Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization

Abstract

[1] We report on an analysis about the spectral shape of the Saturnian kilometric radiation (SKR) recorded by the RPWS/HFR experiment onboard the Cassini spacecraft. The SKR envelope is found to be very variable in timescales covering a large range, from hours to days. Despite this change, we proceed to a classification of the shape in the frequency range from 3.5 kHz to 1.2 MHz taking into consideration the phenomenology and the state polarization of the radio wave. In the considered SKR bandwidth, we make a distinction between three principal components having different spectral shapes and degrees of total polarization. The well-known SKR component exhibits a degree of circular polarization close to 100% and emits in the frequency range from 80-90 kHz to 800-900 kHz. The two other components become visible in the lower and higher parts of the HFR receiver and show a weak linear polarization less than 10%. The generation of the SKR is linked to particles precipitations in the auroral regions, nevertheless the variability of the spectral shape is still subject of investigation. The present analysis is confronted with the theoretical model of SKR envelope spectrum proposed by Galopeau et al. (1989).

Comment on “Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization” by Patrick Galopeau et al.

“…As can be seen in Figures 2, 3, 4, 6, and 7 of G07, component A is characterized by a spectral slope between f −1 and f −2 . Our Figure 1a displays the dynamic spectrum of the data recorded on day 2005‐357 (23 December 2005), when this variable low‐frequency noise and the SKR are well visible.…”

“…The most plausible origin for this noise is plasma impact noise on the spacecraft and antennas in f −2 , as described by Meyer‐Vernet [1985] and Meyer‐Vernet and Perche [1989], and possibly also f −1 noise. The level of the plasma impact noise spectrum depends on the plasma temperature, the presence of beams; thus the thin and thick morphology of component A described by G07 simply reflects the variability of the local plasma conditions.…”

“…The physical origin of these components is questioned without answering for components A and C; component B is the well‐known auroral SKR [see, e.g., Zarka , 1998]. Galopeau et al [2007] describe the SKR spectrum morphology especially of B and C components in reference to the morphological classification of Genova et al [1983], and conclude that the SKR spectrum is variable and unpredictable at time scales of hours to days.…”

An oblate beaming cone for Io‐controlled Jovian decameter emission

“…The Cassini mission provided new results concerning the radiation directivity which is directly related to the emission beam pattern. Galopeau et al [2007b] analyzed the spectral shape of the SKR and showed that the radiation may be considered as the overlapping of hollow cones related to three components. Hence, using radio horizon technique, Farrell et al [2005] brought out the presence of an emission source associated to the Saturnian kilometric radiation.…”

“…However, Cassini investigations also showed the complexity of the interpretation of the SKR observations and the corresponding emission beam which crucially depends on the geometrical conditions of the SKR source with regard to the observer. Galopeau et al [2007bGalopeau et al [ , 2009 addressed the problem about the ellipticity of the polarization of the Saturnian radiation, and Fischer et al [2009] revealed the presence of a strongly elliptically polarized emission when the Cassini spacecraft is at latitudes higher than 30 ∘ . This source was found to be confined to relatively low latitudes and close to midnight, in discrepancy with Voyager observations which reported a source location well away from the high-latitude dayside cusp region.…”