Combined Scattering of Concurrent Unusual High‐Frequency EMIC Waves and MS Waves on Radiation Belt Electrons

We report a concurrent event of unusual high-frequency Electromagnetic ion cyclotron (EMIC) and Magnetosonic (MS) waves in correspondence with butterfly distributions for sub-MeV electrons and flux decay of MeV electrons, observed by Van Allen Probe A on 16 October 2017. The scattering effects are quantitatively investigated by test particle simulations. The results indicate that the unusual EMIC waves mainly scatter electrons from hundreds of keV to several MeV at pitch angles 60 degrees. Their combined scattering effects are suitably analyzed to be justified as the linear superposition of the effect induced by each wave mode. The Fokker-Planck simulations exhibit the formation of butterfly pitch angle distributions for sub-MeV electrons and the obvious flux decay for electrons similar to 1.8 MeV at all pitch angles, showing similar variation trends to the observations. Our study suggests that the occurrence of the two waves can significantly influence the electrons over the whole pitch angle range, which can help us better understand the radiation belt dynamics.