Wave‐Based Quantification of Ion Composition in the Earth's Magnetosphere‐Ionosphere‐Coupling Region

Abstract Ion composition critically determines the basic kinetic characteristics of plasma and affects energy transmission of the magnetosphere. However, it exhibits significant variations with altitude, latitude, local time, longitude, and also with geomagnetic and solar activity. Equatorial noise (EN) emissions, a frequently observed electromagnetic wave in the inner magnetosphere, propagate across magnetic field lines, and thus can reach magnetosphere‐ionosphere‐coupling (MIC) region near the magnetic equator, where their evolution depends on local ion composition. Here we utilize polarization reversal of EN emissions observed near the perigees of Van Allen Probes to in turn derive the compositions of helium and oxygen ions and provide comprehensive ion composition distributions by statistical analysis of EN emissions. Comparisons with the International Reference Ionosphere model and in situ plasma measurements validate the reliability of the estimated results. These findings advance MIC plasma models, and our method provides an indirect tool for plasma diagnostics on other planets.