Acoustic–gravity waves and their role in ionosphere–lower thermosphere coupling

Abstract. The properties of acoustic–gravity waves (AGWs) in the ionospheric D layer and their role in the D layer–lower thermosphere coupling are studied using the dispersion equation and the reflection coefficient. These analytical equations are an elegant tool for evaluating the contribution of upward–propagating acoustic and gravity waves to the dynamics of the lower thermosphere. It was found that infrasound waves with frequencies ω > 0.035 s⁻¹, which propagate almost vertically, can reach the lower thermosphere. Also, gravity waves with frequencies lower than ω < 0.0087 s⁻¹, with horizontal phase velocities in the range 159 m/s < v_h < 222 m/s, and horizontal wavelength 115 km < λ_p < 161 km, are important for the lower thermosphere dynamics. These waves can cause temperature rise in the lower thermosphere and have the potential to generate middle–scale traveling ionospheric disturbances (TIDs). The reflection coefficient for AGWs is highly temperature dependent. During maximum solar activity, the temperature of the lower thermosphere can rise several times. This is the situation where infrasound waves become a prime candidate for the ionospheric D layer–lower thermosphere coupling, since strongly reflected gravity waves remain trapped in the D layer. Knowing the temperatures of the particular atmospheric layers, we can also know the characteristics of AGWs and vice versa.