N-2 LYMAN-BIRGE-HOPFIELD DAYGLOW FROM ATLAS-1

Spectral measurements of the far ultraviolet dayglow were made from th e ATLAS 1 shuttle mission using an intensified-CCD imaging spectromete r array. The instrument imaged relatively large (177 Angstrom) segment s of the spectrum simultaneously and had the capability to image altit ude in the second dimension of the two-dimensional focal plane detecto r when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The FUV channel of the instrument used a CsT e photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also m easured with good sensitivity at FUV wavelengths longer than 1500 Angs trom. In this paper, data are selected of specific interest to a study of the N-2 Lyman-Birge-Hopfield system using two shuttle attitudes: I n the first the field of view was held at a fixed tangent ray height o f similar to 140 km while scanning in wavelength, providing repeated F UV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere bu t covered about 75% of the full wavelength range. This sequence allows comparison of height dependence, including O-2 absorption effects, wi th the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in compar isons in which the line of sight must be integrated along a path that spans a considerable range of conditions (height, local time, latitude , solar zenith angle). In the case of the data sets examined in this s tudy the vibrational population distributions show mixed results. In o ne case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contr ibution. A second case indicates a lower population in v' = 3 and a hi gher population in v' = 6 than would be expected from direct impact ex citation alone. This supports previous indications that conditions res ulting in the cascade mechanism may not always be operative.