Kinetics and thermalization of hot N(4S) atoms in the upper atmosphere

Abstract

The production of energetic N(4S) atoms and their subsequent thermalization by collisions with the ambient gas play an important role in the heat and energy balance of the thermosphère. It has been suggested that nitric oxide formation by the reaction of energetic nitrogen atoms with O2 may account for the NO deficiency in atmospheric models at altitudes of 105–110 km. Using calculated differential scattering cross sections for thermalization of N(4S) atoms by elastic and inelastic collisions with atmospheric bath gases, we determine the energy distribution function of N(4S) atoms and evaluate the importance of the N(4S)+O2 reaction as a source of NO in the thermosphere. We show that the nonthermal component produces large increases in the effective rate coefficient at low altitudes. However, we find that at altitudes below 130 km, where the production of NO by energetic N(4S) atoms substantially exceeds the production by the thermal N(4S) component, there is still a larger contribution from N(2D) atoms, whereas at higher altitudes, where the N(2D) atom source is smaller than the N(4S) atom source, it is the thermal component of the N(4S) distribution that dominates. Thus the production of NO by the reactions of energetic N(4S) atoms cannot be invoked to explain the high abundance of NO.