Abstract
The rates of heat input into the mesosphere and lower thermosphere are calculated and compared with the heat losses. The worldwide average eddy diffusion coefficient required to maintain continuity in the heat budget is calculated and found to vary from about 10 7 cm 2/sec at 120 km down to about 10 5 cm 2/sec at 60 km. From the global asymmetry in heating at the solstice, it is concluded that a systematic pattern of vertical velocities prevails ranging from less than 1 cm/sec in the mesosphere up to 10 cm/sec near 120 km, upward over the summer polar region and downward over the winter polar region. This can be balanced by a wind system towards the winter polar region with velocities near 1 m/sec at 60 km increasing to 30 m/sec at 120 km. Such a wind system provides an explanation for the helium bulge in the upper thermosphere over the winter polar region.
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