SOURCES OF OH(A→X) EMISSION IN THE SPACE SHUTTLE ENVIRONMENT

OH(A→X) emission bands have been observed in Space Shuttle engine exhaust jets using the GLO imager spectrograph located in the payload bay. Spectra were collected at a resolution of 4 A for both day and night solar illumination conditions, all at an altitude of ~390 km. A spectral analysis is presented that identifies and quantifies three OH(A) excitation mechanisms. These include: i) solar-induced fluorescence of the OH(X) in the exhaust flow, ii) solar-induced photodissociation of H2O in the exhaust at Lyman-α and shorter wavelength far-UV lines, and iii) exhaust-atmosphere interactions, most probably through the reaction O+H2O→OH(A)+OH(X). Process i) produces a very rotationally cold and spectrally narrow component due to the rapid cooling of the OH(X) in the supersonic expansion of the exhaust flow. Processes ii) produce extremely excited OH(A), not well characterized by thermal vibrational or rotational distributions. Process iii) has a substantial activation energy, 4.79 eV, and is only slightly above threshold for the ram (180o angle of attack between O and H2O) geometry, consistent with its observation for the night ram but not the night perpendicular exhaust atmospheric interaction. Through the use of a non-equilibrium spectral emission model for OH, the integrated intensity, spectral distribution, and OH(A) internal state characterization for each of the above processes was deduced. Additional confirmation of the analysis is provided through the use of a model simulation of the space experiment to predict the total integrated intensities for processes i) and ii) for which the underlying spectroscopy, absorption cross sections, and solar excitation intensities are well established. Analysis of process (ii) has established an effective value for the fluorescence excitation cross section at ram conditions (·ETO= 5.2 eV) of (1.7 ± 0.9) × 10 -2 A 2 . Evidence for bands attributed to predissociated OH(A) vibrational levels suggest that the associated reaction cross section could be significantly higher.