Interactions between magnetosonic waves and ring current protons: Gyroaveraged test particle simulations

Abstract

Gyroaveraged test particle simulations are implemented to quantitatively investigate interactions between linearly polarized magnetosonic waves (i.e., equatorial noises) and ring current protons inside and outside the plasmasphere at L = 4.5. For magnetosonic waves at the frequency of 33.3 Hz (f_w/f_cp = 6.4 at the magnetic equator, for L = 4.5), it is found that wave-particle interactions at the resonance order corresponding to the lowest resonant proton energy (i.e., N = 6) are dominant. The interactions at other resonance orders make much less contribution. Near the equatorial loss cone at L = 4.5, magnetosonic waves produce strongest proton pitch angle diffusion at 20 keV inside the plasmasphere and at 100 keV outside the plasmasphere, respectively, reaching a rate above 10^-6 s^-1. The corresponding energy diffusion dominates over pitch angle diffusion at high pitch angles; therefore, magnetosonic waves are likely to accelerate protons at a few keV inside the plasmasphere and at 10 keV outside the plasmasphere. Due to the emission equatorial confinement, the effect of transit time scattering also occurs for interactions of magnetosonic waves with ring current protons and tends to be increasingly important outside the plasmasphere, which is consistent with previous studies on interactions of magnetosonic waves with radiation belt electrons.