Thermal Structure of the Mesopause Region (80-105 km) at 40°N Latitude. Part I: Seasonal Variations.
Sodium wind/temperature lidar measurements taken throughout the diurnal and annual cycles at Urbana, Illinois (40°N, 88°W), from February 1996 through January 1998 are used to characterize the seasonal variations of the mesospheric temperature structure between 80 and 105 km. By averaging data over several weeks and over the complete diurnal cycle, the significant 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 and radiative cooling by IR emission. Large seasonal variations of the mean thermal structure are observed. Below 91 km, there is strong adiabatic cooling in summer caused by the mean upward velocities associated with the diabatic circulation system. The maximum amplitude of the annual variation is 9.7 K at approximately 84 km. Above 98 km, increased UV absorption by O2 during 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 lower thermosphere induces a modest 5-km peak to peak annual variation in the mesopause altitude. The mesopause is near 101 km in winter and 96 km in late summer. However, the summer cooling below 91 km is strong enough to define the minimum temperature, causing the mesopause altitude to fall to 87 km from about 7 May to about 15 July (70 days). The mesopause thickness, defined here as the altitude range where the temperature is within 5 K of the minimum, increases dramatically from approximately 7 km in winter to over 16 km in summer. Significant biases can occur in some parameters calculated from nighttime-only observations. The inversion layers that persist between 85 and 96 km in nighttime temperature profiles are virtually eliminated when data are averaged over the complete diurnal period. The strong annual temperature variation present around 84 km is overestimated by 40%, and the strong semiannual variation above 95 km is overestimated by as much as 150% when computed using only nighttime measurements. The low summer mesopause exists for a much longer period (126 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.