Plasma mass loading from the extended neutral gas torus of Enceladus as inferred from the observed plasma corotation lag

Abstract

At Saturn, significant plasma mass loading and radial transport occur together throughout the extended neutral torus, both producing corotation lags. We present a theory that depends upon the ratio of two classes of process: those injecting new mass (simple ionization) and those effectively removing momentum without changing plasma mass density (elastic ion-neutral collisions, like-species charge exchange). Both induce ionospheric torques, but only the former increases total outward mass flux. From an observed angular velocity, our model calculates the distribution of mass loading with radial distance. We present solutions based on Cassini Plasma Spectrometer measurements and theoretical chemistry models for Saturn's magnetosphere. We find a pronounced interaction peak near Enceladus' orbit and a broad ionization region between 5 and 8 Saturn radii (Rs). The total mass outflux at 8Rs is 100 kg/s x (Σ/0.1 S), where Σ is Saturn's ionospheric Pedersen conductance. The reported decrease in lag beyond 8Rs requires that Σ ≠ const or reducing the total mass outflux. Copyright 2009 by the American Geophysical Union.