Impact of substorm time O + outflow on ring current enhancement

Energetic O + ions (tens of keV) rapidly increase in the inner magnetosphere and contribute significantly to the ring current during substorms. Previously, two source regions of the energetic O + ions have been proposed. The first one is the dayside polar region. Ions from the dayside polar region are transported to the lobe; then they are injected to the nightside plasma sheet during substorm expansion phase. The second one is the nightside aurora region. After the substorm onset, energetic O + ions are extracted from the ionosphere with the auroral acceleration processes, and the O + ions are directly supplied to the nightside plasma sheet. We investigated the relative importance of these two regions on supplying the energetic O + ions in the inner magnetosphere. We performed a test particle simulation in global MHD electromagnetic fields. We obtained the following results. (1) During the substorm growth phase, O + ions at tens of eV are extracted from the dayside polar region, resulting in the enhancement of the warm O + ions (hundreds of eV) in the lobe. After the substorm onset, the warm O + ions are nonadiabatically accelerated to tens of keV and injected to the inner magnetosphere. These O + ions contribute to most of the O + ring current. (2) O + ions at < a few keVs are supplied from the nightside aurora region to the plasma sheet. However, their contribution to the O + ring current remains small. From the results, we concluded that the main source of the energetic O + ions is the dayside polar region.