Polarization measurements of Saturn electrostatic discharges during the Voyager era

Abstract:A Saturn Electrostatic Discharge or SED is an intense electromagnetic impulse induced by Saturn's lightning, which escapes into space through the magnetoplasma of the ionosphere. Since previous calculated SED polarizations from Voyager data are inconclusive, this polarization study is the first which properly considers the characteristics of the Voyager Planetary Radio Astronomy (PRA) instrument and the change of the antenna response as a function of spacecraft attitude. SED radio observations of the Cassini spacecraft from 2004 to 2017 showed a hemispheric dependence of their circular polarization sense at frequencies below 2 MHz, which can be explained by left-hand ordinary mode emissions. Here we reinvestigated the SED polarizations of the Voyager radio observations at frequencies below 1.3 MHz. As a result, we found mixed polarizations and a dominance of right-handed polarization, which is difficult to interpret in view of the Cassini result. An equatorial SED source was suggested by its occurrence periodicity, but Voyager imaging observations detected convective cloud features in the northern hemisphere, and their SEDs should be left-handed polarized. In this paper, we show detailed results of the SED polarization analysis and compare them with model simulations of the Voyager PRA signal processing of short SED-like pulses. We found that only SEDs from sources at a very large angle to the antenna plane (elevation angle |β| ≥40°) showed a dominance of the expected left-handed polarization sense. Another interpretation for the results, which is applicable for Voyager 2, is to consider an equatorial storm system as a source for SEDs. Note:This paper was re-submitted to Journal of Geophysical Research: Planets for revision. This page provides the digital data for Figures 3-11 and A1 in this paper; please read a readme file or a single text file in each figure sub-directory.