Calculations have been made of the diurnal variations in minor neutral constituents in the mesosphere and lower thermosphere for the condition of 60° latitude, summer and winter, using the method developed by the authors [Shimazaki and Laird, 1970]. The excited molecular oxygen O2(1Δg) has been added and the effects of absorption of Schumann-Runge bands have been taken into account. The results show significant seasonal differences, which may be interpreted mainly in terms of the difference of penetration of solar radiation and the duration of sunlit hours. However, the comparison of the observed seasonal and diurnal variations in the airglow emissions from hydroxyl, atomic oxygen (5577 Å), and O2(1Δg) with those calculated from models suggests that effects of large-scale meridional circulation (horizontal and vertical transports) may be important in explaining these observations. It is shown that the concentration of the constituents whose main loss mechanism is recombination with atomic oxygen decreases sharply above ˜80 km. These constituents include OH, HO2, H2O2, and NO2 (nighttime) and the extension of this general theory may explain the sudden decrease observed by rockets in the water-cluster ion concentration above ∼80 km. Ion-neutral chemistry should be important in explaining an observed increase in [NO] above ∼85 km.
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