HST Observations of [O I] Emissions from Io in Eclipse

John T. Trauger (JPL), Karl R. Stapelfeldt (JPL), Gilda E. Ballester (University of Michigan), John T. Clarke (University of Michigan), WFPC2 Science Team

We have obtained images of the [O I] emissions from Io during eclipse on 17 May and 22 May 1997 with the Wide Field and Planetary Camera 2 on the Hubble Space Telescope. During these two observing visits, a total of five exposures in the [O I] 630 nm emission line reveal a consistent emission morphology as follows. First, a patchy corona is seen confined along Io's limbs within tex2html_wrap_inline14 400 km of Io's surface, sometimes brighter on the north or south, but generally weaker on the orbital trailing limb (and equivalently the upstream limb relative to plasma flow). Second, relatively brighter emissions are seen in the plasma wake region, where emissions reach 10-20 kRay near the limb. Finally, there is a narrow stream of emissions that appears to originate near 340 tex2html_wrap_inline16 longitude and +20 tex2html_wrap_inline16 latitude, extending linearly eastward to the downstream limb. The [O I] emissions are produced by excited oxygen atoms created by interactions within the flow of plasma ions over neutral target species bound to Io, and must occur in regions that are sufficiently low in density to accomodate the 160-second radiation time of the 630 nm line. We note that these [O I] emissions may bear a close relationship to a number of other recent ground-based, HST, and Galileo observations of Io. These include spatially resolved FOS observations of extended S I] 1900 emission in Io's wake region (Ballester et al. 1997), Galileo in situ plasma observations of Io's wake region (Frank et al. 1996), ground-based infrared measurements of the Loki hot spot (Spencer et al. 1997), Galileo imaging observations of an active volcanic plume tex2html_wrap_inline14 40 tex2html_wrap_inline16 east of Loki (McEwen et al. 1997), Galileo images of Io's visible emission features (Belton et al. 1997), and ground-based spectroscopic observations of [O I] emissions near Io (Scherb et al. 1996). Physical mechanisms leading to the observed [O I] emission morphology will be discussed.