Io's Far-Ultraviolet Emissions as Observed with HST and IUE

G.E. Ballester, J.T. Clarke, M. Combi (U. of Michigan), D.F. Strobel (Johns Hopkins U.), N. Larsen (Carleton C.), M. McGrath (STScI), M. Lenigan (U. of Michigan), J. Ajello (JPL), N.M. Schneider (LASP), D. Rego (UCL)

Spectral, temporal spatial characteristics of the far-UV emissions from Io's atmosphere have been obtained from HST and IUE observations. Observations made with the HST FOS in 1996 when Io was in eclipse, after eclipse egress, and at the following east and west elongations show different spectral line shapes in a bright 1900 A neutral SI line, indicative of different emission distributions within the tex2html_wrap_inline14 aperture. The eclipse data show the first detection of significant and extended emission from Io's wake region, a region shown by Galileo to have an extended ionosphere with energetic processes operating. Other features are observed, such as enhancement at the east and west limbs when Io was at elongation. Nine HST observations have been made of Io in eclipse ingress or egress, and seven show changes with shadow ranging from tex2html_wrap_inline16 % to tex2html_wrap_inline18 %. Large intensity changes have been observed on time scales of a few minutes, independent of sunlight, that may dominate the temporal behavior. Comparison of the observed OI] tex2html_wrap_inline20 1356/OI tex2html_wrap_inline20 1304 emission ratio with that expected for electron excitation of OI shows agreement with tex2html_wrap_inline24 eV. This ratio could also be modified by SO tex2html_wrap_inline26 atmospheric absorption, requiring SO tex2html_wrap_inline26 columns of tex2html_wrap_inline30 cm tex2html_wrap_inline32 for tex2html_wrap_inline34 eV, or greater columns at higher tex2html_wrap_inline36 , which are too large for penetration of the torus electrons and continued excitation. Torus coronal excitation should still be occurring, as suggested by spatial scans of the emissions made with HST's GHRS when Io was near elongation, that showed peak emission near Io accompanied by extended, weak emission approaching expected OI ratios. Electron impact dissociation of SO tex2html_wrap_inline26 , such as in the lower atmosphere or in volcanic plumes, could contribute to the emissions, but the observed spectral characteristics show that this process does not dominate the bulk of the emissions. All these data indicate that more than one excitation mechanism is at play, such as related to the ionosphere and wake region, and to the torus coronal excitation. This work was supported by STScI GO grants 5438, 6008 and 6786, and NASA ADP NAG5-3044 to the U. of Michigan.