Change of energetic ion composition in the plasma sheet during substorms

It has been reported by previous studies that the energetic particle flux of ions of ionospheric origin like O+ ions is more enhanced than that of H+ ions in the near‐Earth tail (X ∼ −6 to −16 RE) 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 ∼ −20 to −30 RE), 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.