Reflection of low-frequency fast magnetosonic waves at the local two-ion cutoff frequency: Observation in the plasmasphere

We investigate the reflection of low-harmonic fast magnetosonic (MS) waves at the local two-ion cutoff frequency (fcutHe+). By comparing the wave signals of the two Van Allen Probes satellites, a distinct boundary where wave energies cannot penetrate inward are found in the time-frequency domain. The boundary is identified as the time series of local fcutHe+. For a certain frequency, there exists a spatial interface formed by fcutHe+, where the incident waves should be reflected. The waves with small incident angles are more likely to penetrate the thin layer where the group velocity reduces significantly and then slow down in a period of several to tens of seconds before the reflection process complete. The cutoff reflection scenario can explain the intense outward waves observed by probe A. These results of MS reflection at fcutHe+ may help to predict the global distribution of MS waves and promote the understanding of wave-particle dynamics in the radiation belt.