Ion Energies Dominating Energy Density in the Inner Magnetosphere: Spatial Distributions and Composition, Observed by Arase/MEP-i

Abstract

We investigate the spatial distributions and composition of contributing energies, which we term an energy range that makes the dominant contribution to energy density, in the inner magnetosphere during the main phase of magnetic storms. We analyze data from the medium-energy particle experiments-ion mass analyzer (MEP-i) on board the Arase satellite during six magnetic storms in year 2017 with the SYM-H minimum smaller than −50 nT. The results show that the inner part (L ≤ 5) is dominated by relatively low-energy ions adiabatically transported from the plasma sheet by enhanced convection. The contributing energies are higher for O+ than for H+ at higher L shells (L > 5), particularly during the storms driven by coronal mass ejections. The results provide in situ evidence of the contribution from mass-dependent/selective acceleration processes associated with substorm activity to the buildup of the outer part the ring current.