Plasmaspheric‐Origin Ions as a Major Source of Plasma Sheet Pressure During Storm Early Main Phase

Abstract The plasmasphere and plasma sheet are physically distinct but dynamically connected through global magnetospheric processes. Using a multifluid magnetohydrodynamic model including solar wind, ionospheric, and plasmaspheric H + and O + components, coupled with ring current and polar wind models, we simulate the 8 September 2017 storm. The modeled plasmaspheric density closely matches RBSP observations on the dayside. Plasmaspheric‐origin H + contributes significantly to plasma sheet mass density throughout the storm and becomes a major pressure source during the early main phase, accounting for up to 60% of the total pressure in localized regions. This pressure dominance occurs when pre‐existing plasmaspheric ions, accumulated in the dawnside plasma sheet due to asymmetric erosion, are energized by storm‐time processes. As the plasmasphere becomes depleted and refilling lags behind convection, this contribution diminishes. These results reveal a previously underappreciated role of plasmaspheric‐origin ions in pressure dynamics and provide new insight into storm‐time mass and energy transport.