Io's equatorial spots: Morphology of neutral UV emissions

The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultrav iolet emissions come from localized regions near Io's equator, designated " equatorial spots." This paper presents a detailed study of the location, sh ape, and brightness of the equatorial spots in near-monochromatic images ob tained using STIS in the first-order long-slit spectroscopy mode. This stud y provides evidence that the equatorial emissions are linked to the interac tion between the Jovian magnetosphere and Io's atmosphere. The morphology o f the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following p rincipal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is n ot 1:1; the equatorial spots are centered 10 degrees -30 degrees longitude downstream from Io's sub-Jovian longitude; the brightness of the emissions in this data set is correlated with Io's distance from the plasma torus cen trifugal equator; and the anti-Jovian equatorial spots are similar to 20% b righter than the sub-Jovian equatorial spots.