THE EFFECT OF PLASMA-HEATING ON SUBLIMATION-DRIVEN FLOW IN IOS ATMOSPHERE

The atmospheric flow on Io is numerically computed in a hat 2-D axisym metric geometry for a sublimation atmosphere on the trailing hemispher e subjected to plasma bombardment, UV heating, and IR cooling. Calcula tions are performed for subsolar vapor pressures of similar to 6.5 x 1 0(-3) Pa (similar to 3 x 10(18) SO2/cm(2)) and 6.8 x 10(-4) Pa (simila r to 4 x 10(17) SO2/cm(2)); the latter approximates the vapor pressure of F. P. Fanale et al. (1982, Satellites of Jupiter, pp. 756-781, Uni v. of Arizona Press, Tucson). The amount of plasma energy deposited in the atmosphere is 20% of the plasma dow energy due to corotation (J. A. Linker et al., 1988, Geophys. Res. Lett. 15, 1311-3141). It is foun d that plasma heating significantly inflates the upper atmosphere, inc reasing both the exobase altitude and the amount of surface covered by more than an exospheric column of gas. This in turn controls the supp ly of the Io plasma torus (M. A. McGrath and R. E. Johnson, 1987, Icar us 69, 519-531). The horizontal flow of mass and energy is also import ant in determining the exobase altitude; and it is shown that IR cooli ng can be important, although our use of the equilibrium, cool-to-spac e approximation for a pure SO2 gas (E. Lellouch et al., 1992, Icarus 9 8, 271-295) may overestimate this effect. The calculated exobase altit udes are somewhat lower than those suggested by McGrath and Johnson (1 987) for supplying the torus, indicating the details of the plasma ene rgy deposition and sputter ejection rate near the exobase, as well as the IR emission from this region need to be examined. In addition, the molecules sublimed (or sputtered) from the surface are transported to the exobase in times short compared to the molecular photodissociatio n time. Therefore, the exobase is dominated by molecular species and t he exobase is supplied by a small region of the surface. (C) 1995 Acad emic Press, Inc.