Electron Cyclotron Maser Instability in the Sources of Saturn’s Kilometric Radiation

Saturn’s Kilometric Radiations (SKRs) are usually observed with characteristics of extraordinary-mode (X-mode) emissions at electron cyclotron frequency, excited via electron cyclotron maser instability (ECMI) of electron inside the source region. Previous studies on ECMI of SKR sources haven’t employed appropriate dispersion relations. When considering the relativistic effect appropriately, the cut-off frequency of X mode could be modified, while the relativistic mode is present with considerable amounts of energetic electrons. Using particle-in-cell simulation, we studied the ECMI process in the SKR sources with parameters obtained with in situ measurement of Cassini mission. According to the previous reports, in the encounter of the SKR source, the density ratio of energetic electrons to total electrons $\frac{{{n_e}}}{{{n_0}}}$ was 24%, and the ratio of plasma frequency to electron cyclotron frequency $\frac{{{\omega _{pe}}}}{{{\Omega _{ce}}}}$ was 0.046. The simulation results suggest that with the observed parameters, X mode emissions cannot be directly amplified, while the non-escaping R mode is most unstable. We carried out further parametric studies with theoretical analysis and simulations, and found that X mode can be excited with the energetic electrons dominating (with density ratio over 90%). The properties of both R and X modes are consistent with the observed SKR emissions. The study provides new insight in to the ECMI processes of SKR.