Effects of Superthermal Plasmas on Hiss Wave-Driven Scattering Loss of Radiation Belt Electrons

Abstract

Plasmaspheric hiss plays an important role in the loss of radiation belt electrons via cyclotron resonant interactions. The cold plasma approximation is widely used in the evaluation of hiss-driven electron losses, which however can break down during disturbed periods of geomagnetic storms and substorms. The kappa particle velocity distribution, characterized by a pronounced high-energy tail, is well-established to model the profile of superthermal plasma under disturbed geomagnetic conditions. In the present study, by calculating the electron bounce-averaged pitch angle diffusion coefficients with kappa plasma dispersion relations, we investigate the sensitivity of hiss-induced cyclotron-resonant electron scattering loss to the spectral index κ under a variety of superthermal plasma conditions. Our results demonstrate that, with increasing κ, the diffusion coefficients of ∼20–100 keV radiation belt electrons significantly decrease at lower pitch angles and increase at higher pitch angles. In contrast, for electrons at higher energies, the diffusion coefficients tend to increase at lower pitch angles and decrease at relatively higher pitch angles. We also find that decrease of L-shell and increase of α* and temperature anisotropy tend to weaken the hiss-driven pitch angle scattering efficiency of electrons at energies from tens to hundreds of keV with a dip at ∼30–50 keV, while the scattering of higher energy electrons can be enhanced. This study confirms the important role of superthermal plasmas in the hiss-driven electron loss processes and should be carefully incorporated in future modeling of radiation belt electron dynamics.