Abstract

Noctilucent clouds (NLC) are observed in the northern latitudes during the summer months. The clouds, thought to be composed of crystalline ice, occur between 80- and 85-km altitude where the temperature of the mesopause can dip to 120 K. These clouds can be seen by ground-based observers when the sun is 6-16° below the horizon. This viewing geometry assures that the clouds will be illuminated by the sun while the lower atmosphere is dark. Imagery collected by Delta Star is presented as well as correlated ground-based and aircraft-based observations. Narrow field of view images obtained by Delta Star resemble NLC photographs made from the ground and from aircraft. Delta Star imaged NLCs on four of the six nights the experiment was attempted, three over central Alaska and one over Finland. Corroborating observations are available for three of these four nights. Altitude measurements made from Delta Star data place the NLCs at 81-km altitude with an estimated thickness of 4 km.

© 1991 Optical Society of America