Detailed spectroscopic analysis of hydroxyl fundamental vibration‐rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110‐km tangent height region. High‐resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ‐doublet populations. A forward predicting, first‐principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration‐rotation Einstein coefficients for the high‐N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

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Earthlimb spectra of thermospheric NO fundamental band emissions, obtained in the CIRRIS 1A Space Shuttle experiment, have been analyzed using nonlinear least‐squares spectral fitting. Absolute NO(υ≥1,J) column densities have been determined in the 100 to 260‐km tangent height region and inverted to yield altitude‐dependent number densities for both a rotationally thermalized and a highly rotationally excited population component. Emissions from high‐J levels are predicted to dominate the Δυ=2 overtone bands during the daytime. The rotationally excited population is found to decrease more at night than the rotationally thermalized component. In addition, radiance from the CO υ=1→0 fundamental band is observed in the NO R‐branch band head region, with greater relative importance at night. The derived CO rotational temperatures are significantly greater than modeled local kinetic temperatures. These results provide important inputs to models of NO(υ,J) formation mechanisms, and of the chemistry, radiative processes, and energy budget of the thermosphere.

The two NO(X²Π, υ=1, Ω=1/2,3/2) spin‐orbit populations in the Earth's thermosphere have been found to depart by more than a factor of 2 from the ratio expected from thermal equilibrium. The effective temperature describing the observed population distribution is as much as 700 K lower than the local kinetic temperature. Absolute NO(υ=1, J, Ω) column densities were derived from high‐resolution (1 cm−1) infrared earthlimb spectra for tangent altitudes up to 200 km, obtained in the CIRRIS 1A Space Shuttle experiment. Nonlinear least‐squares synthetic spectral fitting was used to analyze the NO Δυ=1 fundamental band emissions near 5.3 µm. The spin‐orbit distribution represents a third degree of freedom, along with vibration and rotation, that is not in equilibrium with the local kinetic temperature. These observations may significantly impact the interpretation of band‐integrated measurements of NO in the upper atmosphere, for which equilibrium sublevel distributions have been assumed. The subthermal distribution is most likely produced in the collisional uppumping of NO(υ=0) by O atoms, which is the major source of NO(υ=1) in the thermosphere. This inference suggests that the present effect is related to the subthermal spin‐orbit distributions observed in laboratory studies of NO2 photodissociation.

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