A parametric study on outer radiation belt electron evolution by superluminous R‐X mode waves

Abstract

A parametric study is presented on the temporal evolution of the phase space density (PSD) of the outer radiation belt energetic electrons driven by the superluminous right‐hand extraordinary (R‐X) mode waves at the location L = 4.5. Bounce‐averaged diffusion rates in pitch angle and momentum are calculated by varying the peak wave frequency, the wave normal angle distribution, and the wave latitudinal distribution. Those diffusion rates are used as inputs to solve a 2‐D momentum‐pitch angle diffusion equation. In particular, three cases are considered: momentum diffusion rates alone, momentum +pitch angle diffusion rates, and momentum +pitch angle +cross diffusion rates. Numerical results show that at 24 h, electron PSDs can enhance substantially for 1 MeV energy at higher pitch angles. Momentum diffusion dominates the dynamic evolution of energetic electrons, whereas the contribution of pitch angle or cross‐diffusion rates is insignificant using the specified wave model. In addition, PSD evolutions are sensitively dependent on the assumed different wave normal angle distributions and tend to be located in lower pitch angles when wave normal angles move to smaller regions. Diffusion coefficients and PSD evolution are largely determined by the wave latitudinal distributions. High‐latitude R‐X mode waves primarily contribute to pitch angle scattering of energetic electrons, whereas equatorial (or lower latitude) R‐X mode waves yield efficient acceleration of electrons. This result supports the previous findings that superluminous R‐X mode waves potentially contribute to dramatic variation in the outer radiation belt electron dynamics under appropriate conditions.