An investigation of dissipation processes in a multi-component gas-mixture: Application to acoustic-wave absorption in the thermosphere-ionosphere

Abstract

Infrasonic waves generated in the lower atmosphere can propagate into the thermosphere and perturb the ionosphere. These disturbances are accompanied by a change in the ionospheric electron density, which can be detected via remote sensing methods, such as Global Navigation Satellite System derived measurements of integrated total electron content. The altitude-dependent decrease in density strengthens dissipative phenomena which affect these measurements by reducing the acoustic-wave amplitude. Sutherland and Bass [https://doi.org/10.1121/1.1631937] have described atmospheric absorption, but only up to 160 km, and neglected interspecies diffusion. However, the atmosphere above the mesopause is a mixture of three major gases, namely, N2, O2, and O, where processes associated with mass-fraction-density gradients could affect acoustic-energy dissipation. This work revisits absorption processes via numerical simulations of the equations of fluid mechanics for multicomponent-gas mixtures, under the assumption of a small Knudsen number (i.e., satisfying continuum approximation). More specifically, diffusion due to mole-fraction, pressure, and temperature gradients, and heat diffusion due to concentration gradients are included alongside classical thermo-viscous terms. Their impact is investigated on vertically-propagating acoustic pulses with varying frequencies, <1 Hz, and different amplitudes that match typical values observed from geophysical and anthropogenic sources (e.g., earthquakes, thunderstorms, explosions).