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

Abstract

The Ultraviolet Limb Imaging (UVLIM) experiment flew on STS-39 in the spring oof 1991 to observe the Earth's thermospheric airglow and included a far ultraviolet (1080-1800 A) spect[ometer. We present first results from this spectrometer, including a spectroscopic analysis at 6-A 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•IIg state that differs significantly from those predicted for direct electron excitation and excitation with cascade from the a' 1E•-and w 1A• 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 1 3.1135 and N I 3.1200 limb emission profiles, as expected for emissions excited by N2 dissociation which produces kinetically fast N fragments; however, optically thick components of these features are also observed. Preliminary modeling of the O 1 3.13 56, H I 3.1216, and O I 3.1304 and O 1 3.1641 emissions agrees to within roughly 10% of the observed limb-scan profiles, but the models underestimate the N2 LBH profiles by a factor of 1.4-1.6, consistent with the inferred cascade effect. Other findings include: an O I Z 1152/3.1356 intensity ratio that is inconsistent with the large cascade contribution to O I 3.1356 from np 5p states required by laboratory and nightglow observations; nightglow observations of the tropical ultraviolet arcs exhibit a wide range of O 1 3.1356/3.1304 intensity ratios and illustrate the complicated observing geometry and radiative transfer effects that must be modeled; and we find a 3-0 upper limit of 8.5 R to the total LBH vehicle glow emission. 1. Introduction The spectrum of the FUV and EUV airglow has been extensively studied [Barth and Schaffner, 1970; Meier et al., 1980; Chakrabarti et al., 1983; Eastes et al., 1985; Conway et al., 1988; Morrison et al., 1990] and provides information about the global and vertical distribution of neutral gases, ions, and electrons in the thermosphere. In theory, from simultaneous observations of the O I 3.1304, O I 3.1356, and N 2 Lyman-Birge-Hopfield (LBH) emissions, the O density and photoelectron flux distributions may be uniquely determined [Meier et al., 1985]. When these observations are combined with EUV observations of atomic and ionic species and interpreted through the iterative application of radiative transfer, atmospheric composition, and photoelectron models, a comprehensive understanding of the composition and distribution of all major species of the ionosphere and thermosphere is obtained. However, though the UV airglow, in general, has been extensively Paper number 94JA01543. 0148-0227/94/94JA-01543505.00. measured, our understanding of many particular airglow problems is often based upon an extremely small set of sounding rocket observations that ...

Collision‐induced transitions between the a1Πg, a′ 1Σu−, and w1Δu states of N2: Can they affect auroral N2 Lyman‐Birge‐Hopfield band emissions?

“…The observed vibrational distributions of the N 2 a •II a state, as derived from the N 2 LBH band emissions in the Earth's dayglow and aurora, show variations significantly greater than the statistical uncertainties in the measurements. Some observations clearly agree with theoretical expectations for direct excitation (Xv: o -• a) by electron impact [Meier et al, 1982;Morrison et al, 1990], while others [Eastes et al, 1985;Eastes and Sharp, 1987;Budzien et al, 1994;Torr et al, 1994] clearly exhibit statistically significant departures from the expected values for direct excitation. These observations clearly indicate that direct excitation cannot be the only mechanism influencing the relative vibrational populations.…”

Modeled and observed N2Lyman‐Birge‐Hopfield band emissions: A comparison

“…They found, in observations from above 200 km, significant differences from electron impact excitation of N 2 in the distribution of the vibrational populations, even when including excitation from v > 0 of the ground state. In addition Budzien et al [1994] found that their modeled brightnesses, which did not include cascade, were low by a factor of 1.4–1.6.…”

High‐resolution FUV spectroscopy of the terrestrial day airglow with the Far Ultraviolet Spectroscopic Explorer

“…For example, the Hopkins Ultraviolet Telescope (HUT), which flew on the Astro-1 and 2 space shuttle missions in December 1990 and March 1995, respectively, produced excellent spectra at 3.3 A resolution, looking both upward and down at the bright Earth. These data remain largely unpublished [Feldman et al, 1991[Feldman et al, , 1992 space era yet continue to bear fruit with successive developments in spectroscopic instrumentation [Gladstone, 1988;Meier, 1991;Budzien et al, 1994]. In Figures 8 and 9 we give several examples that bear on some recently discussed problems.…”