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P. E. Geissler, A. S. McEwen (L.P.L., Univ. Arizona), W. Ip (M.P.I., Germany), M. J. S. Belton (N.O.A.O.), T. V. Johnson (J.P.L.), Galileo SSI Team
Galileo images of Io during eclipse reveal diffuse atmospheric emissions which are sometimes distinct in character from those noted in ground-based and HST observations. The airglow has been imaged by the Solid State Imaging (SSI) system over the course of 14 eclipses in 10 orbits during the nominal and extended missions. The emissions can be divided into three morphologic classes: extended limb glows, thin continuous limb glows, and diffuse glows seen against Io's disk, all distinct from the small high-temperature hot spots. The most prominent (clear-filter images, 0.4-1.0 micron) are bright emissions centered on the equator near the sub- and anti-Jupiter points, extending up to 300 km above the sub-Jupiter limb and 700 km above the anti-Jovian limb. These extended limb glows show relatively little variability in location as a function of the geometry of Jupiter's magnetosphere. In contrast, they appear to be closely associated with centers of volcanic activity on Io's surface: Kanehekili and Acala on the sub-Jupiter hemisphere and Prometheus, Culann, and Zamama on the anti-Jupiter side. Images from the earliest orbits show differences on the sub-Jupiter hemisphere, because the Kanehekili and Acala plumes were inactive and the Ra plume was active. Acala is particularly notable in the eclipse data because it has a distinct plume-like morphology (~300 km high) and because the plume has not yet been seen in reflected light. Acala may be a largely gaseous "stealth plume" with little particulate matter. Pele could be another stealth plume, but no emissions have been positively identified in eclipse. The second morphologic class is a thin glow surrounding Io's limb irrespective of latitude or longitude. However, the relative brightness of the northern and southern limbs appears to vary with time, perhaps driven by fluctuations in the tilt of the externally imposed magnetic field. The third class, diffuse glows seen against Io's disk, primarily correlates with the distribution of active plumes on the sub- and anti-Jovian hemisphere and the extended limb glows. One exception is a region on Io's equatorial leading (downstream) hemisphere seen best in SSI's green filter. In contrast, the extended limb glows are relatively bright in the violet bandpass and the continuous limb outline is relatively bright in the red bandpass. These variations with color filter suggest distinct distributions of emitting species in Io's atmosphere. Co-analysis of the Galileo imaging observations with ground-based and HST data (acquired at different wavelengths, observing geometries, and times) offers the possibility of identifying the emitting species, their spatial distributions, and the volcanic-atmospheric-magnetospheric processes.