A review of the photochemistry of selected nightglow emissions from the mesopause

The substantial research activity of recent years on the photochemistry of the hydrogen-oxygen family has improved our understanding of the production of nightglow emissions in the mesopause region of the atmosphere between 80 and 100 km; this work is reviewed with specific attention paid to the emissions of OH, the 557.7-nm emission of O, and the atmospheric bands of O2(b 1Sg). These emissions are closely related to the O atom concentration profile and would provide a wealth of useful photochemical data in studies of noctilucent cloud displays. Ground-based and rocket measurements of the OH nightglow show that the kinetics are well represented by the combination of the production reaction, H + O3 → OH(v′) + O2, with significant collisional deactivation by N2 and O2 for the higher vibrational levels of OH. In contrast, the perhydroxyl radical, HO2, plays a negligible role in the vibrational excitation of OH and the production of OH optical emission in the mesopause region. Atomic oxygen quenching is important for the lower vibrational states of OH(v′). Improved experimental determination of the electric dipole moment function of OH has resulted in a new set of transition probabilities that agrees well with the theoretical set published by Mies for low v rovibrational transitions of OH. The Mies values for the higher overtone transitions do not agree well with the empirical results derived from airglow intensities. Recent papers on the results of the sounding rocket campaign called Energy Transfer in the Oxygen Nightglow (ETON) have demonstrated the substantial progress made once the consequences of mesospheric dynamics are decoupled from the photochemistry of the mesopause region by in situ measurements of the atomic oxygen concentration. The work of recent years utilizing this approach has produced strong evidence for the deduction that the major source of several important oxygen metastable energy states is an intermediate metastable state formed in the first step of a two step reaction chain (known as the Barth process). Energy transfer and quenching reactions involving these metastable states complicate the analysis of the measurements. Continued progress requires laboratory experiments designed to explore the relationship of energy transfer processes to the vibrational development of the precursor species. The production of global maps of atomic oxygen concentration profiles becomes feasible once the details of the mesopause photochemical processes are fully understood.