Abstract
In the inner magnetosphere, fast magnetosonic waves (MS waves) 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 radiation belt. So, when a simultaneous observation of high-frequency EMIC (HFEMIC) waves, anisotropic low-energy protons, and MS waves was first reported, a chain of energy flow from MS waves to HFEMIC waves through proton heating was naturally proposed. In this study, we carry out a statistical analysis using Van Allen Probes data to provide deeper insights into this energy pathway. Our results show that the occurrence of HFEMIC waves exhibits good correlation with the enhanced flux and anisotropy of low-energy protons, but the correlation between the low-energy protons and the concurrent MS waves is rather poor. The latter result is given support by quasilinear diffusion analysis, indicating negligible momentum diffusion rates at sub-keV energies, unless MS wave frequency gets very close to the proton cyclotron frequency (which constitutes only a small number of the cases). The fact that the first chain of the coupling is statistically inconclusive calls for an alternative explanation for the major source of the low-energy anisotropic proton population in the inner magnetosphere.
