The ion mass loading rate at Io

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of ∼10³⁰ amu s⁻¹ = 1.6 × 10²⁸ SO₂ s⁻¹ or approximately 10³ kg s⁻¹(A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., ≤3 × 10²⁷ × (n_e,0/3600 cm⁻³) ions s⁻¹, where n_e,0 is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was ≤3 × 10²⁷ ions s⁻¹, whereas for the Voyager epoch with lower n_e,0 (=2000 cm⁻³), this rate would be ≤1.7 × 10²⁷ ions s⁻¹, consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of ≤1 × 10²⁷ ions s⁻¹. We investigate the processes that control Io’s large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered (10¹⁶-10²¹ m⁻²) and by a factor of ∼100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io’s high conductances and thus generate Io’s large scale electrodynamic interaction such as the generation of Io’s electric current system and the slowing of the plasma flow. The electrodynamic part of Io’s interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io’s atmosphere, which is independent of any particular model for the 3D interaction of torus electrons with its atmosphere.