Mass unloading along the inner edge of the Enceladus plasma torus

A major discovery made by the Cassini spacecraft at Saturn was the substantial mass ejection from the south pole of Enceladus. Previous studies show that this ejected gas can become ionized and subsequently load mass onto the connecting magnetic field lines near the moon. Radial diffusion then allows the mass-loaded field lines to move outward to ∼ 15 R_s and inward to ∼2 R_s, forming a plasma torus. We demonstrate herein that the mass is also "unloaded" along the inner edge of this plasma torus - the edge incident with the plasma-absorbing A-ring. Interpreting down-drifting z-mode tones from active sites along the inner edge of the ion torus as emission near the local electron plasma frequency, f_pe, we can remotely-monitor this reduction in plasma density along the torus inner edge as a function time. We find that the down-drift of the z-mode tones corresponds typically to a plasma density change dn/dt ∼ -5 × 10^-4/cm³-s and when integrated over an annulus defined by the outer edge of the A-ring, corresponds to a mass loss of ∼ 40 kg/s. Using the z-mode tones, we also find locations where plasma mass from the ring-ionosphere is possibly loaded at 1-2 kg/s onto field lines near the Cassini gap.