Resonant Scattering of Radiation Belt Electrons by Saturnian Z-Mode Waves

Abstract

We investigated the resonant interactions between Z-mode waves and radiation belt electrons at Saturn via constructing an empirical model of the frequency spectral and wave normal angle distributions of 5 and 20 kHz Saturnian Z-mode waves using Cassini observations. The results of the quasi-linear bounce-averaged diffusion coefficients show that Saturnian Z-mode waves efficiently scatter radiation belt electrons (with energies of up to several MeV), with the resonant pitch angle coverage narrowing with increasing L-shell. Compared with 20 kHz waves, 5 kHz wave-induced scattering occurs at higher electron energies with stronger efficiency. The results of Fokker-Planck simulations showed that Z-mode waves efficiently accelerate high-energy electrons at intermediate pitch angles, contributing to the formation of a butterfly electron pitch angle distribution. Our results further the current understanding of the role of Z-mode waves in the dynamics of Saturn's electron radiation belt.