A comparison of magnetic field measurements and a plasma‐based proxy to infer EMIC wave distributions at geosynchronous orbit

Abstract

Wave‐particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earth's atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma‐based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single‐wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.