The nonequilibrium calculation for various neutral constituents in the mesosphere and lower thermosphere (Hunt, 1966) was extended by including the effects of molecular and eddy diffusion. Nitrogen and its oxides were added, and more recent laboratory data for chemical reaction coefficients were used. A special numerical technique has been developed to simultaneously solve time‐dependent continuity equations for 14 constituents. The preliminary result indicates that vertical eddy diffusion significantly reduces the irregular variations around 80 km and tends to lower the height of these irregular variations. However, the value of the eddy diffusion coefficient acceptable from the consideration of heat balance below ∼100 km cannot entirely smooth out these irregular distributions for atomic oxygen, ozone, and oxygen‐hydrogen compounds, although the smoothing effect for nitric oxide seems to be strong enough to eliminate the irregular variations above ∼70 km. The role of the transport terms for molecular and eddy diffusion in the continuity equation is illustrated explicitly and discussed for some representative cases. We show that the dynamic model fits the observational results for ozone and nitric oxide concentrations better than the static model and also offers a more adequate explanation for the observed diurnal variation in the OH airglow emission.

Diurnal variations of number densities of oxygen, nitrogen, hydrogen, carbon, and ionic minor constituents in the mesosphere and lower thermosphere are investigated using the simultaneous solution of photochemical‐diffusive equations. Special account is taken of the mutual coupling of nitrogen compounds with ions. Quantum yield of N(²D) production in the dissociative recombination of NO+ and the photodissociation of N2, which are the main sources of NO in the lower thermosphere, must be ≳0.5 in order to reproduce the NO density around 100 km deduced from γ band airglow measurements. The NO density shows no substantial diurnal variation between the heights of 75 and 110 km and shows the day‐to‐night variation of a factor of greater than 2 above 120 km, whereas NO changes almost completely into NO2 at night below ∼70 km. The calculated NO density is smaller than that observed in the height range of 60–80 km, although it explains the observed electron density according to currently known ion chemistry. A larger rate coefficient recently measured for the conversion of NO+ into NO+·N2, which eventually will be hydrated to NO+·H2O, can be used to adequately model the height where the dominant ion changes from hydrated ions to NO+. Disagreement of the calculated atomic oxygen density profiles with the observed ones in the lower thermosphere is discussed in connection with the eddy diffusion coefficient and the solar flux. Dynamical transport of atomic oxygen such as the general circulation may be invoked to resolve this disagreement.

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At extremely high latitudes (>60°), ionospheric conditions are very complicated, and there are sometimes irregularities of electron density due to which spread F is considered to occur in the F regions, although its presence cannot always be detected on ionograms. Paying special attention to the occurrence of these conditions, we have considered three possible definitions of the occurrence probability of spread F. On the basis of each of these definitions, the latitudinal, diurnal, seasonal, and solar‐cycle variations are calculated, and the results are discussed. The effects of geomagnetic and auroral activity are also discussed and are compared with respect to the three definitions mentioned above. The results seem to suggest that irregularities, the origin of spread F, may sometimes exist when the F region cannot be observed from the ground because of blackouts, etc.

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