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

Abstract

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 $E\times B$ drift. Interestingly, the modulated ion fluxes appear at near $90°$ 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 $E\times B$ effects. The present results provide a deeper understanding of the interactions between ULF waves and plasmaspheric plume ions in the inner magnetosphere.