The Faraday rotation effect in Saturn Kilometric Radiation observed by the CASSINI spacecraft

Abstract

Non-thermal radio emissions from Saturn, known as Saturn Kilometric Radiation (SKR), are analyzed for the Faraday rotation effect detected in Cassini RPWS High Frequency Receiver (HFR) observations. This phenomenon, which mainly affects the lower-frequency part of SKR below 200 kHz, is characterized by a rotation of the semi-major axis of the SKR polarization ellipse as a function of frequency during wave propagation through a birefringent plasma medium. Faraday rotation is found in 4.1% of all HFR data recorded by Cassini above 20 degrees northern and southern magnetic latitude, from mid-2004 to late 2017. A statistical visibility analysis shows that elliptically polarized SKR from the dawn source regions, when beamed toward high latitudes into the noon and afternoon local time sectors, is most likely to experience Faraday rotation along the ray path. The necessary conditions for Faraday rotation are discussed in terms of birefringent media and sharp plasma density gradients, where SKR (mostly R-X mode) gets split into the two circularly polarized modes R-X and L-O. By means of a case study we also demonstrate how Faraday rotation provides an estimate for the average plasma density along the ray path.