Journal Article

Scattering of Radiation Belt Electrons by Fast Magnetosonic Waves: Considering the Kinetic Effects

Zhang, Shuai, Xiong, Ying, Yu, Xiongdong, Yao, Fei, Liu, Kaijun, Wang, Yan

2023

Overview

When assessing the scattering of radiation belt electrons by fast magnetosonic (MS) waves, it is traditionally assumed that the waves follow the MS/whistler branch of the cold plasma dispersion relation (CPDR) in magnetohydrodynamics. However, MS waves are essentially ion Bernstein modes following a distinct kinetic dispersion relation. This study calculates the MS wave‐induced electron diffusion rates with the kinetic dispersion relation for the first time and compares the results with that obtained with the CPDR. It is found that the kinetic effects lead to a lower minimum resonant energy around 100 eV and a broader resonant pitch angle range. Kinetic effects also result in power spectral density attenuation when transforming wave frequency spectra into wavenumber spectra, so the diffusion rates are overall smaller than the ones obtained using the CPDR. Our results demonstrate that kinetic effects can significantly affect the role that MS waves play in the radiation belt dynamics. Plain Language Summary Magnetosonic (MS) waves belong to the kinetic ion Bernstein modes essentially. But when the cold plasma is dominating, the waves also approximately follow the MS/whistler branch of the cold plasma dispersion relation (CPDR) in magnetohydrodynamics. Subsequently, studies of the electron scattering by MS waves have traditionally assumed the CPDR for simplicity. Motivated by recent studies which involved both satellite observations and kinetic theory revealing that the lower harmonic MS waves clearly follow the kinetic dispersion relation, we assess how the differences between the kinetic and cold plasma dispersion relations affect the MS wave‐induced electron scattering rates. Our results indicate that the kinetic dispersion relation produces relatively lower parallel phase speeds for MS waves, leading to a lower minimum resonant energy and subsequently a broader resonant pitch angle range for electrons (of a given energy). The kinetic effects also decrease the overall diffusion rates by attenuating the wave power spectral density in wavenumber space when mapped from a given frequency spectrum. Key Points Linear kinetic dispersion relation indicates lower phase speeds and a broader range of group speeds for fast magnetosonic (MS) waves The lower phase speeds of MS waves result in a broader range of resonant pitch angles and lower minimum resonant energies of electrons Kinetic effects reduce the wavenumber power spectral density and thus result in smaller electron diffusion rates

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Cold / cold plasma dispersion relation / Cold plasmas / Diffusion / Diffusion rate / Dispersion / Electron diffusion / Electrons / Energy / fast magnetosonic waves / Frequency spectra / Frequency spectrum / group speed / kinetic dispersion relation / Kinetic theory / Magnetohydrodynamics / phase speed / Phase velocity / Pitch (inclination) / Plasma / Power spectral density / quasi‐linear diffusion coefficient / Radiation / Radiation belt electrons / Radiation belts / Satellite observation / Scattering / Scattering of radiation / Spectra / Wave attenuation / Wave frequency / Wave power / Wavelengths / Waves