Altitudes of polar mesospheric clouds observed by a middle ultraviolet imager

The first images of Earth limb in the middle ultraviolet (235-263 nm) have revealed in detail the altitude structures of polar mesospheric clouds (PMC s). The images were obtained from the Ultraviolet and Visible Imaging and S pectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) spacecraft during the austral summer of 1997-1998. The satellite made multiple passes over the Antarctic and obtained over 750 images of PMCs at latitudes poleward of 70 degrees S. Even without correction for scene back grounds, the imager easily observed PMCs distinct from the atmospheric back grounds. The clouds appeared as discrete, filamentary structures having alt itudes between 80 and 85 km, although most PMCs appeared at altitudes betwe en 82.0 and 83.0 km with a mean of 82.3 +/- 0.8 km. The clouds were randoml y distributed on a trans-polar scale of similar to 1000 km, although in som e instances the clouds clustered for distances of 200-300 km across the pol ar mesosphere. In other instances, PMCs were wholly absent on the mesospher ic horizon. The imager also noted enhanced radiances on the topsides of the PMC altitude profiles; this excess radiance may be caused by "subvisible" particles not apparent at visible wavelengths. The PMC altitudes do not app ear correlated with latitude or local time on the scale of the observations discussed here.