Transpolar structure of polar mesospheric clouds

Middle-ultraviolet (210- to 252-nm) images have revealed the transpolar str ucture of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument o n the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid- UV images of PMCs during 26 passes over the north and south polar regions d uring the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analys is of PMC radiance projected to an 83-km altitude reveals periodic structur es with wavelengths ranging from similar to 100 to similar to 3000 km. In e ither hemisphere, more PMCs have features with wavelengths shorter than 100 0 km than longer, and a crude spectrum of the PMC structures suggests a spe ctral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of similar to 100 km, and the longer wavelengths g enerally have more spectral "power" than the shorter wavelengths. PMC struc tures do not remain stable over time periods of weeks, bur may retain simil ar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmo sphere to the mesosphere and modulate the appearance of PMCs.