Modeling of Cassini's charging at Saturn orbit insertion flyby

Abstract

[1] Spacecraft-plasma interactions are studied in the regime relevant for Cassini during the Saturn orbit insertion (SOI) flyby. Three-dimensional particle-in-cell self-consistent code has been employed to calculate the spacecraft potential in a complicated plasma environment of Saturn's magnetosphere for a wide range of distances (for dipole shells between L ∼ 4 and L ∼ 10). Plasma parameters derived from Cassini plasma spectrometer (CAPS) measurements at the SOI flyby have been used as input data. Modeling includes photoemission due to the solar UV radiation, two populations of ions, plasma flows and the flyby geometry. Numerical results highlight significant differences in the plasma conditions along the SOI trajectory. Outside Rhea's shell, at L ∼ 10, the positive spacecraft potential is primary due to the photoelectron production on the Sun-exposed surface of the orbiter. In the inner plasmasphere (L < 8) the spacecraft charges to a negative potential with values up to a few electron temperatures. The dependence of the potential versus distance from Saturn demonstrates behavior qualitatively similar to that deduced from the CAPS data acquired during the SOI period. Simulations display different spatial structures of the plasma particle densities and the self-consistent potential surrounding the SC at different distances from Saturn. Three plasma constituents (electrons, water group ions and protons) produce a new kind of plasma wake with a self-consistent separation between the plasma components in the electric field of the orbiter. Ion focusing occurs in the inner magnetosphere only for protons while the heavy water group ions always form a geometric wake downstream of the spacecraft. The main factor defining the spatial configurations of the potential and of the plasma density perturbations is the energy distribution of the surrounding plasmas. The obtained results can be of importance for understanding the main physical processes occurring in Saturn's magnetosphere, including dust-plasma interactions in the planetary rings, and for reliable interpretations of the electric field and plasma parameter measurements.