N2 triplet band systems and atomic oxygen in the dayglow

New spectrographic observations of the Earth's dayglow have been acquired by the Arizona Airglow Experiment (GLO) flown on the Space Shuttle. GLO is an imaging spectrograph that records simultaneous vertical profiles of prominent Earth limb emissions occurring at wavelengths between 115 and 900 nm. This study addresses the measured emissions from the N2 triplet states (first positive, second positive, and Vegard-Kaplan band systems) and their excitation by the local photoelectron flux. The triplet state population distributions modeled for aurora by Cartwright (1978) are modified for dayglow conditions by changing to a photoelectron-flux energy distribution and including resonance scattering by the first positive system. Modeled and observed intensities are in excellent agreement, in contrast to the well-studied auroral case. This work concentrates on dayglow conditions at 200 km altitude near the subsolar point. Several atomic oxygen dayglow emission features were analyzed to complement the N2 analysis. The photoelectron-excited O I (135.6, 777.4 nm) lines were found to be 3 to 4 times weaker than predicted while the O I (630.0, 844.6 nm) lines were in close agreement with the model prediction. (Author)