Diurnal variations of odd nitrogen and ionic densities in the mesosphere and lower thermosphere: Simultaneous solution of photochemical-diffusive equations

Abstract

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.