Reanalysis of SKR Source Crossed by Cassini: Loss Cone Electrons as a Secondary Source of Free Energy

Abstract

Saturn's Kilometric Radiation (SKR) is a powerful non‐thermal radio auroral emission produced by the Cyclotron Maser Instability (CMI), which generates waves close to the local cyclotron frequency in a highly magnetized plasma from mildly relativistic electrons. The source of free energy for SKR was identified by Cassini to be the monoenergetic shell (horseshoe) distributions of 6–12 keV. Since then, Juno identified loss cone and shell distributions as the main and secondary CMI sources of free energy at Jupiter. We reanalyze Cassini measurements during SKR source crossings. We identify similar empty loss cones between 1.6 and 8.9 keV on 6 occasions alongside shell distributions. We show that loss cone distributions are unstable by computing the CMI growth rate. The amplified waves are predicted to have frequencies between 1.003 and 1.017 with a beaming angle close to . Finally, we compare our findings with those obtained at Jupiter.