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R.C. Woodward, F.L. Roesler (U Wisc), R.J. Oliversen (NASA/GSFC), W.H. Smyth (AER), H.W. Moos (JHU), F. Scherb (U Wisc)
On 14 October 1997 six spectrally resolved images of Io were acquired by the HST Space Telescope Imaging Spectrograph (STIS)\@. Although primarily intended to target Io [Roesler et al.\ 1999], these images include \ion{S}{2} \lambda\lambda1204,1256, \ion{S}{3} \lambda\lambda1194,1712, and \ion{S}{4} \lambda\lambda1406,1425 from the Io plasma torus in a 2'' \times 25'' rectangle centered on Io. Simultaneously with these HST/STIS observations, broadband images of the entire plasma torus in [\ion{S}{2}] \lambda6731 were aquired at the McMath-Pierce west auxiliary telescope [Woodward et al.\ 1999]. In this work these data are described, and a semiempirical model of the plasma torus [Woodward and Smyth 1994] is applied to both data sets jointly: to characterize the plasma torus in general, and in particular to determine if these torus emission lines undergo an intrinsic brightening at ~\,06:00 UT, as do the neutral O and S lines near Io [Scherb et al.\ 1998, Roesler et al.\ 1999]. Evidence of near-Io ionic emission will also be discussed.
Because Io was near western (receding) elongation when these data were acquired, the torus emission observed by STIS was strongly influenced by line-of-sight geometry, and therefore by the precise location of the torus. Since the torus radial position and scale height are known to vary with magnetic longitude, local time, and other factors in ways that are not well understood, the groundbased torus images are crucial in interpreting the spatially limited STIS data and extrapolating it to the full torus.
This work was supported in part by NASA grant #NAS5-30131.
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