SPA-GRL Issue Highlights

The addition of the Space Telescope Imaging Spectrograph to the Hubble Space Telescope (HST) in February 1997 allowed for ultraviolet imaging spectroscopy of Io and its surroundings. Examining spectra observed during HST orbits in 1997-1998, Feldman et al. detect an enhanced Lyman Alpha emission of hydrogen near Io's poles that appears to be uncorrelated with emissions of atomic oxygen and sulfur. They explore the possibility that this emission is reflected solar radiation attenuated by a sulfur dioxide atmosphere concentrated largely near Io's equator, and find it to be a plausible explanation. The Lyman Alpha images are able to show an important feature of Io's atmosphere: a nonuniformity dominated by a freezing out of sulfur dioxide near the poles.

Terdiurnal (8-hour) atmospheric tidal oscillations constitute a property of the wind field in the mesosphere and lower thermosphere at northern mid-latitudes. Pendleton et al. examine 10 nights of high-precision measurements of OH Meinel rotational temperatures above the Bear Lake Observatory, Utah, in October 1996, and find a mean nighttime temperature of 203 K. They also find that the terdiurnal component dominated the nighttime temperature variability in the OH Meinel region near the autumnal equinox in 1996; this result is supported by incoherent-scatter-radar data, which independently established the presence of 8-h tidal oscillations in the 95-130-km region at the time of interest. The authors report that the daily amplitude of the 8-h component of nocturnal temperature variability in the OH Meinel region can approach 15 K.