Radial dependence of O+ and direct entry into the near-Earth plasma sheet (5 to 26 RE)

Abstract:

The plasma sheet is populated from the ionosphere from one of two sources, either the dayside cusp or the nightside aurora. Ions sourced from the cusp flow over the polar cap and into the lobes before accessing the plasma sheet, while ions from the aurora have direct access to the plasma sheet. These nightside auroral ions exhibit temporal dispersions consistent with time-of-flight from the ionosphere (a 100 eV O⁺ ion takes 18 minutes to travel to 6 R_E, whereas a 1 keV ion will take 6 minutes). Dispersed signatures exhibited by nightside auroral ions are often observed following magnetic dipolarization events, suggesting a burst of outflow following substorm onset. Since field lines convect toward the neutral sheet and earthward in the nightside plasma sheet, lower energy ions are expected to be observed closer to the neutral sheet (and accordingly, Earth) because they take longer to transport up the field line. Accordingly, the energy range of dispersed signatures from the nightside auroral source has a radial dependence, with minimum energy increasing with radial distance. We identify substorm onsets through observation of dispersionless ion injections from the SOPA instrument on Los Alamos National Laboratory geosynchronous satellites in conjunction with magnetometer data from MMS and Van Allen Probes (RBSP). By comparing these observations with dispersed O⁺ and H⁺ signatures from the HOPE instrument on Van Allen Probes (L=5-6.5) and HPCA instrument on MMS (apogee 12 R_E for 2016 and 26 R_E for 2017 tail seasons), we show the radial dependence of ionospheric outflow within the near-Earth plasma sheet and correlation with substorm activity.