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[53.06] Model Comparison of Io's Atmosphere at Eastern and Western Elongation

M. C. Wong, W. H. Smyth (AER)

Model calculations are performed for Io's atmosphere, with sublimation as the primary atmospheric gas source, for cases where the satellite is at eastern elongation (dawn) and western elongation (dusk). It is found that the structure and composition of the atmosphere change significantly between these two locations of Io on its orbit around Jupiter. The differences can be primarily attributed to the varying interaction of the atmosphere with the corotating plasma ions that preferentially bombard the trailing side of the satellite. At western elongation (dusk) where the sublimation atmosphere is centered on the trailing hemisphere this interaction is maximized, producing a day-night total column ratio of ~ 70 whereas it has a value of ~ 500 at eastern elongation. In these calculations the ions are reaccelerated after collisions with the neutrals if they are above the region where the ionosphere is collisionally bound. This reacceleration leads to a subtantially higher heating rate (~ a factor of two higher than that used in Wong and Johnson 1996, {\sl J. Geophys. Res.} {\bf 101}, 23,243-23,254 ) which further raises the exobase and drives stronger dynamic transport of SO₂ to the nightside. It is also found that the abundance and the relative composition at the exobase of the species vary significantly between the dayside and nightside atmosphere. While SO₂ is still the dominant species in the dayside, other species such as SO and O are comparable in abundance in the nightside. With the introduction of small amount of Na and Na-bearing compounds at the surface, it is shown that atomic sodium is far more abundant in total column and, in addition its relative composition at the exobase, than molecular sodium.