Core electron temperature and density in the innermost Saturn's magnetosphere from HF power spectra analysis on Cassini

Abstract

We analyze the large-scale structures of electrons in Saturn's inner magnetosphere equatorial plane, from 2.8 to about 10 Saturnian radii (R_S). The electron total density and core temperature are obtained using the quasi-thermal noise spectroscopy method, based on the HF power spectra measurements acquired with the Cassini/Radio and Plasma Wave Science instrument around the local plasma frequency from July 2004 to May 2012. The results reveal the existence of two regions. An inner region around Enceladus orbit (3-5 R_S) is characterized by a high variability of the electron density, an increasing core temperature profile (∝ R^2.7), and a strong correlation of the density and temperature. An outer region, beyond 5 R_S, is characterized by a decrease of the density (∝ R^-4.19) and a slight decrease of the core temperature with distance from the planet (∝ R^-0.3). The electron temperature profile is consistent with heating by thermalization of the electrons with the ions in the inner region and with thermalization balanced by cooling due to outward radial transport in the outer region. In the outer region, we identify a local time asymmetry of both the density and the temperature: higher core temperatures are observed in the nightside while higher densities are observed in the dayside. We finally estimate the plasma scale height from a few orbits at quasi-constant altitude. We find that it varies as R^1.53 inside Dione's orbit and displays a bell-shaped profile between 7 and 9 R_S, consistent with the maximum in the corotation lag previously observed.