Properties of the thermal ion plasma near Rhea as measured by the Cassini plasma spectrometer

Abstract

Rhea is the second largest Saturnian satellite orbiting at 8.74 Saturn radii within the magnetosphere's near corotating thermal plasma. Rhea's orbital speed is less than the corotation speed, so the thermal plasma forms a wake in the direction of Rhea's orbital motion. During 26 November 2005, Cassini passed within 500 km of Rhea and through this wake, with a subsequent flyby an order of magnitude higher on 30 August 2007. The thermal plasma moments during these encounters are investigated here utilizing the Ion Mass Spectrometer (IMS) sensor of Cassini and analyzed by a forward model technique. Owing to the brevity of flybys, IMS is only able to sample a single slice of phase space at high time resolution throughout, rather than actuating to allow sampling of a variety of pitch angles but only providing a few data points. Even with this restriction, the moments before/after the encounter are in good agreement with other nonflyby actuating calculated moments. It is found that the plasma is dominated by water ions, with plasma velocities ≈30% slower than would be expected by rigid corotation, and local plasma densities decrease when passing through the wake. During the encounters, the results show that on the Saturn side of Rhea, there is no radial component of plasma flow, yet there is a radial component of ≈10 km/s outward on the anti‐Saturn side.