Thermal structure of the mesopause region (80-105 km) at 40 degrees N latitude. Part II: Diurnal 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 behavio r of solar thermal tides in the mesopause region between 80 and 105 km. The 24-, 12-, 8-, and 6-h tides are investigated. Between 80 and 92 lan the di urnal temperature variation is influenced mainly by in situ heating associa ted with solar UV absorption by O-3 (similar to 5 K amplitude at 85 km) and by an upwardly propagating wave originating from tidal sources in the trop osphere and stratosphere. Above 100 lan, there is also a strong diurnal sig nature due to in situ heating from solar UV absorption by O-2 (similar to 3 K amplitude at 102 km). The phase analysis shows this oscillation to be ev anescent or downward propagating with maximum amplitude near local noon. Th e middle region between 92 and 100 km contains very little diurnal variatio n, which appears to be caused by destructive interference of the direct sol ar heating, which is maximum during the day with the migrating tidal pertur bations and chemical heating (similar to 1.5 K amplitude at 96 lan), which are both maximum at night. This feature of the diurnal oscillation is preva lent throughout the annual cycle. The diurnal variation is significantly re duced during winter. The Global Scale Wave Model (GSWM) consistently undere stimates the diurnal amplitude especially at altitudes below 90 km. The sem idiurnal tide is characterized by increasing amplitude with increasing alti tude throughout the mesopause region at all times of the year. The GSWM pre dicts a similar structure, but with a smaller amplitude growth length than the observations reveal. The phase of the 12-h tide is generally downward f or all seasons, indicating this component is excited below the mesopause re gion. The 8- and 6-h tides exhibit consistently small amplitudes (similar t o 3 K) below 97 km and sharp amplitude increases above 100 km. An analysis of a mean day averaged over the entire annual cycle highlights the fundamen tal structure of mesosphere and lower thermosphere solar tides. Evanescent 24-h variations are dominant below 93 km with maximum temperature near loca l noon, while upwardly propagating 12-h oscillations dominate above 95 km w ith maximum amplitudes near local midnight and noon. The total rms temperat ure variability associated with the combined effects of gravity waves, tide s, and seasonal variations averages about 13 K between 80 and 105 km.