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F. Herbert (LPL/U.Arizona), A. Hendrix (JPL), F. Bagenal, N.M. Schneider (LASP/U.Colorado)
We analyze 74 FUV (1150 \le \lambda \le 1720\thinspaceÅ) HST/STIS observations of the dawn ansa of the Io plasma torus made during Galileo's C23, I24, and I27 encounters with Io. These data include G140L spectral scans from 6 HST orbits observing near System III longitudes 39\degr, 149\degr, 208\degr, 267\degr, 325\degr, and 337\degr, plus 2 undispersed images at ~90\degr\ from 1 additional orbit. Special problems include the faintness (\leq 200\thinspace R) of the torus at these wavelengths and the presence of a spatially inhomogeneous dark-count background with intensity comparable to the signal.
To minimize sensitivity to photon noise, we fit the data with a special constrained-nonnegative least squares fitting procedure [JGR 105:16035], using a simple plasma emission model integrated over the line of sight. This procedure inverts the line-of-sight density superposition (``peels the onion'') using the assumption of local azimuthal symmetry at the dawn ansa and estimates model parameter uncertainty by Monte Carlo noise simulation.
The undispersed images show that the ``ribbon'' feature seen at visible wavelengths in ground-based observations [ Science 226:337] is also clearly present in the FUV. Although the analysis is still in its first stages, the fits to the S\scriptscriptstyle +\thinspace1256\thinspace Å, S\scriptscriptstyle ++\thinspace1297\thinspace Å, and S\scriptscriptstyle +++\thinspace1410\thinspace Å\ multiplets in the spectral scans indicate that the S\scriptscriptstyle ++ and S\scriptscriptstyle +++ densities peak ~0.1\thinspace R\rm J further from Jupiter than does the S\scriptscriptstyle + density. Moreover, the System III variation of torus scale height relative to the centrifugal equatorial plane that was first observed in ground-based images [Science 450:450, JGR 100:21683] also appears in our results. It also appears that the S\scriptscriptstyle +/\thinspace S\scriptscriptstyle ++ and S\scriptscriptstyle +/\thinspace S\scriptscriptstyle +++ ratios dropped by a factor of ~2 between the I24 and I27 epochs, a variability that has also been seen in EUVE observations [F. Herbert et al., JGR 106, in press].
We thank NASA and STScI for support under grant #s NAG5-8952, NAG5-6362, and NAG5-8051.
The author(s) of this abstract have provided an email address for comments about the abstract: herbert@vega.LPL.arizona.edu