Ion composition variations in the plasma sheet observed by Cluster/RAPID

Abstract

Based on the energetic particle measurements obtained by Cluster/RAPID (Research with Adaptive Particle Imaging Detectors), ion composition variations in the plasma sheet have been investigated. By comparing observations during quiet and storm intervals, it is found that, in addition to the enhanced energy density for all ion species, the ratio of energetic O+ ions to protons shows an increase during geomagnetic active periods in the near‐Earth plasma sheet (−15 RE < X < −12 RE). However, the ratio shows a decrease in the middle plasma sheet (−19 RE < X < −15 RE). These observation results have been further confirmed by a statistical study of all the plasma sheet crossing events from 2001 to 2003. Observations also show that energetic particles embedded in the earthward high‐speed streams observed in the plasma sheet have an obvious low abundance of heavy ions compared with surrounding plasmas. It is implied that energetic O+ ions could be distributed in a limited region in the plasma sheet and much less ionosphere origin particles could reach the region beyond X = −15 RE. Bursty Bulk Flows (BBFs) could not supply more energetic O+ ions to the near‐Earth plasma sheet, whereas the ionospheric supplement, together with local acceleration processes, leads to an enhanced oxygen abundance during active times.