Io's ultraviolet aurora: Remote sensing of Io's interaction

With the recent observations of Io's very striking equatorial aurora by Roe sler et al. [1999], it is now possible to remotely sense Io's local interac tion with the Io plasma torus. Here we present an explanation for the forma tion of Io's aurora which implies that Io's electron flow is strongly rotat ed and shielded around Io. The Hall effect close to Io breaks the interacti on symmetry to produce a brighter anti-Jovian radiation spot, and it also r esults in rotated Alfven wings. Observations of Io's aurora also impose new constraints on Io's atmosphere. We find, among other things, an atomic oxy gen to sulfur dioxide ratio of similar to 20%.