Concept of a two-level mesopause: Support through new lidar observations

Abstract

Regular Doppler temperature lidar measurements at Fort Collins, United States (41°N, 105°W), and Kühlungsborn, Germany (54°N, 12°E), during 1996/1997 have provided two new data sets to further detail the annual variation of the thermal structure of the mesopause region, i.e., the 80- to 105-km altitude range. We present 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, between 70°S and 70°N. These data permit us to deduce what we consider to be a genuine global picture depicting the thermal structure of the mesopause region, a prominent and globally unified feature is that the mesopause exists at only two altitudes: A winter state at an altitude near 100 km and a summer state near 88 km. In the winter state, the mesopause is at a nearly constant temperature of about 180 K, independent of latitude. The summer state mesopause does not seem to exist in the equatorial region; its temperature decreases drastically 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 that the winter high mesopause lasts longer at a given geographical latitude and covers more geographical latitudes at a given time, we estimate the globally averaged altitude of the mesopause to be 98 km.