The Colorado State Na temperature lidar has been in regular nighttime operation since 1991. By the end of January 1993, 94 nights of quality data with more than 4 hrs each night have been collected. Analysis of a total of 22,999 photocount files gives rise to high resolution seasonally and nightly averaged temperature and Na density profiles, from which the thermal structure of a midlatitude mesopause emerges. Contrary to the reference atmosphere of CIRA 1986, two prevailing temperature minima are seen at altitudes 86.3¿2.5 km and 99.0¿2.9 km with the lower minimum having the dominating effect. The trend of the mean nightly temperature of the lower minimum is observed to vary monotonically between a low of 172 K at summer solstice and a high of 212 K at nearly one month following winter solstice. As in the polar regions, this is a clear signature of a wave-driven diabatic circulation observed at a midlatitude.

The temperature of the upper minimum fluctuates around 189¿8 K year-round. Like the polar mesopause, the mesopause altitude follows a bistable pattern, although the seasonal variation of the mesopause temperature is much more complicated. Based on the seasonal variations of these two temperature minima, the observed patterns of mesopause altitude and temperature can be explained. Although the mechanisms responsible for the formation of double temperature minima are not clear at present, the lower temperature minimum with a robust 40 K annual temperature change plays the role of the mesopause commonly known. ¿American Geophysical Union 1993