Io's Sodium Corona and Spatially Extended Cloud: A Consistent Flux Speed Distribution

Abstract

A data set composed of different groundbased observations for Io's sodium corona and spatially extended sodium cloud and covering the spatial 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 investigate 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 ∼1.4 to ∼10 R Io, the 1985 east–west emission data of Schneider et al.acquired from ∼4 to ∼40 R Io, and sodium cloud image data acquired near Io's orbital plane from ∼10 to ∼100 R Ioby a number of different observers in the 1976 to 1983 time frame. A one-dimensional east–west profile that contains Io is constructed from the data set and is analyzed using the sodium cloud model of Smyth and Combi (1988, Astrophys. J. Supp.66, 397–411; 1988, Astrophys. J.328, 888–918). When the directional feature in the trailing cloud is either north or south of this east–west line (i.e., not at the null condition), an isotropic modified [incomplete (α = 7/3) collisional cascade] sputtering flux speed distribution at the satellite exobase with a peak at 0.5 km sec −1provides an excellent fit to the data set for a sodium source of 1.7 × 10 26atoms sec −1. In particular, the model calculation reproduces (1) the essentially symmetric column density distributions exhibited by the eclipse measurements about Io within the Lagrange sphere radius (5.85 R Io, i.e., the gravitational grasp of the satellite), (2) the change in the slope of the column density observed just beyond the Lagrange sphere radius in the east–west profile of the forward cloud, but not in the trailing cloud, and (3) the distinctly different east–west brightness profiles exhibited by the forward and trailing clouds in the emission data at the more distant (∼ ±20–100 R Io) portions of the cloud. In contrast, the speed dispersion at the exobase for either an isotropic Maxwell–Boltzmann flux speed distribution or an isotropic classical (α = 3) sputtering flux speed distribution (which has a higher velocity-tail population than the Maxwell–Boltzmann, but not as high as the incomplete collisional cascade sputtering distribution) is shown to be inadequate to fit the data set. To fit the enhanced trailing east–west profile observed when the directional feature is at the null condition, an additional enhanced high-speed (∼15–20 km sec −1) sodium population is required which is nonisotropically ejected from the satellite 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 features (Pilcher et al., 1984, Astrophys. J.287, 427–444), in the more recent lower-velocity component required in modeling the sodium zenocorona (Smyth and Combi, 1991, J. Geophys. Res. 96, 22711–22727; Flynn et al., 1992, Icarus99, 115–130), and in the very recent modeling of the directional feature reported by Wilson and Schneider (1995, Bull. Am. Astron. Soc.27, 1154). A complete sodium source rate speed distribution function at Io's exobase from 0–100 km sec −1is then constructed by combining the isotropic modified [incomplete (α = 7/3) collisional cascade] sputtering flux speed distribution, the nonisotropic directional feature (lower-velocity zenocorona) source (∼15–20 km sec −1) , and the higher-speed (∼20–100 km sec −1) charge-exchange source required to simulate the sodium zenocorona far from Jupiter.