Quasilinear Diffusion of Protons by Equatorial Magnetosonic Waves at Quasi‐Perpendicular Propagation: Comparison With the Test‐Particle Approach

Abstract

AbstractAlthough some plasma waves exhibit the largest growth rate and amplitude at 90° wave normal angle (WNA), particle scattering by these waves in a quasilinear (QL) sense has seldom been examined previously. Using test‐particle calculation and QL theory and assuming a uniform background magnetic field, the present study investigates the proton scattering by equatorial fast magnetosonic waves (MSWs; a.k.a equatorial noise) with varying WNAs including 90°. Comparison with the diffusion coefficients in momentum space obtained from the test‐particle approach indicates that the QL diffusion coefficients due to MSWs are consistent with those estimated from the test‐particle method up to 90° WNA, provided that MSWs described conform to the usual QL theory assumptions. The test‐particle dynamics due to MSWs at 90° WNA are examined in detail. Although in the QL picture, perpendicularly propagating MSWs are only supposed to resonate with protons at integer harmonic frequencies, waves at slightly off‐integer harmonic frequencies as part of a narrowband discrete spectrum of incoherent MSWs play an important role in making the proton scattering stochastic. Considering the recent test‐particle result of bounce‐averaged resonance of energetic protons, non‐zero wave power at the WNAs ≳89.5° typically excluded in QL diffusion can be important for ring current proton dynamics.