A resolution of the N2 Carroll‐Yoshino (c4′ ‐ X) band problem in the Earth's atmosphere

Abstract

In the study of UV airglow from the Earth's atmosphere, the N2 Carroll‐Yoshino (CY) c4′ 1Σu+ ‐ X 1Σg+(0,0) and (0,1) Rydberg band emissions near 958 Å and 980 Å, respectively, are found to be weak relative to the c4′ (0) excitation rate. This result is surprising because laboratory measurements show that CY(0,0) and CY(0,1) are the brightest N2 emission features between 910‐1010 Å even under optically thick conditions [Zipf and McLaughlin, 1978]. In order to investigate the cause of this weak emission quantitatively, we have developed a resonant fluorescent scattering model for CY(0,0) and CY(0,1). The model is intended to be comprehensive, including multiple scattering, extinction, branching, escape to space, predissociation, and temperature effects. Results show CY(0,0) photons are radiatively trapped and undergo resonant fluorescent scattering accompanied by substantial loss in the atmosphere. Indeed, the model predicts weak CY(0,0) intensities, consistent with observations. We find that the most important loss processes for the CY(0,ν″) system in the Earth's dayglow are predissociation and branching to CY(0,1) followed by absorption by the overlapping, 100% predissociated Bridge‐Hopfield I (BH I) b1Πu(2) ‐ X1Σg+(0) band. Near solar minimum, model CY (0,1) and (0,2) dayglow zenith intensities between 160‐170 km range between 4‐9 R and 0.5‐1.5 R, respectively, where the lower number assumes 16.5% predissociation of the c4′(0) state and the higher number assumes 1% predissociation. These intensities are all consistent with observations reported by Morrison et al. [1990]. For the Earth's aurora, model CY(0,1) and (0,2) intensities averaged between 88°‐96° from the zenith at 118.5 km range between 60‐180 R and 150‐390 R, respectively, and CY(0,2) intensities at 170 km range between 200‐360 R. These results are consistent with upper limits from Feldman and Gentieu (1982) if the probability of c4′ (0) predissociation is at least 9%. We also present qualitative arguments to explain the relatively bright CY(0,ν″) emission on Titan and Triton.