Cusp and Nightside Auroral Sources of O+ in the Plasma Sheet

Abstract

Energetic O+ outflow is observed from both the dayside cusp and the nightside aurora, but the relative importance of these regions in populating the plasma sheet and ring current is not known. During a storm on 16 July 2017, the Arase and MMS satellites were located in the near-earth and midtail plasma sheet boundary layers (PSBL). During the storm main phase, Arase and MMS both observe O+ in the lobe entering the PSBL, followed by a time period with energy-dispersed bursts of tailward-streaming O+. The ions at MMS are at higher energies than at Arase. Trajectory modeling shows that the ions coming in from the lobe are cusp origin, while the more energetic bursty ions are from the nightside aurora. The observed and simulated energies and temporal dispersion are consistent with these sources. Thus, both regions directly contribute O+ to the plasma sheet during this storm main phase.