Thermal upper limit on eddy diffusion in the mesosphere and lower thermosphere

The thermodynamic stability of the mesosphere and lower thermosphere requires that there be a downward transport of heat associated with any turbulent mixing. The maximum amount of heat that is available for transportation downward through any level by eddy conduction can be determined from the absorption of solar energy above that level; any heat losses by infrared radiation or large-scale circulation can only reduce the available heat. This evaluation of the available heat can be used to place an upper limit on the vertical eddy diffusivity, and this limit is substantially below values frequently quoted on the basis of vapor or meteor trail studies. The upper limit is about 106 cm2/sec above 80 km and about 4×105 cm2/sec from 50 to 70 km. The fact that diffusive equilibrium distributions of atmospheric constituents prevail no lower than 105 km, where the molecular diffusion coefficient is 106 cm2/sec, indicates that the eddy coefficient at that altitude has the limiting value obtained from thermal considerations.