Observations of elevated power near the Brunt‐Väisälä frequency

Abstract

High‐frequency peaks close to the Brunt‐Väisälä frequency (N) are observed in the spectra of the nightglow brightness derived from OH Meinel airglow measurements made over the foothills of the Rocky Mountains in Boulder (40°03′N, 105°14′W), Colorado during the summer of 1999. Over a 57‐day period there were 28 nights of sufficient duration and continuity to support reliable spectral analyses. Measurements for these nights yield persistently spectral features near N. The periods of the spectral peaks are in the range ∼5.6–5.8 min, usually appearing as a double‐peaked feature. These periods are compared with values of the Brunt‐Väisälä period (τN) derived from concurrent lidar soundings of the temperature of the upper mesosphere and lower thermosphere over Fort Collins (40°35′N, 105°03′W), Colorado, about 49 km distant from Boulder. Detailed comparisons of the observed periods and the profiles of N suggest that the double‐peaked features in the spectra may arise because the OH M nightglow emissions is double‐peaked, as observed by Evans and Llewellyn (1972) and by Llewellyn et al. (2002). Several theories are explored for the spectral enhancement. The data do not support a definitive answer, but we favor an explanation in terms of orographically excited waves related to the proximity of our airglow observations to the Rocky Mountains. A strong possibility is the parametric excitation described by Klostermeyer (1990). This explanation requires the presence of a large‐amplitude low‐frequency primary wave and is supported by the observed frequent occurrence of strong wave activity in the approximately hour period range.