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A. R. Vasavada, A. Bouchez, A. P. Ingersoll (Caltech), The Galileo SSI Team
Several imaging sequences during the tenth and eleventh orbits of the Galileo spacecraft were dedicated to observing auroral phenomena on the dark side of Jupiter at visible wavelengths. Observations of both polar hemispheres were made over a range of solar phase angles, magnetic longitudes, and magnetic local times, at resolutions as high as 25 km/pixel. This data set refines measurements of the brightness and location of visible auroral emissions made earlier in the mission (Ingersoll et al. 1998, Icarus Galileo special issue). Its breadth allows comparison with the variety of phenomena described in Earth-based studies of the FUV and IR aurora on the dayside.
The most surprising result is the low altitude of the emission, 220 km +/- 30 km above the 1 bar pressure level. Height was measured perpendicular to the jovian limb as seen by Galileo, where the aurora crosses the limb, using background stars as reference points. A preliminary survey of the images shows fine structure along the main auroral oval as well as the presence of faint, secondary arcs and patches north and south of the main oval. Limb emission (independent of the auroral ovals) is consistent with a diffuse emission covering the region enclosed by the main oval. Some images clearly show Io flux tube footprint emission including a prograde tail. We will present measurements of the emitted power, variability, and geometry of all phenomenon and comparisons with the VIP4 magnetic field model (Connerney et al. 1998).