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L.M. Trafton (U. Texas), M.A. McGrath (STScI), P.D. Feldman (JHU), K.-L. Jessup (U. Mich.), A. Hendrix (U. Colorado), G.E Ballester (U. Mich), S.A. Stern (SWRI)
Two HST/STIS spectra of Io spanning 1700-3100 Å\ were obtained in Jupiterís shadow on August 18, 1999 as part of a campaign to make coordinated HST and Galileo observations of the Jovian system. The first observation began at 0654 UT, a minute after total eclipse entry, and lasted 800 sec. The second began at 0710 UT and lasted 700 sec., still in shadow. During acquisition, immediately prior to the eclipse, Io (diameter 1."12) was centered in the 2"x2" aperture. The spectra were obtained with grating G230L (R ~740) in the ACCUM mode. Owing to the close proximity of Jupiter, scattered light from the planet dominated Ioís eclipsed spectrum and had to be removed by carefully interpolating and subtracting the sky background in the aperture. To confirm the validity of the weak emission features extracted, the two observations were reduced independently; and opposite halves of the aperture for the combined observations were also reduced independently. Prominent emission features were found below 2200 Å\ whose identity is under investigation. Above 2200 Å, Ioís spectrum rises and appears to contain a diffusely reflected Jovian or solar continuum, perhaps sunlight refracted or scattered by Jupiterís upper atmosphere. Emission from SO appears to be detected, provided that Ioís atmosphere absorbs at 2700 Å; i.e., that a spectral feature observed at that wavelength for each observation is real. We will discuss the possibility that this feature is an artifact. If SO-SO2 dominates the emission below 2000 Å, excitation by 200 eV electrons is favored over 25 eV electrons, based on a comparison with laboratory spectra.