On the possibility of fast neutral production of the inner Io torus

摘要

The Galileo spacecraft passed through the Io torus and its inner boundary on 7 December 1995 and 5 November 2002 during the J0 and A34 passes, respectively. Electron densities inferred from plasma wave measurements on both passes indicated a steep gradient at the inner boundary of the cold Io torus located between 4.5 and 5 R J (Io is at 5.9 R J ), as well as the expected much less steep gradient in the outer torus, with an outer boundary at distances greater than 8 R J . We attempt to model the inner torus with the fast neutral production model of Wang et al. (2001). A reduced ion pickup velocity at Io can prevent fast neutrals from penetrating inside the observed inner edge of the cold torus. Outward radial convection that increases with radial distances enables a steady state to be maintained. Differences between the passes are not entirely explained by varying pickup velocity at Io and convective transport. Factors including variable mass loading, diffusive transport mechanisms, dawn-dusk torus asymmetry, and spacecraft trajectory effects may also be important.