Energy Coupling from Magnetosonic Waves to High-Frequency Electromagnetic Ion Cyclotron Waves: Statistical Analysis

In the inner magnetosphere, fast magnetosonic waves (MSWs) are known to resonantly interact with ring current protons, causing these protons to gain energy preferentially in the direction perpendicular to the background magnetic field. An anisotropic distribution of enhanced ring current protons is a necessary condition to excite electromagnetic ion cyclotron (EMIC) waves which are known to facilitate a rapid depletion of ultra-relativistic electrons in the outer Van Allen belt. So, when a simultaneous observation of high-frequency EMIC (HFEMIC) waves, anisotropic low-energy protons, and MSWs was first reported, a chain of energy flow from MSWs to HFEMIC waves through proton heating was naturally proposed. Here, we carry out a statistical analysis using Van Allen Probes data to provide further insights into this energy pathway. Our results show that the occurrence of HFEMIC waves exhibits good correlation with enhanced flux and anisotropy of low-energy protons which is marginally stable to the HFEMIC instability. In contrast, the correlation between the low-energy protons and the occurrence and power of MSWs is rather poor, casting doubt on the first chain of energy flow. Furthermore, quasilinear diffusion analysis indicates that MSWs are not very effective for low-energy proton heating for most of the cases. We suggest that an alternative explanation for the origin of these protons in the inner magnetosphere is in need.