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G. E. Ballester (U. Mich.), L. Ben-Jaffel (IAP, Paris), J. T. Clarke (U. Mich.), R. Gladstone (SWRI), S. Miller (UCL, London), L. M. Trafton (U. Texas, Austin), J.T. Trauger (JPL)
An excess of H-Ly\alpha emission from Uranus' sunlit hemisphere was detected by the IUE satellite in 1982, and some excess was confirmed with the Voyager 2 UVS during the 1986 encounter with Uranus. Radiative transfer modeling has shown that the Voyager H-Ly\alpha observations did require emission additional to the scattered solar and IPM H-Ly\alpha, and thus produced by internal processes in the upper atmosphere, such as aurora or other unidentified mechanisms. Subsequent IUE observations showed very large short- and long-term intensity variations that support an auroral source. However, although Voyager did identify UV auroral emissions by H2 in the sunlit hemisphere, it did not detect a large H-Ly\alpha auroral emission there, making it impossible to provide conclusive evidence that the H-Ly\alpha enhancements observed by IUE are due to aurora. Auroral emissions are spatially confined, and resolution of the emission distribution could yield the needed evidence, or could alternatively provide observational clues to other possible causes of dayglow variations in the upper atmosphere. Uranus intrinsically weak H-Ly\alpha emission (~600~R on average) had not allowed for such an experiment in the past, but the high sensitivity in the FUV of the Space Telescope Imaging Spectrograph (STIS) on HST has now provided first images of Uranus in the FUV. The observations made on 29-30 July 1998 consisted of a FUV MAMA image in the open mode (25MAMA) and a consecutive image filtering out the H-Ly\alpha (F25SRF2) to measure and subtract the disk reflected sunlight above 1250 Ang. A quick look at the data shows the H-Ly\alpha emission and disk-reflected sunlight, with additional noise from the geocoronal background. We will present the results from these data, taking advantage of the time-tagging information to subtract the geocoronal background, and modeling of the underlying disk background. Four new observations will hopefully be made before October 1998 which will cover the full planet in longitude, and will use a different technique to improve the s/n of the H-Ly\alpha detection.
The author(s) of this abstract have provided an email address for comments about the abstract: gilda@umich.edu