Nocturnal Variation of the Intensity and Rotational Temperature of the OH (8,3) Band in the Airglow

THE OH airglow is a sensitive indicator of conditions in the mesosphere. We report the results of measurements of the intensity and excitation temperature in the OH (8,3) vibrational rotational band, carried out with a spectrophotometer of simple design. Light entering a 3 mm diameter iris is collimated with a 170 mm focal length lens and impinges on a narrow band interference filter of 5 cm diameter; the filter has a bandwidth of approximately 3 Å and can be tilted to scan the band between 7300 Å and 7403 Å. The filtered light is detected with a cooled photomultiplier ‘EMI 9558A (S20)’; photon counting is accomplished with a multichannel analyser ‘Laben 100’. An entire scan, including dark current measurements, is carried out automatically in about 20 min. After removal of sky background and dark current the experimental spectra are compared with synthetic spectra obtained by considering the theoretical contribution of all lines of the OH (8,3) band under the assumption of thermal equilibrium. The χ2criterion is utilized to optimize the fitting; best values for the relative intensity and the rotational temperature are obtained at intervals of one hour every night. Observations reported here were carried out for a total of 22 nights and 276 scans during July to December 1969 and for a total of 7 nights and 72 scans during July to December 1968. Data were taken near l'Aquila (42° N, 13° E) at two locations: at Campo Imperatore (2,200 m above sea level) until August 1969 and at Preturo (700 m above sea level) after that date.