HST/STIS FUV Observations of the Io Plasma Torus

Details HST/STIS FUV Observations of the Io Plasma Torus HST/STIS FUV Observations of the Io Plasma Torus

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.1710h&link_type=abstract

American Astronomical Society, DPS Meeting #33, #17.10; Bulletin of the American Astronomical Society, Vol. 33, p.1064

Astronomy and Astrophysics

Astronomy

Scientific paper

We analyze 74 FUV (1150 <= λ <= 1720 Å) 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°, 149°, 208°, 267°, 325°, and 337°, plus 2 undispersed images at ~90°\ from 1 additional orbit. Special problems include the faintness (<= 200 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+ 1256 Å, S++ 1297 Å, and S+++ 1410 Å multiplets in the spectral scans indicate that the S++ and S+++ densities peak ~0.1 R J further from Jupiter than does the S+ 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+/S++ and S+/S+++criptstyle; 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.

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