+ energy density ratio is significantly larger during storms (0.2–0.6) than during quiet times (0.05–0.1). To explain these results we suggested a current sheet acceleration mechanism in which ions are energized by the dawn-to-dusk convection electric field in a mass-dependent way in the course of interaction with the current sheet.