The Cassini Radio Science Observations of the Enceladus Plasma Cloud

Cassini Radio Science Subsystem (RSS) observations of the plasma environment of Enceladus from the radio occultation of 26 January 2006 are reported. The radio occultation technique was used to measure the electron content as a function of distance along a the orbital path of Enceladus with the radio line-of-sight passing about 45 km below the South Pole. The RSS observations, when applied to a toroidal model of a plasma cloud along the orbit of Enceladus show a significant enhancement of the electron density, extending out to distances from the satellite in excess of 88 Enceladus radii in both directions. Simple estimates show that this is consistent with a photoionization plasma source together with transport away from the satellite due to the plasma flow. In addition, the occultation data shows an electron density depletion near the plume, in agreement with other instruments, and this is possibly due to electron attachment to grains.