IOS SODIUM CORONA AND SPATIALLY EXTENDED CLOUD - A CONSISTENT FLUX SPEED DISTRIBUTION

A data set composed of different groundbased observations for Io's sod ium corona and spatially extended sodium cloud and covering the spatia l range from Io's nominal exobase of 1.4 satellite radii to east-west distances from Io of +/- 100 satellite radii (R(Io)) is used to invest igate the velocity distribution of sodium at the exobase, The data set is composed of the novel 1985 eclipse measurements of Schneider Et al . (1991, Astrophys. J. 368, 298-315) acquired from similar to 1.4 to s imilar to 10 R(Io), the 1985 east-west emission data of Schneider et a l. acquired from similar to 4 to similar to 40 RI,, and sodium cloud i mage data acquired near Io's orbital plane from similar to 10 to simil ar to 100 R(Io) by a number of different observers in the 1976 to 1983 time frame. A one-dimensional east-west profile that contains Io is c onstructed from the data set and is analyzed using the sodium cloud mo del of Smyth and Combi (1988, Astrophys. J. Supp. 66, 397-411; 1988, A strophys. J. 328, 888-918). When the directional feature in the traili ng cloud is either north or south of this east-west line (i.e., not at the null condition), an isotropic modified [incomplete (alpha = 7/3) collisional cascade] sputtering flux speed distribution at the satelli te exobase with a peak at 0.5 km sec(-1) provides an excellent fit to the data set for a sodium source of 1.7 x 10(26) atoms sec(-1). In par ticular, the model calculation reproduces (1) the essentially symmetri c column density distributions exhibited by the eclipse measurements a bout Io within the Lagrange sphere radius (5.85 R(Io), i.e., the gravi tational grasp of the satellite), (2) the change in the slope of the c olumn density observed just beyond the Lagrange sphere radius in the e ast-west profile of the forward cloud, but not in the trailing cloud, and (3) the distinctly different east-west brightness profiles exhibit ed by the forward and trailing clouds in the emission data at the more distant (similar to +/- 20-100 R(Io)) portions of the cloud, In contr ast, the speed dispersion at the exobase for either an isotropic Maxwe ll-Boltzmann flux speed distribution or an isotropic classical (alpha = 3) sputtering Aux speed distribution (which has a higher velocity-ta il population than the Maxwell-Boltzmann, but not as high as the incom plete collisional cascade sputtering distribution) is shown to be inad equate to fit the data set. To fit the enhanced trailing east-west pro file observed when the directional feature is at the null condition, a n additional enhanced high-speed (similar to 15-20 km sec(-1)) sodium population is required which is nonisotropically ejected from the sate llite exobase so as to preferentially populate the trailing cloud. The need for such a nonisotropic high-speed population of sodium has also been recognized in the earlier modeling analysis of the directional f eatures (Pilcher et al., 1984, Astrophys. J. 287, 427-444), in the mor e recent lower-velocity component required in modeling the sodium zeno corona (Smyth and Combi, 1991, J. Geophys. Res. 96, 22711-22727; Flynn et al., 1992, Icarus 99, 115-130), and In the very recent modeling of the directional feature reported by Wilson and Schneider (1995, Bull. Am. Astron. Sec. 27, 1154). A complete sodium source rate speed distr ibution function at Io's exobase from 0-100 km sec(-1) is then constru cted by combining the isotropic modified [incomplete (alpha = 7/3) col lisional cascade] sputtering flux speed distribution, the nonisotropic directional feature (lower-velocity zenocorona) source (similar to 15 -20 km sec(-1)), and the higher-speed (similar to 20-100 km sec(-1)) c harge-exchange source required to simulate the sodium zenocorona far f rom Jupiter. (C) 1995 Academic Press.