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
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 , 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  (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 , 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 , 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.
- Roesler, F. L., H. W. Moos, R. J. Oliversen, R. C. Woodward, Jr., K. D.
Retherford et al..
Imaging Spectroscopy of Io's Atmosphere with HST/STIS."
283, 353 (1999).
- 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
American Astron. Soc.,
32, 1059 (2000).
- 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."
81, S290 (2000).
- 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."
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.