ULF Wave‐Driven Energization of Plasmaspheric Plume Ions During Multi‐Step Geomagnetic Storms

We report direct observations of the energization of plasmaspheric plume ions by ultralow frequency (ULF) waves during a multi‐step geomagnetic storm from 25–29 August 2015. Based on Van Allen Probes observations, we identify multiple plasmaspheric plume regions in the afternoon‐dusk sector during the storm, characterized by highly irregular density structures. Concurrently, the cold ( V) ion fluxes exhibit intermittent enhancements within these regions. A detailed analysis reveals that the ion flux variations result from the modulation of the ULF wave‐driven drift. Interestingly, the modulated ion fluxes appear at near pitch angles and occur only in high‐density plumes; no modulation signatures appear in low‐density plasma troughs. These observations strongly support that ULF waves can perpendicularly modulate plasmaspheric plume ions within high‐density regions via effects. The present results provide a deeper understanding of the interactions between ULF waves and plasmaspheric plume ions in the inner magnetosphere.