31st Annual Meeting of the DPS, October 1999
Session 78. Io: Neutral Atmosphere, Ionosphere, Magnetospheric Interactions, and Plasma Torus
Contributed Oral Parallel Session, Friday, October 15, 1999, 4:00-5:30pm, Sala Pietro d'Abano

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[78.06] Lyman Alpha Observations of Io with STIS/HST

G.E. Ballester (U Mich, U Wisc), P.D. Feldman (JHU), F.L. Roesler (U Wisc), H.W. Moos (JHU), R.J. Oliversen (GSFC), R.C. Woodward, F. Scherb (U Wisc), W.H. Smyth (AER), L. Ben Jaffel (IAP, France), D.F. Strobel, B.C. Wolven, K.D. Retherford (JHU)

\noindent Imaging-spectroscopic observations performed with the STIS instrument on HST have revealed a surprising signature from Io at the H Lyman-\alpha wavelength (Roesler et al., Science 283, 353, 1999). With the novel technique of centering Io in the 2\times52'' aperture using the G140L (1150-1730Å) grating, Io's ~\,kR Ly-\alpha signal was imaged and separated from the extended geocoronal and interplanetary medium (IPM) background emissions. The first data, taken on 14 Oct 97, showed very different properties in the Ly-\alpha than in the O and S emissions known to arise from the complex Io-torus-Jupiter interaction. The Ly-\alpha was confined inside the limb, enhanced at the polar and mid latitudes, non-uniform, and stable with time. New data acquired on 23 and 27 Aug 98 are confirming these general properties (Feldman et al., MOP meeting, Paris 1999). Various possible explanations have been put forth, but the new data are strengthening the interpretation of the signal as mainly produced by solar Ly-\alpha reflected by the surface, with a strong modulation by absorption by the molecular atmosphere dominated by SO2. The Ly-\alpha images may thus provide the first map of Io's molecular atmosphere, showing larger opacity and abundance at low latitudes. Results from in-depth analysis of the data will be presented, characterizing the localized roles of both the atmospheric absorption and surface reflectance. (Significant emission and opacity effects in the Io signal from the IPM and geocorona will be accounted for.) We will also evaluate the postulated emission contribution by resonant scattering of solar Ly-\alpha by atomic H in Io's atmosphere, the existence of which has been suggested from Galileo plasma and wave data showing an enhanced proton population near Io with a pickup signature. Our first estimates of an atmospheric H contribution indicate it could be present, but not dominate the Ly-\alpha signal. Other possible emission sources are being considered, as well as whether the possible neutral H source for the pickup protons could be Io itself, the torus, and/or Jupiter.

The author(s) of this abstract have provided an email address for comments about the abstract: gilda@umich.edu

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