Acoustic Wave Heating of the Thermosphere

A numerical model is used to study the dissipation in the thermosphere of upward propagating acoustic waves. Whereas dissipating gravity waves can cool the upper atmosphere through the effects of sensible heat flux divergence, it is found that acoustic waves mainly heat the thermosphere by viscous dissipation. Though the amplitudes of acoustic waves in the atmosphere are poorly constrained, the calculations suggest that dissipating acoustic waves can locally heat the thermosphere at rates of tens of kelvins per day and thereby contribute to the thermospheric energy balance. It is shown that viscous heating cannot be calculated from the divergence of the wave mechanical energy flux. Acoustic waves that are barely detectable at mesopause heights can become significant heaters of the atmosphere high in the thermosphere. We suggest that acoustic waves might be responsible for heating the equatorial F region to produce the hot spot observed in the O I 630 nm airglow over the Andes Mountains.