Model calculations for Io's atmosphere at eastern and western elongations

Numerical calculations are performed for Io's sublimation atmosphere at eas tern and western elongations using an improved version of the multispecies hydrodynamic model first developed by Wong and Johnson (1996). Subsolar SO2 frost temperatures of 113 and 120 K are adopted to cover a range of plausi ble atmospheric abundances that are consistent with recent observations. In the model, the incoming plasma ions are allowed to reaccelerate to the unp erturbed corotational speed after colliding with a neutral in the atmospher e above the ion gyropause where the local ion gyration frequency is higher than the neutral collision frequency. This reenergization of ions gives an energy deposition rate per ion that can be up to 2.5 times its upstream cor otational energy rate, depending on the local atmospheric column. It is fou nd that, in general, SO2 is the dominant species in the dayside atmosphere while the noncondensibles, O-2 and SO2 are the dominant species in the nigh tside atmosphere. The gas-phase reactions among these noncondensibles can p roduce a substantial amount of SO2 in the nightside atmosphere. In all four cases considered here (high and low density; eastern and western elongatio ns) we find that there exists a global exobase above all of the surface and that the ratio of dayside to nightside average SO2 abundance is surprising ly limited to the range 17-95, even though surface vapor pressure on the ni ghtside is many orders of magnitude lower. The dayside [SO]/[SO2] mixing ra tio is similar to 3.2-7.1%, consistent with recent observations by Lellouch (1996) if SO is colocated with SO2. We have also included simple NaX chemi stry in the model and found that atomic sodium is far more abundant than mo lecular sodium because of fast reactions of NaX with O. Depending on the so lar zenith angle and Io's orbital location, the exobase altitude ranges fro m 30 to 465 km while its temperature ranges from 220 to 2800 K. It is also found that at western elongation the dayside atmospheric flow is enhanced b y the impinging plasma ions, resulting in an overall hotter and bigger atmo sphere which has an average SO2 column that is about twice as high as what would result from a hydrostatic atmosphere in vapor pressure equilibrium wi th the surface. At eastern elongation, the plasma energy is added to the ni ghtside atmosphere, increasing its pressure and causing the formation of a standing oblique shock that occurs farther upstream in the day-to-night gas flow than in the case for western elongation. Consequently, the dayside at mospheric temperature is lower, and the day-to-night transport rate is redu ced, which results in lower relative abundance for all other species except SO. The results from the high density case at western elongation give a li ne-of-sight tangential column density at the terminator for S, O, and Na, r espectively, of 1.7 x 10(15), 2.5 x 10(15), and 4.3 x 10(13) cm(-2). These profiles above the surface are reasonably consistent with those in Io's cor ona inferred from recent HST measurements for S and O (F. Roesler 1999, pri vate communication) and from eclipse measurements for Na (Schneider et al. 1991a). (C) 2000 Academic Press.