下部熱圏に於ける太陽─日潮汐波動による平均風生成

Abstract

The generation of the mean zonal winds in the lower thermosphere is discussed, on the basis of momentum transport by dissipating solar diurnal tidal waves. The most important factor to control the amplitude of tidal waves and the magnitude of the induced mean winds in the lower thermosphere is the dissipative effect due to small scale motions in the mesosphere and the lower thermosphere which is parameterized in the form of eddy diffusion or the form of Rayleigh friction. In both cases of parameterizations, the behavior of the tidal wave agrees well with observations. The induced mean zonal winds in case of eddy diffusion-type parameterization are found to be three times larger than those of the case of Rayleigh friction-type parameterization. The weaker zonal flow is interpreted to result from the removal of net momentum. As the observed mean zonal winds distribution is not definitive, we cannot decide which parameterization is suitable for the present problem. However, the present results confirm that the dissipating solar diurnal tidal wave contributes greatly to the configulation of the mean zonal winds in the lower thermosphere.