Saturn's Magnetic Field Periodicity: Spectral Sidebands, their Origin and Significance

Magnetic field measurements during the first 15 Cassini orbits inside 10 Saturn radii yield spectra containing a central peak, Saturn's periodicity, and sidebands with difference frequencies corresponding to the orbital period. The sidebands are interpreted as a phase modulation of the periodicity caused by the periodic changes in longitude and distance from Saturn. The sideband amplitudes are used to determine a single parameter characterizing the modulation. This interpretation is consistent with outward propagation of a wave from an inner source rotating differentially with respect to the magnetosphere as proposed in the 'Cam' model. The modulation parameter coincides with a wave speed of 30 km/sec. This speed agrees with that derived by Cowley et al. (2006) who used the Cam model to reproduce the observed phase shift during a single orbit. Future studies of the periodicity and the wave properties should benefit from analyses based on this phase modulation.