Far-ultraviolet imaging spectroscopy of Io's atmosphere with HST/STIS.

Well-resolved far-ultraviolet spectroscopic images of O I, S I, and previously undetected H ILyman-alpha emission from Io were obtained with the Hubble space telescope imaging spectrograph (STIS). Detected O I and S I lines (1250 to 1500 angstroms) have bright equatorial spots (up to 2.5 kilorayleighs) that shift position with jovian magnetic field orientation; limb glow that is brighter on the hemisphere facing the jovian magnetic equator; and faint diffuse emission extending to approximately 20 Io radii. All O I and S I features brightened by approximately 50 percent in the last two images, concurrently with a ground-based observation of increased iogenic [O I] 6300-angstrom emission. The H ILyman-alpha emission, consisting of a small, approximately 2-kilorayleigh patch near each pole, has a different morphology and time variation.