Preliminary results on Saturn's inner plasmasphere as observed by Cassini: Comparison with Voyager

Abstract

We present an analysis of Saturn's inner plasmasphere as observed by the Cassini Plasma Spectrometer (CAPS) experiment during Cassini's initial entry into Saturn's magnetosphere when the spacecraft was inserted into orbit around Saturn. The ion fluxes are divided into two sub‐groups: protons and water group ions. We present the relative amounts of these two groups and the first estimates of their fluid parameters: ion density, flow velocity and temperature. We also compare this data with electron plasma measurements. Within the plasmasphere and inside of Enceladus' orbit, water group ions are about a factor of ∼10 greater than protons in number with number densities exceeding 40 cm−3. Within this inner region the spacecraft acquires a negative potential so that the electron density is underestimated. The electron and proton temperatures, which could not be measured in this region by Voyager, are T ∼ 2 eV at L ∼ 3. Also, within this inner region the protons, because of a negative spacecraft potential, appear to be super‐corotating. By enforcing the condition that protons and water group ions are co‐moving we may be able to acquire an independent estimate of the spacecraft potential relative to that estimated when comparing ion‐electron measurements. Using our estimates of plasma properties, we estimate the importance of the rotating plasma on the stress balance equation for the inner magnetosphere and corresponding portion of the ring current.