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T.H. Morgan (NASA HQ), A.E. Potter (National Solar Observatory), J.B. Corliss (Univ. of Wisconsin, Madison), R.M. Killen (Univ. of Maryland, College Park), F. Scherb, R.C. Woodward (Univ. of Wisconsin, Madison)
When Io emerges from eclipse by Jupiter, the sodium D-line emissions are weak, partly because of the small Doppler shift of Io relative to the Sun, but also because the neutral sodium density is a factor of two to four times smaller than when Io has been out of the shadow and exposed to sunshine for an hour or two. We have observed four cases of post-eclipse brightening of the sodium emission, using a 10 arc second aperture image slicer to obtain images of sodium around Io. These show that initially, most of the sodium is confined to the �banana cloud� near Io. As time passes, the sodium density close to Io increases, and after and hour or two, most of the sodium is found surrounding Io. We have thought of two possible explanations for the post-eclipse sodium growth. One has to do with condensation of sulfur dioxide during the eclipse, covering particles of NaCl, which are thought to be the source of sodium (Hunten, 2003). As Io emerges from eclipse, the sulfur dioxide sublimes, exposing the NaCl to ion sputtering. Another possibility is that the sputtering process, which yields sodium, is strongly temperature dependent, such that yields are low during cold time during the eclipse, and then increase rapidly as Io emerges into warming sunshine. Laboratory experiments by Yakshinskiy et al. (2004) have shown that sputtering yields of sodium are, in fact, strongly temperature dependent. The NASA Planetary Astronomy Program supported this work.
Hunten, D.M. (2003) Nature 421, 30-31. Yakshinskiy, B.V. and Madey, T.E. (2004) Icarus 168, 53-59.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.