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M. H. Burger, N. M. Schneider (University of Colorado/LASP), I. de Pater (Berkeley), M. Brown, A. Bouchez (Cal Tech), T. Mallama (STZ)
Between Io's atmosphere and extended neutral clouds exists a corona or exosphere composed of neutral atoms and molecules bound by Io's gravity. Although sodium is probably only a minor constituent of the corona, the fact that it is has a high cross section for resonant scattering makes it very easy to detect. However, close to Io's surface, sodium abundance can not be measured using standard earth-based techniques. This is because earth-based limitations on resolution make it impossible to distinguish reflected light off Io's surface from sodium emission.
We have measured the density profile of the sodium corona using a combination of two sets of observations which can circumvent the problems associated with standard techniques. The first are Galileo Solid State Imager (SSI) observations taken of the sodium cloud. These images provide spatial resolution not obtainable from earth, and light originating above Io's surface (from either atomic emission or scattering off plume materials) can be separated from light reflected off the moon's surface. The second method for measuring the density profile relies on observations of mutual events of the Galilean satellites observed using the Keck HIRES Spectrograph. As Io eclipses another moon (e.g. Callisto), light from the sun passes through Io's corona. A spectrum taken of Callisto will reveal an absorption feature superimposed on the solar Na D Fraunhofer lines. A series of these spectra taken at short intervals during the eclipse can provide measurements of the coronal density at different radial densities from Io.
Combined analysis of the Galileo and Keck data will give the density profile in the corona within approximately six Io radii. By comparing the two different data sets, a search can then be made for spatial asymmetries, and the results will be compared with measurements from mutual events in 1985 (Schneider, et al., Ap.J., 368, 298, 1991) with an aim for discovering temporal variability.
This work is supported by the NASA Planetary Astronomy Program.