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

For the middle and outer magnetosphere of Saturn (>5.5 R-S), 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 R-S) 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 (similar to 75% of corotation) are found in the vicinity of the Enceladus orbit and the radial extent of the sub-corotation is about 1.0 R-S, in reasonable agreement with the simulated radial dimension of the Enceladus torus. Citation: Wilson, R. J., R. L. Tokar, and M. G. Henderson (2009), Thermal ion flow in Saturn's inner magnetosphere measured by the Cassini plasma spectrometer: A signature of the Enceladus torus?, Geophys. Res. Lett., 36, L23104, doi:10.1029/2009GL040225.