Circulation and temperatures at 70- to 100-kilometer height

The flow and thermal structure adjacent to the mesopause (80 km) appears to be far more complex and diversified than is suggested by earlier models. A well-pronounced semiannual variation of the wind and horizontal temperature gradient is present in the 70- to 100-km layer, which is in phase with similar semiannual oscillations of temperature and density in the 350- to 650-km height range. It may be hypothesized that the 70- to 100-km layer responds to variations in the solar corpuscular radiation in a way that is characteristic of the middle and higher thermosphere and acquires the highest seasonal temperatures during the equinoxes (with the principle of warm winter pole still preserved). A rather complex pole-to-equator distribution of the wind flow and temperature, having several zonal belts of distinct character, can be postulated. The day-to-day variations of the wind at 92 km tend sometimes to be in phase with the circulations at 16 and 5 km for periods of 10 days or longer. However, since the data from the 70- to 100-km layer are still exceedingly sparse and can fit a variety of models, the concepts discussed in this paper must be regarded as only tentative and provisional.