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L. M. Trafton (Univ. TX at Austin)
We report an upper limit of 70 R to Io's disk-averaged ambient H Ly-\alpha Emission based on HST observations obtained during Aug 20-29, 1994 using the Goddard High Resolution Spectrograph. The data were obtained with the Ech-A grating and the square Large Science Aperture (1."74 on a side) with the solar-blind Digicon detector. The spectral resolution corresponds to ~53mÅ\ for a square object the size of Io (0."90). The observations were taken when Io was near western elongation and at low magnetic latitudes; i.e., when io was well-immersed in the plasma torus. At the time of the observations, the interstellar wind was inclined 36\deg to Jupiter's line of sight. Both the geocoronal and ISM H emission profiles were blue-shifted essentially clear of the central wavelength of Io's ambient H Ly-\alpha profile; or of the diffusely reflected solar profile. We did not detect this reflected solar component. Our upper limit for iogenic H emision is 2.7 times lower than the corresonding disk-averaged intensity of 211 R for H Ly-\alpha detected by Roesler et al. (Science 283, 353; 1999) using the STIS instrument on HST during 1997, also near western elongation. This result, and our failure to detect the reflected solar line, suggests that Io's detected H Ly-\alpha emission is not diffusely reflected solar light but is temporally variable. Upper limits are also set on SI fluorescence from Io.