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 < -50 nT) occurred when Geotail was located in the near-Earth plasma sheet (X greater than or equal to -10 R-E) For each of the storms, we have selected a col responding quiet interval from time periods when Geotail revisited the near-Earth plasma. sheet under the condition 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 similar to0.01; 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.