Characteristics and Occurrence of Magnetosonic Waves in Plasmaspheric Plumes

Magnetosonic waves, or equatorial noise, are whistler-mode emissions distributed near the Earth's magnetic equator between proton cyclotron frequency and lower hybrid resonance frequency. Their origin and characteristics inside and outside the plasmasphere have been investigated due to their potential role in scattering energetic electrons and protons. However, their characteristics in plasmaspheric plumes remained undocumented. This study, for the first time, statistically investigated magnetosonic waves in the plasmaspheric plumes based on the entire mission period (2012-2019) of Van Allen Probes A and B. Results showed that the occurrence rate of magnetosonic waves in plumes was 25%, with an average amplitude and wave normal angles of 44 pT and 84 degrees-88 degrees, respectively. Increased geomagnetic activity enhanced its amplitude and ratio of plasma frequency to electron cyclotron frequency. Approximately 78% of selected magnetosonic wave events were simultaneously observed with plasmaspheric hiss that is most effective in scattering electrons in the plume.