Thermal structure of the mesopause region (80-105 km) at 40 degrees N latitude. Part I: Seasonal variations

Sodium wind/temperature lidar measurements taken throughout the diurnal and annual cycles at Urbana, Illinois (40 degrees N, 88 degrees W), from Febru ary 1996 through January 1998 are used to characterize the seasonal variati ons of the mesospheric temperature structure between 80 and 105 km. By aver aging data over several weeks and over the complete diurnal cycle, the sign ificant effects of gravity waves, tides, and planetary waves are surpressed . The observed mean annual temperature structure is largely consistent with the assumption of radiative equilibrium between direct solar UV heating an d radiative cooling by DR emission. Large seasonal variations of the mean t hermal structure are observed. Below 91 gm, there is strong adiabatic codin g in summer caused by the mean upward velocities associated with the diabat ic circulation system. The maximum amplitude of the annual variation is 9.7 K at approximately 84 km. Above 98 km, Increased UV absorption by O-2 duri ng summer drives an annual oscillation in this region with an amplitude of approximately 5 K. These two phenomena determine the seasonal variation of the mesopause altitude. The annual variation in solar UV heating in the low er thermosphere induces a modest 5-km peak to peak annual variation in the mesopause altitude. The mesopause is near 101 km in winter and similar to 9 6 km in late summer. However, the summer cooling below 91 km is strong enou gh to define the minimum temperature, causing the mesopause altitude to fal l to similar to 87 km from about 7 May to about 15 July (similar to 70 days ). The mesopause thickness, defined here as the altitude range where the te mperature is within 5 K of the minimum, increases dramatically from approxi mately 7 hm in winter to over 16 km in summer. Significant biases can occur in some parameters calculated from nighttime-only observations. The invers ion layers that persist between 85 and 96 km in nighttime temperature profi les are virtually eliminated when data are averaged over the complete diurn al period. The strong annual temperature variation present around 84 km is overestimated by 40%, rind the strong semiannual variation above 95 km is o verestimated by as much as 150% when computed using only nighttime measurem ents. The low summer mesopause exists for a much longer period(similar to 1 26 days) in the nighttime observations. The mesopause temperature averaged over the annual cycle is 188 K compared to 190 K for the nighttime average. This bias is most pronounced during summertime when the difference is 7 K.