Quasi thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: Electron temperatures and density

Abstract

On 1 July 2004, the Cassini spacecraft performed its Saturn orbit insertion, twice crossing the equatorial plane between the G and F rings. The radio HF receiver observed a peak at the upper‐hybrid frequency and weakly banded emissions having well‐defined minima at gyroharmonics. We show that through most of the encounter, these emissions do not result from instabilities, but instead are the quasi‐thermal noise that can be calculated from the classical theory of plasma fluctuations. The spectroscopy of this noise yields the electron density, the core and the halo temperatures in the range 2.3 < L/RS < 7, −1.2 < z/RS < +0.1. For the first time, we measure the core temperature of the Kronian plasma torus to be about 0.5 eV in the ring plane at ∼2.5RS, and increasing to ∼6 eV at 7RS. From the noise minima at the gyroharmonics, we also deduce the magnetic field strength, which agrees with the Cassini's magnetometer data to better than 2%.