Relaxational dissipation in atmospheric waves—II. Application to earth's upper atmosphere

Abstract

A dispersion relation is derived for acoustic-gravity waves propagating in a multiconstituent gas whose atoms and molecules are subject to relaxational exchanges of thermal energy between translational and internal forms. The relation employs a complex thermal capacitance, derived in a companion paper, which incorporates the relaxational effects. Approximations to this relation permit ready assessment of the attenuation of wave amplitude and energy that relaxation produces, both in absolute amount and relative to viscous attenuation. In Earth's upper atmosphere, relaxational attenuation is found to be of greatest potential consequence in the middle and upper E region. Both electronic relaxation in atomic oxygen and vibrational relaxation in molecular oxygen and nitrogen appear to be capable of playing a significant role, but it is found that the actual significance of their roles cannot be assessed adequately because of present uncertainties of governing parameters (and, in the case of vibrational relaxation, of governing collisional interactions). Similar uncertainties are found to surround the role that relaxation, notably in atomic oxygen, may play in the termination of turbulence near the 110 km level.