Analysis of narrowband emission observed in the Saturn magnetosphere

Narrowband emission is observed at Saturn centered near 5 kHz and 20 kHz and harmonics of 20 kHz. This emission appears to be in many ways similar to Jovian narrowband emission observed at higher frequencies. We analyze one example of this emission near a possible source region. In situ electron distributions suggest narrowband emission has a source region associated with electrostatic cyclotron harmonic and upper hybrid emission. Linear growth rate calculations indicate that the observed plasma distributions are unstable to the growth of electrostatic harmonic emissions. In addition, it is found that when the local upper hybrid frequency is close to 2 f(ce) or 3 f(ce) (f(ce) is the electron cyclotron frequency), electromagnetic Z mode and weak ordinary (O mode) emission can be directly generated by the cyclotron maser instability. In the presence of density gradients, Z mode emission can mode-convert into O mode emission, and this might explain the narrowband emission observed by the Cassini spacecraft.