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 gf 1991 to observe the Earth's thermospheric airglow and included a far ultraviolet (1080-1800 Angstrom) spectrometer. We present first results from this spectrometer, including a spectroscopic analysis at 6-Angstrom resolution of H, O, N, and N-2 dayglow 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 a(1)II(g) state that differs significantly from those predicted for direct electron excitation and excitation with cascade from the a' (1) Sigma(u)(-) and w (1) Delta(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 NI lambda 1135 and NI lambda 1200 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 OI lambda 1356, HI lambda 1216, and OI lambda 1304 and OI lambda 1641 emissions agrees to within roughly 10% of the observed limb-scan profiles, but the models underestimate the N, LBH profiles by a factor of 1.4-1.6, consistent with the inferred cascade effect. Other findings include: an OI lambda 1152/lambda 1356 intensity ratio that is inconsistent with the large cascade contribution to OI lambda 1356 from np P-5 states required by laboratory and nightglow observations; nightglow observations of the tropical ultraviolet arcs exhibit a wide range of OI lambda 1356/lambda 1304 intensity ratios and illustrate the complicated observing geometry and radiative transfer effects that must be modeled; and we find a 3-sigma upper limit of 8.5 R to the total LBH vehicle glow emission.