Cassini Observation of Relativistic Electron Butterfly Distributions in Saturn’s Inner Radiation Belts: Evidence for Acceleration by Local Processes

Abstract

AbstractThe morphology of electron pitch angle distributions (PADs) helps to identify dynamic processes in Saturn’s magnetosphere. Previous studies demonstrated convective transport being important for relativistic electron acceleration at L > 4 in the inner magnetosphere, whereas closer to Saturn the situation is not as well established. We have investigated the PADs of relativistic electrons throughout radiation belts, using 13‐years measurements from Cassini spacecraft. Our results show that at 3.5 < L < 6, the PADs peak near 90°. However, at 2.5 < L < 3.5, the PADs transform to butterfly distributions, dominating over an energy range of one order of magnitude, from 800 keV to >7 MeV. These results suggest that convective transport continues to govern down to L = 3.5. For even smaller L‐shells, the independence of butterfly PADs from energy suggests that the acceleration by resonant interactions of electrons with waves likely serves as an additional key mechanism regulating the dynamics in the core region of radiation belts.