Change of energetic ion composition in the plasma sheet during substorms

It I-las been reported by previous studies that the energetic particle flux of ions of ionospheric origin like O+ ions is more enhanced than that of Hf ions in the near-Earth tail (X similar to-6 to -16 R-E) during substorms. To explain this strong O+ flux enhancement, some studies have surmised that thermal O+ ions in the plasma sheet boundary layer or the lobe are strongly accelerated at the magnetic reconnection region (X similar to-20 to -30 R-E), and are subsequently transported into the near-Earth plasma sheet; with earthward plasma flows. However, other studies have supposed that the strong O+ flux enhancement is caused by local magnetic field reconfiguration (local dipolarization). In the present study, we used Geotail/EPIC measurements of energetic (60 keV to 3.6 MeV) ion flux to test the above two scenarios. We investigated ion composition in the plasma sheet while earthward plasma flows and/or dipolarization signatures were observed. In terms of energy density ratio of oxygen ions to protons, the observational results can be summarized as follows: (1) earthward plasma flows without dipolarization signatures did not accompany large increases of the ratio in most cases; (2) when earthward plasma flows appeared with dipolarization signatures, they accompanied increases of the ratio; and (3) most of dipolarization events were associated with large increases of the ratio. These results suggest that the strong increase in the energetic oxygen constituent in the near-Earth plasma sheet is due to acceleration of ions during dipolarization, consistent with the latter scenario.