CONCEPT OF A 2-LEVEL MESOPAUSE - SUPPORT THROUGH NEW LIDAR OBSERVATIONS

Regular Doppler temperature lidar measurements at Fort Collins, United States (41 degrees N, 105 degrees W); and Kuhlungsborn, Germany (54 d egrees N, 12 degrees E), during 1996/1997 have provided two new data s ets to further detail the annual variation of the thermal structure of the mesopause region; i.e., the 80- to 105-km altitude: range. We pre sent a quantitative analysis of these data sets taken at two stations with considerable north-south and east-west differences. In addition, we expand this observational base by including data taken earlier, bet ween 70 degrees S and 70 degrees N. These data permit us to deduce wha t we consider to be a genuine global picture depicting the thermal str ucture of the mesopause region. a prominent and globally unified featu re is that the mesopause exists at only two altitudes: A winter state at an altitude near 100 lan and a summer state near 88 lan. In the win ter state, the mesopause is at a nearly constant temperature of about 180 K, independent-of latitude. The summer state mesopause does not se em to exist in the equatorial region; its temperature decreases drasti cally as one goes from midlatitudes toward the poles. Relatively sharp transitions take place from the winter state to the summer state and vice versa in spring and autumn, respectively. Since our data show tha t the winter high mesopause lasts longer at a given geographical latit ude and covers more geographical latitudes at a given time, we estimat e the globally averaged altitude of the mesopause to be 98 km.