The Radio Wave Polarization of Saturn Lightning Observed by Cassini

Abstract

We investigate the polarization of the radio waves emitted by lightning in Saturn's atmosphere, which are referred to as SEDs for Saturn Electrostatic Discharges. Using the complete SED data set retrieved by Cassini's RPWS (Radio and Plasma Wave Science) instrument we found that the circular polarization of SEDs below a frequency of 2 MHz depends on the latitudinal hemisphere of the lightning storm. SEDs from several storms located at $35°$ South latitude display a strong right–hand sense of circular polarization, whereas SEDs from storms located at $35°$ and $50°$ North are overwhelmingly left–hand polarized. A single storm located at Saturn's equator in June 2005 shows 4 right–handed SEDs when Cassini was in the northern hemisphere and 4 left–handed SEDs when Cassini was in the southern hemisphere. The polarization characteristic is consistent with an emission in the L-O mode, consistent with the theory of Fischer, Gurnett, et al. (2007, JGR 112, A12308, https://doi.org/10.1029/2007JA012592) that the R-X mode is largely absorbed in Saturn's ionosphere below a frequency of 2 MHz. The sense of circular SED polarization below 2 MHz is opposite to the polarization of Saturn kilometric radiation (SKR), and thus this can be used to distinguish SEDs from SKR and to determine the hemispherical origin of the causative lightning storm.