Hydroxyl Rotational Temperatures in the Airglow

Abstract

Nightglow spectra of the OH Meinel bands have been analysed to determine the rotationa1 temperature. It has been found that the rotational temperatures determined from different rotational lines in a single band are not constant but increase with increasing rotational quantum number. A deviation from isothermal rotational temperatures for excited molecules in rotational equilibrium is caused by the variation of kinetic temperature in the OH emission region, but is much less than that observed. It has been concluded that nonequilibrium processes are important. A rotational relaxation model has been proposed to describe the spectrum emitted by molecules that are not in rotational equilibrium. The model was used to calculate the OH nightglow spectrum for bands originating from the eighth and ninth vibrational levels. The calculated rotational temperatures are in good agreement with those derived from observed spectra. The effects of vibrational relaxation in the excited OH molecules have been considered qualitatively and are in agreement with the observed behaviour of the measured intensities and rotational temperatures. It is suggested that the application of the relaxation model to other airglow emissions may give new information on atmospheric processes.