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%.

The role of higher-order modes on the electromagnetic whistler-cyclotron wave fluctuations of thermal and non-thermal plasmas

“…The analysis of electromagnetic wave scattering is, in fact, one of the most efficient methods of plasma diagnostics used in laboratory fusion research devices and in space-plasma measurement (e.g., Refs. [13][14][15].…”

The electron density of Saturn's magnetosphere

“…Since the dipole magnetic field follows R −3 , leading to A ∝ R 3 , and H is proportional to R as reported by Moncuquet et al (2005), the density depends on R −4 (equivalent to L −4 in the equatorial plane). This relation matches the distribution at L < 9 by Persoon et al (2006) and the averaged distribution of the density in this study.…”

“…Recent observations have made it possible to study the magnetospheric electron densities in more detail (Moncuquet et al, 2005;Persoon et al, 2005Persoon et al, , 2006Wahlund et al, 2005. Persoon et al (2005) used upper hybrid resonance emissions to measure the equatorial electron density, and found that the electron density varies significantly inside 5 R S , but consistently drops off exponentially with increasing distance from Saturn beyond 5 R S .…”

Sensitivity, quantum limits, and quantum enhancement of noise spectroscopies

“…Conclusions -We have derived the sensitivity of noise spectroscopies from estimation theory, finding simple expressions for the Fisher information matrix in terms of the spectral model. The result enables rigorous use of noise spectra in cell biology [5,6], molecular biophysics [7,8], geophysics [9], space science [10], and quantum in- formation processing [12][13][14][15][16]. For optically-probed particulate systems that show line-broadening, e.g.…”

“…Noise spectroscopies, in which naturally occurring fluctuations of a system of interest are recorded by a noninvasive probe, have applications in a wide range of disciplines including atomic [1] and solid state physics [2,3], surface science [4], cell biology [5,6], molecular biophysics [7,8], geophysics [9], space science [10], quantum optomecanics [11], and quantum information processing [12][13][14][15][16]. By the fluctuation-dissipation theorem, the noise spectrum under thermal equilibrium gives the same information as do driven spectroscopies, with the advantage of characterizing the system in its natural, undisturbed state [17].…”