Electron energetics in the Enceladus torus

Abstract

The inner magnetosphere of Saturn contains both neutral gas and plasma whose composition is dominated by water group ion species. This gas is distributed in a torus-shaped region located near the orbit of Enceladus. The source of this gas is the predominately water vapor plume located in the south polar region of Saturn's icy satellite Enceladus. The electron distribution in the torus comprises two populations: a relatively cold (<3 eV) thermal population with a less dense, hot (>10 eV) suprathermal tail. This paper describes model calculations of the electron energy distribution and energetics in the torus in order to explore how the observed electron distributions can be explained. A thermal electron energy equation is numerically solved, and electron temperatures are calculated. A separate electron energy deposition model determines suprathermal electron fluxes as functions of energy. The main suprathermal electron population is due to photoelectron production from the absorption of solar radiation by water and other neutral species, and this is demonstrated by a comparison of the model results with recently published Cassini data. The model includes heating of thermalized electrons by fresh photoelectrons and by hot pickup ions and also includes cooling by collisions with water and other neutral species. Calculated electron temperatures for the thermal population are about 104 - 2 × 104 K (i.e., 1-2 eV), values which are somewhat lower than recently published Cassini Langmuir probe electron temperature values of 2-3 eV. Copyright 2011 by the American Geophysical Union.