Assessing the Sources of the O⁺ in the Plasma Sheet

To study the average contributions of the cusp outflow through the lobes and of the nightside auroral outflow to the O + in the plasma sheet (PS), we performed a statistical study of tailward streaming O + in the lobes, plasma sheet boundary layer|the plasma sheet boundary layer (PSBL) and the PS, using MMS/Hot Plasma Composition Analyzer (HPCA) data from 2017 to 2020. Similar spatial patterns illustrate the entry of cusp‐origin O + from the lobes to the PS through the PSBL. There is an Y GSM ‐dependent energy pattern for the lobe O + , with low‐energy O + streaming closer to the tail center and high energy (1–3 keV) O + streaming near the flanks. Low energy (1–100 eV) O + from the nightside auroral oval is identified in the near‐Earth PSBL/PS with high‐density (>0.02 cm −3 ), and energetic (>3 keV) streaming O + with similar density (∼0.013 cm −3 ) is observed further out on the duskside of the PSBL/PS. The rest of the nightside auroral O + in the PSBL is mixed with O + coming in from the lobe, making it difficult to distinguish the source. We estimated the contributions of the different sources of H + and O + ions through the PS between 7 and 17 R E , using estimates from this work and data extracted from previous studies. We conclude that, during quiet times, the majority of the near‐Earth PS H + are from the cusps, the polar wind and Earthward convection from the distant tail. Similarly, while the O + in the same region has a mixed source, cusp origin outflow provides the highest contribution.