Johns Hopkins University Applied Physics Laboratory
Sources, sinks, and transport of energetic particles in Saturn's magnetosphere
Saturn's magnetosphere is filled with plasma. We analyze the high energy part of this plasma, namely protons and electrons at energies between several tens of keV and several tens of MeV. Measurements are used that were obtained by the MIMI/LEMMS instrument onboard the Cassini spacecraft during seven years in orbit. These data are averaged over the mission. We aim to understand the resulting distribution in space and energy. The inner part of the magnetosphere, which is the radiation belts, is successfully modeled. This allows constraining the parameters of the main processes. Several pieces of evidence suggest that the protons of the belts are tertiary particles from the interaction of cosmic rays with Saturn's atmosphere (CRAND). After production, they are subject to radial diffusion, and are lost at the moon orbits. Neutral gas, which has an unusually high abundance at Saturn compared to other planets, does not play a major role in this cycle. The middle magnetosphere, radially outward from the radiation belts, is also studied. A large variety of physical processes act there. We focus on a region seven Saturn radii away from the planet's center, where the strength of the processes is estimated. This includes a proper treatment of particles losing energy in macroscopic ice grains of the tenuous E ring, which was not done before. We find that energetic protons below 1MeV are lost via charge exchange with the neutral gas environment, not the E ring. Source processes, potentially related to injection events, are found to be active in this region and counter the losses.