Mutual Event Observations of Io’s Sodium Corona

We have measured the column density pro—le of Ios sodium corona using 10 mutual eclipses between the Galilean satellites. This approach circumvents the problem of spatially resolving Ios corona directly from Ios bright continuum in the presence of atmospheric seeing and telescopic scattering. The primary goal is to investigate the spatial and temporal variations of Ios corona. Spectra from the Keck Observa- tory and McDonald Observatory from 1997 reveal a corona that is only approximately spherically sym- metric around Io. Comparing the globally averaged radial sodium column density pro—le in the corona with pro—les measured in 1991 and 1985, we —nd that there has been no signi—cant variation. However, there appears to be a previously undetected asymmetry: the corona above Ios sub-Jupiter hemisphere is consistently more dense than above the anti-Jupiter hemisphere. Subject headings: planets and satellites: individual (Io) ¨ techniques: spectroscopic On-line material: machine-readable table