Gravity Waves in the Thermosphere Simulated by a General Circulation Model

By using a general circulation model that contains the region from the ground surface to the upper thermosphere, characteristics of gravity waves in the mesosphere and thermosphere are examined. At 100 km height, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λx ≈ 2000 km), 40(λx ≈ 1000 km) and 80 (λx ≈ 500 km) are 6 h, 3 h and 1.5–2 h, respectively. For the individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force. This means that gravity waves with larger horizontal phase velocity (larger vertical wavelength) can penetrate into the lower thermosphere. The vertical energy flux due to gravity waves indicates that upward propagation from the lower atmosphere to the thermosphere is dominant. Fluctuations of the horizontal wind associated with gravity waves and its relation to the ionospheric variation are discussed. Short‐period gravity waves into the thermosphere induce day‐to‐day variations of the zonal wind in the upper thermosphere. This result suggests that day‐to‐day variability of the zonal wind caused by gravity waves is expected to contribute to the ionospheric variability.