Seasonal and Diurnal Variations in the Temperature and Sodium Density Structure of the Middle Atmosphere

140 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1998.More than 1000 hours of high-resolution sodium lidar observations were taken at the Urbana Atmospheric Observatory from February 1996 to April 1998. Temperature and Na density were measured in the mesosphere and lower thermosphere (MLT) during both day and night throughout the annual cycle. Coverage over the complete diurnal cycle produces unbiased, background profiles and enables an accurate characterization of tidal oscillations. The thermal structure was determined from 80 to 105 km, and the Na density structure from 76 to 108 km. The annual mean temperature structure is determined largely by radiative equilibrium. Indirect heating mechanisms such as chemical heating, quenching, and turbulent heating are mostly balanced by dynamic cooling associated with dissipating gravity waves. The mesopause (altitude of minimum temperature) is controlled by two independent seasonal changes in the thermal structure. The mesopause occupies a high state (>96 km) during most of the year, when it is determined by in situ thermospheric heating, except during a 2-month summer period when strong adiabatic cooling below 92 km forces the mesopause to approximately 87 km. The temperature inversion layer frequently observed in this region from nighttime campaigns is suppressed when averaging over the complete diurnal period. Daytime photolysis and photoionization has a considerable effect on Na chemistry in the MLT. A phase analysis of the diurnal (24-h) oscillation reveals that daily changes in temperature and Na density are caused by a combination of dynamics (propagating tides from the troposphere and stratosphere), in situ forcing (heating by absorption of solar UV radiation), and chemistry (exothermic reactions that influence temperature and photochemical reactions that affect Na density).U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD