Thermal Structure of the Mesopause Region (80–105 km) at 40°N Latitude. Part II: Diurnal Variations

Abstract

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 behavior 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 km the diurnal temperature variation is influenced mainly by in situ heating associated with solar UV absorption by O3 (?5 K amplitude at 85 km) and by an upwardly propagating wave originating from tidal sources in the troposphere and stratosphere. Above 100 km, there is also a strong diurnal signature due to in situ heating from solar UV absorption by O2 (?3 K amplitude at 102 km). The phase analysis shows this oscillation to be evanescent or downward propagating with maximum amplitude near local noon. The middle region between 92 and 100 km contains very little diurnal variation, which appears to be caused by destructive interference of the direct solar heating, which is maximum during the day with the migrating tidal perturbations and chemical heating (?1.5 K amplitude at 96 km), which are both maximum at night. This feature of the diurnal oscillation is prevalent throughout the annual cycle. The diurnal variation is significantly reduced during winter. The Global Scale Wave Model (GSWM) consistently underestimates the diurnal amplitude especially at altitudes below 90 km. The semidiurnal tide is characterized by increasing amplitude with increasing altitude throughout the mesopause region at all times of the year. The GSWM predicts a similar structure, but with a smaller amplitude growth length than the observations reveal. The phase of the 12-h tide is generally downward for all seasons, indicating this component is excited below the mesopause region. The 8- and 6-h tides exhibit consistently small amplitudes (<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 fundamental structure of mesosphere and lower thermosphere solar tides. Evanescent 24-h variations are dominant below 93 km with maximum temperature near local noon, while upwardly propagating 12-h oscillations dominate above 95 km with maximum amplitudes near local midnight and noon. The total rms temperature variability associated with the combined effects of gravity waves, tides, and seasonal variations averages about 13 K between 80 and 105 km.