DISCUSSIONS

These observations are closely related the recent HST WF/PC2 images of [O I] 6300Å emission, obtained by J. Trauger (1997) when Io was in eclipse. However, our different orbital phase angles imply that the bright equatorial spots we observed were not caused by Io's wake, since the wake reported by Trauger would have been obscured by Io in our images. However, the diffuse limb glow was enhanced on the side towards the plasma torus in both sets of observations.

The Galileo science team reports similar equatorial emissions from unfiltered CCD images taken of Io in eclipse. They attribute the increased equatorial emissions to tidal interactions with Jupiter and associated volcanic activity. Although it is not clear how such a mechanism would account for a sudden change in neutral emissions as seen in this and the companion ground-based data (see poster 70.01).

The discovery of Ly-alpha emission in the polar regions, apparently at or near the surface, and whose emission is not correlated with the neutral oxygen emission, may indicate the destructive by-product of a hydrogen ice (e.g. hydrogen sulfide or water). The detection of the hydrogen gas may be the result of solar Ly-alpha resonant scattering. Alternatively, Ly-alpha (emission may be the result of hydrogen transported from Jupiter as part of the Birkiand currents.