Heat and momentum sources of mean circulation at an altitude of 70 to 100 km

Provided that the mean zonal and meridional wind components are known the average vertical velocity component and the momentum and heat sources of the mean meridional circulation can be evaluated applying the steady state equations of the forced mean circulation. Monthly meridional cross sections of pressure, temperature and wind given by Groves (1971, 1969) have been used to determine the mean vertical motion and the rates of momentum and heat changes which are required to maintain the mean circulation as described by the cross sections in the region from 70–100 km altitude. Naturally, the results completely depend on the empirical wind model chosen. A striking feature of this model is the occurrence of mean meridional wind components nearly as large as the zonal components. The strong mean meridional winds, often ascribed to an insufficient amount of observations, implies large vertical velocity components, which would lead to effects contradictory to the actual behaviour of the atmosphere if the wind model were very inaccurate. Yet the calculated results correspond with circulation processes postulated from various phenomena such as for instance, the radio wave absorption of the lower ionosphere and seasonal composition changes in the thermosphere. Therefore it is supposed that large mean meridional velocities as given in Groves’ (1969) model on the whole reflect the behaviour of the upper mesosphere and lower thermosphere. The resulting seasonal and spatial variations of the heat and momentum sources lead to the assumption that very effective transport processes exist in the region between 70 and 100 km height and that they are caused by fluctuations (for instance waves of different types) which cannot simply be simulated by an eddy diffusion coefficient.