Variability of the neutral mesospheric and lower thermospheric composition in the diurnal cycle

A non-steady one-dimensional model atmosphere, including N2, oxygen, hydrogen and carbon compounds, suitable for the interpretation of the processes occurring during twilight, has been developed for mid-latitude and equinox conditions at altitudes from 60 to 220 km. The temporal dependence of the photodissociation coefficients has been calculated in detail, paying special attention to twilight conditions. Photochemical processes for the different compounds are considered using the latest reaction rate coefficients. Vertical transport effects are also included, considering molecular diffusion and a semiempirical eddy diffusion coefficient profile. The Variability of the different compounds throughout a complete day is obtained and compared with other theoretical models and experimental measurements. Special emphasis is paid to the study of the rapid changes occurring during morning and evening twilight times in the concentration of various minor compounds. The results obtained have been applied to the study of the diurnal evolution of the O2(1Δg) and OH emissions derived from appropriate photochemical schemes. A reasonable agreement is achieved between different measurements and our predictions and this may be taken to validate the model. © 1991.