Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state

Abstract

[1] The Cassini mission crossed the source region of the Saturn kilometric radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave Science (RPWS) experiment detected both local and distant radio sources, while plasma parameters were measured in situ by the magnetometer and the Cassini Plasma Spectrometer. A goniopolarimetric inversion was applied to RPWS three-antenna electric measurements to determine the wave vector k and the complete state of polarization of detected waves. We identify broadband extraordinary (X) mode as well as narrowband ordinary (O) mode SKR at low frequencies. Within the source region, SKR is emitted just above the X mode cutoff frequency in a hot plasma, with a typical electron-to-wave energy conversion efficiency of ∼1% (2% peak). The knowledge of the k vector is then used to derive the locus of SKR sources in the kronian magnetosphere, which shows X and O components emanating from the same regions. We also compute the associated beaming angle at the source ′ = (k, −B) either from (1) in situ measurements or a model of the magnetic field vector (for local to distant sources) or (2) polarization measurements (for local sources). Obtained results, similar for both modes, suggest quasi-perpendicular emission for local sources, whereas the beaming pattern of distant sources appears as a hollow cone with a frequency-dependent constant aperture angle: ′ = 75°± 15°below 300 kHz, decreasing at higher frequencies to reach ′ (1000 kHz) = 50°± 25°. Finally, we investigate quantitatively the SKR polarization state, observed to be strongly elliptical at the source, and quasi-purely circular for sources located beyond approximately two kronian radii. We show that conditions of weak mode coupling are achieved along the raypath, under which the magnetoionic theory satisfactorily describes the evolution of the observed polarization. These results are analyzed comparatively with the auroral kilometric radiation at Earth.

Kinetic simulation of the electron-cyclotron maser instability: effect of a finite source size

“…This conclusion is generally supported by the observations of the planetary auroral radio emissions (Zarka 1998;Treumann 2006;Ergun et al 2000;Lamy et al 2011). To investigate a possible contribution of other wave modes, we performed a simulation considering four modes simultaneously: fundamental and harmonic extraordinary modes (X1 and X2), and fundamental and harmonic ordinary modes (O1 and O2).…”

“…Although the fraction of the particle energy flux that is transferred to waves is relatively low (about 1% for the state shown in Figs. 1b and 2b), this conversion efficiency can be sufficient to provide, e.g., the observed intensity of terrestrial and Saturnian auroral kilometric radio emissions (Gurnett 1974;Benson & Calvert 1979;Kurth et al 2005;Lamy et al 2011).…”

Dynamic regimes of cyclotron instability in the afterglow mode of minimum-Belectron cyclotron resonance ion source plasma

“…( ). The most evident influence of thermal corrections can be observed in the shift of cut-off point of the dispersion relation, which allows the fast extraordinary waves to escape from the rarefied plasma [29]. Being quite typical for space plasma this specific case could hardly be realized in the present experimental conditions.…”

Direction Finding and Polarization Measurements of SKR (invited)

“…The estimated locations were found to match that of UV and IR auroras. On the other hand, Lamy et al [2010Lamy et al [ ,2011 took benefit of spacecraft crossing (2008 October 17) of some SKR southern sources, for measuring CMI emission angle and polarisation near the source. They found a mixture of X and O-mode with different spectral coverages, a large beaming angle and a strong elliptical polarisation of the SKR near 10 kHz.…”