AbstractIn Earth's inner magnetosphere low‐energy‐density plasma in the plasmasphere (PS) partially overlaps with higher‐energy plasma in the ring current (RC). Auroral emissions with zonally elongated arc‐like shapes called Stable Auroral Red (SAR) arcs occur in the subauroral latitudes that are connected to the inner magnetosphere. There has yet to be a statistical analysis of the PS‐RC overlap region in which SAR arcs occur. We conducted a statistical analysis of the occurrence characteristics of this PS‐RC overlap region for SAR arcs, using the RBSP‐B satellite over a 4‐year period from 1 January 2015 to 31 December 2018. We defined the PS‐RC overlap region based on ground‐satellite conjugate observations of SAR arcs on the same geomagnetic field line. The overlap region in which SAR arcs develop is located at pre‐midnight for high‐energy oxygen ions and post‐midnight for low‐energy hydrogen ions. Superposed epoch analysis using the SYM‐H index shows that the PS‐RC overlap occurs near the end of the main phase of magnetic storms for both H+ and O+ ions, indicating that the SAR arc source develops at the particle injection during the end of the storm's main phase. Superposed epoch analysis using the SML index revealed that several substorms occurred near the onset time of the SAR arc source for both ion species. This suggests that particle injection by substorms at the end of the magnetic storm is important for the formation of the SAR arc source.
