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M.E. Brown, A.H. Bouchez (Caltech)
Electrons in Jupiter's magnetosphere continuously bombard Ganymede and excite emissions from atmospheric species, as first detected by Hall et al. (1998) through HST observations of UV oxygen lines. These emissions are a tracer of the interaction between the magnetospheres of Jupiter and Ganymede and their study can yield details about this interaction and its time variability. Long term study of such emissions, however, requires a ground-based capability of detecting Ganymede's atmosphere. The relatively strong UV emissions are not detectable from the ground, and the weaker visible emissions are overwhelmed by the reflected sunlight from the surface of the satellite. To circumvent this difficulty, we have observed Ganymede while it is in eclipse in Jupiter' shadow. Electrons still impact the satellite at the same rate, so the atmospheric glows are still active, but reflected sunlight is gone, so the atmosphere is observable.
We have observed Ganymede in eclipse from the Keck telescope on two separate dates and detected the 6300 and 6363Å\ emission lines of atomic oxygen. We analyze the emissions in analogy with the auroral emissions on the earth which emit the same lines. Ganymede is marginally resolved in the images and the emissions appears to be concentrated at the poles of the satellite. During the several-hours-long course of the eclipses, Ganymede moves through a wide range of magnetospheric locations in Jupiter. Analysis of the emission intensity and morphology of the emission as it changes over time will give insight into the dynamic interactions taking place within these two magnetospheres.