The Spatial Localization of Storm‐Time ULF Waves Due To Plasmaspheric Plumes

Abstract The generation and propagation of Ultra Low Frequency (ULF) waves are intrinsically coupled to the cold plasma population in the terrestrial magnetosphere. During geomagnetic storms, extreme reconfigurations of the cold plasma create a complex and dynamic system that drastically modifies this coupling. The extent and manner in which this coupling is affected remains an open question. In this report, we assess the coupling between ULF waves and cold plasmaspheric plumes during geomagnetic storms. We present a series of event studies of Van Allen Probes observations that show selected times that clear coupling between plumes and ULF waves was observed. For each event, we use inferred measurements of the cold plasma density during plume crossings, in combination with magnetic and electric field observations of ULF waves. The event studies show very different, and at times contrasting, wave behavior. This includes events where ULF waves appear to be spatially confined within plume structures. We discuss potential physical mechanisms that could generate the observed coupling. The results demonstrate that understanding cold plasma‐ULF wave coupling is a critical area of future investigations.