Observations of the far ultraviolet airglow by the Ultraviolet Limb Imaging Experiment on STS‐39

The Ultraviolet Limb Imaging (UVLIM) experiment flew on STS‐39 in the spring of 1991 to observe the Earth's thermospheric airglow and included a far ultraviolet (1080‐1800 Å) spectrometer. We present first results from this spectrometer, including a spectroscopic analysis at 6‐Å resolution of H, O, N, and N 2 day glow emissions and modeling of the observed limb‐scan profiles of dayglow emissions. The observed N 2 Lyman‐Birge‐Hopfield (LBH) emission reflects a vibrational population distribution in the a¹Π g state that differs significantly from those predicted for direct electron excitation and excitation with cascade from the a′ ¹Σ u − and w ¹Δ u states. The Vibrational population distribution and LBH brightnesses suggest a total cascade rate 45% that of direct excitation, in contrast to laboratory measurements. For the first time, pronounced limb brightening is observed in both the N I λ1135 and N I λ1200 limb emission profiles, as expected for emissions excited by N 2 dissociation which produces kinetically fast N fragments; however, optically thick components of these features are also observed. Preliminary modeling of the O I λ1356, H I λ1216, and O I λ1304 and O I λ1641 emissions agrees to within roughly 10% of the observed limb‐scan profiles, but the models underestimate the N 2 LBH profiles by a factor of 1.4‐1.6, consistent with the inferred cascade effect. Other findings include: an O I λ1152/λ1356 intensity ratio that is inconsistent with the large cascade contribution to O I λ1356 from np 5 P states required by laboratory and nightglow observations; nightglow observations of the tropical ultraviolet arcs exhibit a wide range of O I λ1356/λ1304 intensity ratios and illustrate the complicated observing geometry and radiative transfer effects that must be modeled; and we find a 3‐σ upper limit of 8.5 R to the total LBH vehicle glow emission.