Nonlinear Wave–Wave Interactions of Ion Cyclotron Waves in Saturn’s Magnetosphere: Cassini Observations and Theoretical Predictions

Ion cyclotron waves (ICWs) are commonly observed in Saturn’s inner magnetosphere, especially near Enceladus. Quasi-parallel ICWs are often thought to be excited by temperature-anisotropic ions, and these waves can drive nonlinear interactions when their amplitudes become sufficiently large. This study focuses on an ICW event observed by Cassini during its flyby of Enceladus, in which highly oblique emissions accompanied quasi-parallel ICWs near the oxygen ion gyrofrequencies. Firstly, the observed highly oblique emissions are identified as whistler mode with right-hand polarizations. We then calculate the bicoherence index to demonstrate nonlinear coupling between highly oblique and quasi-parallel waves. Finally, using the cold plasma dispersion relation, we identify possible wave sets that satisfy the necessary conditions for three-wave interactions, further supporting the conclusion that the highly oblique whistler waves are nonlinearly generated by ICWs. Our results provide insight into nonlinear processes in Saturn’s magnetosphere and those of other planets.