Pick-Up ion Distributions in the Inner and Middle Saturnian Magnetosphere

Abstract

Based on the entire dataset collected by the Cassini Plasma Spectrometer, we provide a comprehensive picture of the pitch angle (PA) and velocity distributions of pick-up ions (PUIs) in Saturn's inner and middle magnetosphere. We investigate the dependence of these distributions on Saturnian Local Time and magnetic latitude. We also search for correlations with the signatures of ion cyclotron waves (ICWs) observed by the Cassini magnetometer. Our survey reveals that ion PA distributions have a pancake shape and their full width increases monotonically with magnetic latitude. This increase in angular width is anti-correlated with the observed amplitudes of ICWs that are generated during the thermalization of the PUI distribution. We find no evidence of the previously observed, non-monotonic change of wave amplitudes with magnetic latitude mapping into the width of the PA distributions. This suggests that only a small fraction of the energy deposited into the waves is transferred back to the ions to broaden the distribution. We find that the PA scattering time is several times the bounce period, meaning PUIs become PA scattered only after completing several cycles of bounce motion and, hence migrating to higher latitudes by the time they become more isotropic. When moving away from Saturn's magnetic equatorial plane, the observed half-width of the velocity distributions does not evolve appreciably with latitude and $L$ shell value. This behavior changes only outside the orbit of Rhea where the observed velocity distributions begin to broaden due to elevated plasma temperatures.