Abstract
The formation of the mean zonal wind in the lower thermosphere is discussed from a viewpoint of induction and transmission of zonal mean momentum by tidal waves.
As a preliminary discussion, in Section 3, the momentum of the mean zonal wind averaged meridionally along isobaric surface is shown theoretically to be equal to the tidal wave momentum E/C as is in the case for other atmospheric waves. The momentum of the meridionally averaged Eulerian mean zonal wind is not equal to the wave momentum. In Section 4, numerical integration with a simplified atmospheric model is performed as an initial value problem to show that the results are consistent with those in. Section 3, and that in case of the first mode of the semidiurnal tide, the direction of the mean zonal wind
averaged meridionally is opposite to that of the wave momentum if the vertical wave length is longer than 4πH (scale height) as suggested by Nakamura (1976).
In Section 5, the similar numerical integration is performed with somewhat realistic atmospheric model including ion drag and molecular viscosity and conductivity to discuss how much zonal winds are induced by tidal waves in the lower thermosphere. It is shown that the first mode of the solar semidiurnal tide excited in lower layers (zonal component of tidal wind is 10m/sec in amplitude at about 100km height over the equator) induces easterly winds in the lower thermosphere with a maximum speed of a few m/sec at about 150km height over the equator.
Based on the above result and a rough order estimation, it is suggested that tidal waves excited in lower layers, especially, the first mode of the solar diurnal tide and the second symmetric mode of the semidiurnal tide, may induce easterlies with a maximum speed of an order of -100m/sec at a height of about 110km over the equator.
