31st Annual Meeting of the DPS, October 1999
Session 70. Ganymede and Callisto
Contributed Oral Parallel Session, Friday, October 15, 1999, 10:30-12:00noon, Sala Pietro d'Abano

## [70.07] HST Imaging of Polar Aurorae on Ganymede

M. A. McGrath (STScI), P. D. Feldman, H. W. Moos, K. Retheford, D. F. Strobel, B. C. Wolven (JHU)

We report on new observations made with the Hubble Space Telescope on October 30, 1998 that provide unambiguous evidence for the existence of polar aurorae on Ganymede. The observations used the Space Telescope Imaging Spectrograph (STIS) with a 2'' wide slit to provide an objective spectrum covering the range 1160--1720Å. We detect emission concentrated at the poles of the satellite in two spectral lines, O\,{\sc i}\,\lambda1304 and 1356\,Å, confirming the interpretation of earlier HST observations by Hall et al. (1998). Reflected solar continuum is detected and fitted to a solar spectrum yielding an albedo of 2.3% over the range 1400--1720Å, in good agreement with the value of 2.6% derived by Hall et al. from the C\,{\sc ii}\,\lambda1335 multiplet. The atomic oxygen emissions appear in both hemispheres, generally (but not completely) at latitudes above 45\circ, in accordance with the model of Kivelson et al. (1997) that predicts Jovian magnetic field lines are linked to Ganymede's atmosphere only at high latitudes. Both the brightness and relative north-south intensity of the emissions varied considerably over the 5.5 hour span of the observations, presumably due to the changing Jovian magnetic field at Ganymede. The mean fluxes, as well as the O\,{\sc i} \lambda1356/O\,{\sc i} \lambda1304 ratio, are comparable to those observed by Hall et al., lending support to the identification of the excitation mechanism as electron impact on O2. In addition to observed H\,{\sc i} Lyman-\alpha reflected from the disk, extended Lyman-\alpha emission resonantly scattered from a hydrogen exosphere is detected out to beyond two Ganymede radii from the limb. The brightness is consistent with the value reported from Galileo measurements by Barth et al. (1997).

Barth, C. A., et al.~1997, Geophys.~Res.~Lett., 24, 2147.

Hall, D. T., et al.~1998, Ap.~J., 499, 475.

Kivelson, M. G., et al.~1997, Geophys.~Res.~Lett., 24, 2155.