Frequency drift of Saturn chorus emission compared to nonlinear theory

The fine structure of nonlinear drifting-frequency chorus is observed at Saturn by the Cassini Radio and Plasma Wave Investigation. During a high-inclination orbit in which Cassini is at near-constant L-shell within about 10°of the magnetic equator, moderately intense nonlinear chorus is observed. Cassini observed a region of intense chorus and large bandwidth a few degrees on either side of the magnetic equator, with lower intensities and bandwidths observed nearest the magnetic equator. Using the observed plasma wave spectra and electron phase space distribution, we have measured plasma parameters within or near the chorus generation region and evaluated the theoretical value of the frequency sweep rate, ∂f/∂t, based on the nonlinear wave growth theory of Omura et al. and the backward wave oscillator theory of Trakhtengerts. Both theories produce rates that are within a factor of 2 of the observed values, but the nonlinear wave growth theory values are closer to the observations for the cases examined. The work presented is consistent with nonlinear theory in the generation of chorus, but also reveals a distinct region of weaker or linear chorus growth nearest to the magnetic equator at Saturn. Key Points Chorus wave growth appears linear nearest the magnetic equator. Nonlinear chorus with frequency drift occurs away from the magnetic equator. Frequency drift rates are consistent with nonlinear theory.