Harmonic growth of ion-cyclotron waves in Saturn's magnetosphere

Abstract

Ion-cyclotron waves have been observed by the Cassini spacecraft in Saturn's middle magnetosphere. Waves have frequencies near the gyrofrequencies of water-group ions, they are left handed, and in most regions they propagate at small angles to the ambient magnetic field. Their origin is explained in terms of the ion-cyclotron instability generated by water-group ions picked up from Saturn's neutral cloud. Additionally, in some regions dynamic spectra reveal the existence of fluctuations at approximately twice the value of the ion gyrofrequency. These waves have a significant compressional component and propagate at oblique angles to the magnetic field. Their characteristics suggest that these fluctuations can be identified as a harmonic mode (n = 2) generated by the ring distributions of pickup ions. In this work we study in detail the characteristics of these harmonic mode waves. We also evaluate growth rates using a kinetic dispersion analysis.