Analysis of the Astro-1/Hopkins Ultraviolet Telescope EUV-FUV dayside nadir spectral radiance measurements

[1] The Hopkins Ultraviolet Telescope (HUT) was one of three ultraviolet Astro-1 observatory instruments on space shuttle Columbia in December 1990 (STS-35), covering the 830 - 1850 Angstrom wavelength interval (first order) at similar to3.3 Angstrom spectral resolution. Satellite altitude was 360 km. Dayside nadir measurements were performed during a single orbit on 7 December 1990 under solar maximum (F-10.7 = 222), geomagnetically quiet (Ap = 4) conditions, covering late morning local times ( solar zenith angle < 48 degrees). These data constitute a reference dayglow radiance spectrum comprising a number of thermospheric emission features, including several weak features neighboring bright optically thick emissions, that have not yet received adequate explanations, in part owing to questions regarding spectral intensity calibration, dynamic range, etc., associated with older data sets. In this paper, the HUT extreme ultraviolet (EUV) - far ultraviolet (FUV) nadir dayglow spectrum is presented along with the results of a modeling analysis using a development version of the Atmospheric Ultraviolet Radiance Integrated Code ( AURIC). The analysis relies on currently available laboratory data and first-principles excitation and transport modeling codes and utilizes constraints drawn from the FUV spectral region data to study several EUV emissions of interest for which past airglow data are sparse. The focus is on the airglow in the 980 - 1200 angstrom region, which is particularly rich in emission features affected by thermospheric conditions. Emissions investigated in detail include ( 1) the optically thick OI 989 angstrom multiplet and associated 1172 angstrom fluorescence, ( 2) the Oi 1026 angstrom sextuplet blended with atomic hydrogen Lyman beta, (3) the atomic nitrogen multiplets at 1134 and 1200 angstrom consisting of optically thick components excited by e(-) + N and optically thin components excited by e(-) + N-2 and h nu + N-2, ( 4) a number of other Ni and N+ features (e.g., N+ 1085 angstrom) excited by N-2 dissociative ionization, (5) the Birge-Hopfield I (N-2 BH-1) system, and (6) the resonance lines of argon ( Ar 1048 and 1067 angstrom).