Abstract
Whistler mode chorus plasma wave emissions have been observed at Saturn near the magnetic equator and the source region. During crossings of the magnetic equator along nearly constant L shells, the Cassini Radio and Plasma Wave Science Investigation often observes a local decrease in whistler mode intensity and bandwidth closest to the magnetic equator, where linear growth appears to dominate, with nonlinear structures appearing at higher latitudes and higher frequencies.
We investigate linear growth rate using the Waves in a Homogeneous, Anisotropic, Multi-component Plasma dispersion solver and locally observed electron phase space density measurements from the Electron Spectrometer sensor of the Cassini Plasma Spectrometer Investigation to determine the parameters responsible for the variation in chorus intensity and bandwidth. We find that a temperature anisotropy (T-perpendicular to/T-parallel to similar to 1.3) can account for linear spatiotemporal growth rate of whistler mode waves, which provides a majority of the observed frequency-integrated power.
At the highest frequencies, intense, nonlinear, frequency-drifting structures (drift rates similar to 200 Hz/s) are observed a few degrees away from the equator and can account for a significant fraction of the total power. Chorus emission at higher frequencies is distinct from lower frequency whistler mode emission and is sometimes correlated with simultaneously observed low-frequency electromagnetic ion cyclotron waves.
These electromagnetic ion cyclotron waves appear to modulate a slow frequency drift (similar to 15 Hz/s) which develops into nonlinear growth with much larger frequency drift associated only with the higher-frequency chorus.