A dynamical‐chemical model of wave‐driven fluctuations in the OH nightglow

Abstract

Wave‐induced temporal fluctuations in the intensity of the OH nightglow are related to the temperature oscillations of the wave field by a model that incorporates a five‐reaction photochemical scheme and the complete dynamics of linearized acoustic‐gravity waves in an isothermal, motionless atmosphere. The intensity I and rotational temperature T oscillations, δI and δT, are conveniently related by the ratio, where the overbar refers to time‐averaged quantities. The ratio η is a complex quantity that depends on the properties of the basic state atmosphere (temperature, thermodynamic parameters, major constituent O2, N2 and minor constituent O, O3, OH, H, HO2 concentrations, and scale heights), chemical reaction rate constants, wave period, horizontal wavelength, and direction of wave energy propagation (upward or downward). The intensity‐temperature oscillation ratio η is evaluated for a nominal case corresponding to an altitude of about 83 km in a nightside model atmosphere with an atomic oxygen scale height of −2.8 km; horizontal wavelength λx is 100 km, and wave energy propagation is upward. Over a broad range of acoustic periods |η| varies between 7 and 8, and η is approximately in phase with the temperature fluctuations. At gravity wave periods, |η| decreases with increasing period from a maximum value of about 7.0; at a period of about 3 hours, |η| is about 1.8. The phase of η and δT are within 45° in the gravity wave regime. The main effect of order of magnitude changes in λx is the modification of the location and width (in period) of evanescent regions. At hour periods, |η| increases as the magnitude of atomic oxygen scale height decreases; at periods of several hours, |η| is about 1/3 greater for an atomic oxygen scale height of −2 km than for the nominal scale height. The amplitude of η is essentially independent of the direction of wave energy propagation, but the phase of η relative to that of δT depends on the upward or downward sense of energy propagation at periods in close proximity to the evanescent regime. The magnitude of η at gravity wave periods can depend sensitively on the altitude of the OH emission layer; higher OH emission heights give smaller values of |η| at 10‐min periods, providing the O3 scale height is not too great. Neglect of minor constituent photochemistry in computing η is a tolerable approximation at acoustic wave periods, but it is entirely inadequate at gravity wave periods. Inclusion of dynamical effects is absolutely essential for a valid assessment of η at any period.