Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21943210t&link_type=abstract
American Astronomical Society, AAS Meeting #219, #432.10
Astronomy and Astrophysics
Astronomy
Scientific paper
HST/STIS observations of Io obtained in Aug 1999 shortly after umbral ingress into Jupiter's shadow reveal a mid-UV to visual emission spectrum of SO2 excited by impact from Jovian plasma torus electrons (illumination of Io by sunlight refracted by Jupiter's atmosphere is negligible). This spectrum peaks near 3200 Å at 27 Rayleighs/Å. The excitation-dissociation byproducts SO, S I, O I, and potentially S2, are also observed to emit over this range. Two tandem 12-13 min mid-UV exposures obtained with the STIS/MAMA detector beginning 1 min after umbral ingress showed significant weakening of the emission spectrum, which we attribute to partial freezing out of the atmospheric column and the loss of energetic photo-electrons. Similar exposures obtained in the near-UV to visual wavelength range with the STIS/CCD detector beginning 13 min after umbral ingress showed little change in the emission intensity, indicating that most of the freezeout had already occurred. With several minutes between exposures, this time scale is consistent with Io's eclipse light curve taken with the Cassini ISS camera through the clear filter (mid-UV to near-IR), which showed a decline in the disk-averaged intensity in the first 18 min, a relatively flat plateau, then a rise to the pre-eclipse level just prior to egress (Geissler et al. 2004). An unidentified emission source is needed to explain the emission observed longward of the SO2 emission. The low signal level required binning of pixels resulting in only a few spatial resolution elements over Io's disk. Specific plume activity is not well constrained through examination of the disk-averaged MUV emission spectrum. The simulated best fit upstream electron temperature accounting for the peak SO/ SO2 intensity ratios and the absolute intensities is a thermal temperature of 4-5 eV and a non-thermal 30 eV electron density that is 2--5% of the thermal density.
Goldstein David B.
McGrath Melissa A.
Moore Chris H.
Trafton Laurence M.
Varghese Philip L.
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