Oxygen Pickup Acceleration in the Plasma Mantle Associated With Traveling Flux‐Rope Structures

Abstract It is well known that, in the high‐altitude cusp and mantle regions, ionospheric‐origin outflowing oxygen (O + ) ions are often energized through non‐adiabatic processes, among which cyclotron resonance is considered a major mechanism. Here, based on Cluster observations, we present evidence that pickup acceleration also operates as a non‐adiabatic energization process in the plasma mantle. Abrupt O + energization from ∼1 keV to several tens of keV was observed within a tailward‐convecting flux rope and the traveling compression region surrounding another flux rope, both likely generated by high‐latitude magnetopause reconnection. In both cases, the O + velocity distribution functions exhibit signatures consistent with pickup acceleration. These reconnection‐related structures can trigger non‐adiabatic pickup acceleration of O + ions by locally reducing magnetic scale lengths. These results suggest that localized reconnection‐related structures can provide an efficient pathway for transferring solar‐wind energy to ionospheric O + ions in the high‐altitude magnetosphere.