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

Electromagnetic ion cyclotron (EMIC) waves and Magnetosonic (MS) waves are commonly observed in the Earth's magnetosphere and play important roles in energetic electron dynamics. Usually, EMIC waves and MS waves scatter electrons at different energies and pitch angles. Recently, a different type of H+ band EMIC waves, named unusual high frequency EMIC waves have been reported. Studies on the unusual high frequency EMIC waves found that these waves are capable of scattering sub-MeV and MeV electrons which can also be influenced by MS waves. In this study, we report a concurrent event of unusual high frequency EMIC and 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° while the MS waves mainly influence sub-MeV and MeV electrons at pitch angles > 60°. 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 ~ 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.