A study of mid -latitude 5577angstrom OI dayglow emissions.

Abstract

The green line (5577A) is a bright, persistent component of the visible airglow that is produced by an electric quadrupole transition from the meta-stable second excited state ($\sp1$S$\sb0)$ to the first excited state ($\sp1$D$\sb2)$ of atomic oxygen. In this thesis, production and loss mechanisms important to the F-region dayglow 5577A emission are investigated. Four major source reactions need to be incorporated in the modeling of the emission profile, photoelectron impact on atomic oxygen, dissociative recombination of O$\sb2\sp+,$ quenching of $\rm N\sb2(A\sp3\Sigma\sb{u}\sp+)$ by atomic oxygen, and photo-dissociation of O$\sb2$ which becomes more important below the emission peak. For some of the reactions, the properties of the rate coefficients, branching ratios, and cross sections are not well known. Models are used to determine the rate coefficients, branching ratios, and cross sections for these reactions based on observations. The impact of photoelectrons on atomic oxygen is the primary source of 5577A dayglow emission in the thermosphere. The observed profiles of 5577A emission are best reproduced by the impact cross sections of Henry et al. (1969). This is in good agreement with the latest theoretical cross sections of Berrington and Burke (1991, Personal Communication) but about a factor of two smaller than the cross sections measured by Shyn et al. (1986). The quenching of N$\sb2$(A) by atomic oxygen is an important source of the 5577A emission at the peak in the layer. The total quenching rate was determined using a vibrational model and a band model for N$\sb2$ to study emissions at 3371A from the Atmosphere Explorer satellite. The value of the rate coefficient deduced here agrees well with experimental values by Piper and Caledonia (1981) and Thomas and Kaufman (1985). The effective branching ratio determined by this study tends to support the results from Piper (1982) and De Souza et al. (1985). The effect of the distribution of the vibrational population of the $\rm N\sb2(A\sp3\Sigma\sb{u}\sp+)$ state on the branching ratio is also discussed. The extension of the dayglow photochemistry into the twilight is also investigated. The model developed for the dayglow can reasonably reproduce the rapidly changing twilight emissions.