The Structure and Variability of Extended S II 1256Å Emission Near Io

R. Carey Woodward, Jr.,1 Fred L. Roesler,1 Ronald J. Oliversen,2 William H. Smyth,3 H. Warren Moos,4 and Fran Bagenal5

This material was originally presented at the Spring 2001 meeting of the American Geophysical Union. The abstract (below) was published in Eos 82(20), S250, 2001; please use that citation when referencing this work. A HTML version of the full paper will appear here eventually; a one-sheet printed version is available now, as PostScript and PDF.


Since the first Space Telescope Imaging Spectrograph (STIS) observations of Io in 1997 [1], 32 spectrally dispersed STIS images of Io containing the S II 1256Å line have been obtained during eight visits (observing sequences). Each image is a 2" × 25" rectangle containing Io, which includes emission out to 15-40 Io radii from the moon, depending on viewing geometry. After carefully removing contamination from spectrally adjacent lines, the variable dark current in the STIS FUV MAMA, and the contribution of the foreground/background plasma torus, we have examined the S II 1256Å emission away from the surface of Io in each image. We have also compared these data with the overall plasma torus, as seen in [S II] 6731Å groundbased images [2] (which have been acquired throughout this time period, and overlap three of the eight visits in particular). We find that the S II 1256Å emission is quite different from the neutral O and S UV emission observed simultaneously. It falls off more slowly and less symmetrically, and has greater temporal variability; these effects cannot adequately be explained as a simple function of phase, viewing geometry, and System III magnetic longitude, although a System III dependence is present. Earlier [3], we reported a large, highly asymmetric brightening in the extended S II 1256Å emission on 14 October 1997, correlated with brightenings in neutral O and S UV lines in the same STIS data and with [O I] 6300Å observed from the ground; this brightening is now seen to be unique in the full dataset, both in brightness and in asymmetry. (This is consistent with the much larger groundbased [O I] 6300Å dataset [4], in which features comparable to the 14 October 1997 brightening are rare.) These and other results, and their implications for the Io-torus interaction, will be discussed. This work was supported in part by NASA grants NAS5-30131 and NAG5-6546, and RTOP 344-32-30.


  1. Roesler, F. L., H. W. Moos, R. J. Oliversen, R. C. Woodward, Jr., K. D. Retherford et al.. "Far-Ultrviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS." Science 283, 353 (1999).
  2. Woodward, Jr., R. C., R. J. Oliversen, F. Scherb, F. Roesler. "Synoptic Broadband Imaging of the Io Plasma Torus in [S II] 6731Å: Long-term Variability." Bull. American Astron. Soc., 32, 1059 (2000).
  3. Woodward, Jr., R. C., F. L. Roesler, R. J. Oliversen, W. H. Smyth, H. W. Moos, F. Scherb. "Simultaneous HST/STIS and Groundbased Observations of Sulfur in the Io Plasma Torus." Eos, 81, S290 (2000).
  4. Oliversen, R. J., F. Scherb, W. H. Smith, M. E. Freed, R. C. Woodward, Jr., M. L. Marconi, K. D. Retherford, O. L. Lupie, J. P. Morgenthaler. "Sunlit Io Atmospheric [O I] 63000Å Emission and the Plasma Torus." J. Geophys. Res., in press.

1University of Wisconsin--Madison (Dept. of Physics)
2NASA Goddard Space Flight Center
3Atmospheric & Environmental Research, Inc.
4Johns Hopkins University (Dept. of Physics & Astronomy)
5University of Colorado.

This site was last updated 31 July 2001. Its contents are copyright © 2001 R. Carey Woodward, Jr. and/or the Board of Regents of the University of Wisconsin, as their interests lie. All rights reserved. Academic use permitted with appropriate citation.