Thermal ion flow in Saturn's inner magnetosphere measured by the Cassini plasma spectrometer: A signature of the Enceladus torus?

Abstract

For the middle and outer magnetosphere of Saturn (. >5.5 RS), Cassini plasma spectrometer ion counting data provides thermal ion moments by assuming Maxwellian phase space density distributions for each ion species. However, in the inner magnetosphere (<5.5 RS) within Saturn's extended neutral cloud and proposed Enceladus torus, there is fresh ion production via charge exchange, yielding a complex ion velocity distribution. In this study, ion flow velocities in the inner region are obtained using the assumption that the ion phase space distributions are gyrotropic. Significantly sub-corotating ion flow velocities (∼75% of corotation) are found in the vicinity of the Enceladus orbit and the radial extent of the sub-corotation is about 1.0 RS, in reasonable agreement with the simulated radial dimension of the Enceladus torus. Copyright 2009 by the American Geophysical Union.