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 obtaine d by the suprathermal ion composition spectrometer (STICS) sensor of the en ergetic particle and ion composition (EPIC) instrument on the Geotail space craft. In 1998 four magnetic storms (minimum Dst < -50 nT) occurred when Ge otail was located in the near-Earth plasma sheet (X greater than or equal t o -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. sh eet under the condition Dst > -20 nT, The energy density of the H+, He+, an d O+ ions was computed from the EPIC/STICS data for these storm and quiet-t ime events. We obtained. the following results: (1) The energy density is h igher during storms than during quiet times for all ion species (H+, He+, a nd O+); (2) the He+/H+ energy density ratio during storms is 0.01-0.02, whi le that, during quiet times is similar to0.01; and (3) the O+/H+ energy den sity ratio is significantly larger during storms (0.2-0.6) than during quie t times (0.05-0.1). To explain these results we suggested a current sheet a cceleration mechanism in which ions are energized by the dawn-to-dusk conve ction electric field in a mass-dependent way in the course of interaction w ith the current sheet.