OBSERVED THERMAL STRUCTURE OF A MIDLATITUDE MESOPAUSE

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 seas onally and nightly averaged temperature and Na density profiles, from which the thermal structure of a midlatitude mesopause emerges. Contra ry to the reference atmosphere of CIRA 1986, two prevailing temperatur e 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 monotoni cally 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 observ ed at a midlatitude. The temperature of the upper minimum fluctuates a round 189 +/- 8 K year-round. Like the polar mesopause, the mesopause altitude follows a bistable pattern, although the seasonal variation o f the mesopause temperature is much more complicated. Based on the sea sonal variations of these two temperature minima, the observed pattern s 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 common ly known.