With the recent observations of Io's very striking equatorial aurora by Roesler et al. [1999>, it is now possible to remotely sense Io's local interaction with the Io plasma torus. Here we present an explanation for the formation of Io's aurora which implies that Io's electron flow is strongly rotated and shielded around Io. The Hall effect close to Io breaks the interaction symmetry to produce a brighter anti-Jovian radiation spot, and it also results in rotated Alfv¿n wings. Observations of Io's aurora also impose new constraints on Io's atmosphere. We find, among other things, an atomic oxygen to sulfur dioxide ratio of ~20%. ¿ 2000 American Geophysical Union