Electron temperatures in Saturn's plasma disc

We present new results concerning electron temperature (Te) mapping in the inner magnetosphere of Saturn (<7 RS). The data are obtained by the Langmuir Probe (LP) instrument of the Radio and Plasma Wave Science (RPWS) investigation on board Cassini. The study is based on the fourteen first orbits around Saturn, and focus is on cold (<10 eV) and dense (>5 cm−3) electron populations in the inner plasma torus (plasma disc). Apart from a general increase with distance from Saturn and away from the equatorial plane (latitude), we identify a possibly anisotropic electron temperature in the plasma disc, as well as the presence of photoelectrons from the spacecraft or E-ring dust particles. A polytropic index of γ≈+0.4 is determined from the observations, which can be used in theoretical models of the plasma disc. We also infer local variations in Te near the equatorial plane, where the E-ring of dusty plasma and enhancements of water rich neutral gas exist, and determine constraints on the energy source required to heat the electrons in the plasma disc. We suggest that Joule heating or heating by low-frequency plasma waves as well as cooling by neutral gas and E-ring dust are involved in the energy balance, together with Coulomb collision heating by co-rotating ions.