The Spatial Extent of Sodium Emission Around Io

The sodium D emissions associated with Io's spectrum also originate in a large volume of space surrounding the satellite, extending more than 10 arc sec in radius. Although this region extends well beyond the equilibrium point between Jupiter and Io, no emission torus has been detected around Jupiter. The emission, however, appears to be stronger close to Io's orbital plane and especially on the Jovian side of Io. Resonant scattering is a viable excitation mechanism in the cloud, and in any case provides an upper limit to the column abundance.