Noctilucent cloud observed in late April at South Pole Station: Temperature anomaly or meteoritic debris?

Abstract

A sunlit cloud was observed near the horizon at South Pole Station (90°S), four months after summer solstice in 1992, at a solar depression angle of 14.6°. The angular location of the transition from sunlit to twilit cloud in the photograph establishes the cloud height at about 80 km. The cloud extended horizontally at least from 81° to 85°S and from 40°W to 20°E. The probable origin of this cloud by formation of water-ice crystals near the mesopause suggests that mesospheric temperatures occasionally deviate by at least 70 K from their climatological means. An alternative explanation that the cloud was a debris cloud formed by disintegration of a reentering man-made satellite is ruled out. A debris cloud from an entering meteoroid is a possible explanation but would be unprecedented. Hydroxyl airglow emissions at the south pole in May give a frequency of 1 % for mesopause temperatures below 155 K, consistent with the rarity of out-of-season sightings of noctilucent clouds in Antarctica.