Resonant diffusion of energetic electrons by narrowband Z mode waves in Saturn's inner magnetosphere

Abstract

AbstractBanded emissions near 5 and 20 kHz are observed by the Cassini Radio and Plasma Wave Science (RPWS) instrument and are identified as Z mode waves in the regions where the plasma frequency is below the electron gyrofrequency. Using wave parameters derived from Gaussian fits to the measured power spectral density for narrow‐band Z mode waves and the ambient density and magnetic field information at L = ~4, we compute the bounce‐averaged diffusion coefficients for the Saturnian radiation belt electrons. Our results indicate that 5 kHz Z mode emissions can cause resonant scattering of electrons from ~2 MeV to tens of megaelectron volts, while 20 kHz waves can interact with electrons from hundreds of kiloelectron volts to ~2 MeV. In principle, the rate of momentum diffusion can be an order of magnitude larger than that of pitch angle diffusion and cross diffusion for electrons at equatorial pitch angles > ~10°, suggesting that Z mode waves on Saturn are responsible for local acceleration of radiation belt energetic electrons at intermediate pitch angles.