Wave Normal Angle Distribution of Magnetosonic Waves in the Earth's Magnetosphere: 2-D PIC Simulation

Abstract

Radiation belt electrons can be accelerated and scattered by magnetosonic waves in the Earth's magnetosphere, and the scattering rate of electrons is sensitive to the wave normal angle. However, observationally it is difficult to identify the wave normal angle within a few degrees. In this study, using 2-D particle-in-cell (PIC) simulations, we investigate the wave normal angle distribution of magnetosonic waves excited by ring distribution protons. Both the linear theory and simulations have shown that the wave normal angles are distributed over a narrow range (82 degrees -89 degrees) with a major peak at about 85 degrees during the linear growth stage when the proton ring velocity is close to the Alfven speed. In addition, 2-D PIC simulations further demonstrated that the waves tend to have larger wave normal angles (84 degrees -89 degrees) during the saturation stage since the waves with smaller wave normal angles are dissipated faster. It is also found that wave normal angles decrease with the increase of wave frequency. With the increase of the ring velocity of the proton ring distribution, the perpendicular wavenumber of excited magnetosonic waves decreases, which leads to the decrease of the wave normal angle. The simulation results provide a valuable insight to understand the property of magnetosonic waves, and the findings are useful for the global simulations of radiation belt dynamics. Key Points Using 2-D PIC simulations, the wave normal angle distribution of magnetosonic waves has been investigated The wave normal angles are distributed over a narrow range (82 degrees -89 degrees) during the linear growth stage The wave normal angles decrease with the increase of the ring velocity of the proton ring distribution