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The equation for the diffusion velocity in the mesosphere and the lower\udthermosphere (MLT) includes the terms for molecular and eddy diffusion. These\udterms are very similar. For the first time, we show that, by using the\udsimilarity theory, the same formula can be obtained for the eddy diffusion\udcoefficient as the commonly used formula derived by Weinstock (1981). The latter\udwas obtained by taking, as a basis, the integral function for diffusion\udderived by Taylor (1921) and the three-dimensional Kolmogorov kinetic energy spectrum. The\udexact identity of both formulas means that the eddy diffusion and heat\udtransport coefficients used in the equations, both for diffusion and thermal\udconductivity, must meet a criterion that restricts the outer eddy scale to\udbeing much less than the scale height of the atmosphere. This requirement is\udthe same as the requirement that the free path of molecules must be much\udsmaller than the scale height of the atmosphere. A further result of this\udcriterion is that the eddy diffusion coefficients Ked, inferred\udfrom measurements of energy dissipation rates, cannot exceed the maximum\udvalue of 3.2 × 106 cm2 s−1 for the maximum value of\udthe energy dissipation rate of 2 W kg−1 measured in the mesosphere and the lower\udthermosphere (MLT). This\udmeans that eddy diffusion coefficients larger than the maximum value\udcorrespond to eddies with outer scales so large that it is impossible to use\udthese coefficients in eddy diffusion and eddy heat transport equations. The\udapplication of this criterion to the different experimental data shows that\udsome reported eddy diffusion coefficients do not meet this criterion. For\udexample, the large values of these coefficients\ud(1 × 107 cm2 s−1) estimated in the Turbulent Oxygen\udMixing Experiment (TOMEX) do not correspond to this criterion. The\udKed values inferred at high latitudes by Lübken (1997) meet this\udcriterion for summer and winter polar data, but the Ked values\udfor summer at low latitudes are larger than the Ked maximum value\udcorresponding to the criterion. Analysis of the experimental data on\udmeteor train observations shows that energy dissipation with a small rate of\udabout 0.2 W kg−1 sometimes can induce turbulence with eddy scales very\udclose to the scale height of the atmosphere. Our results also explain the\uddiscrepancy between the large cooling rates calculated by Vlasov and\udKelley (2014) and the\udtemperatures given by the MSIS-E-90 model because, in these cases, the\udmeasured eddy diffusion coefficients used in calculating the cooling rates\udare larger than the maximum value presented above

Eddy diffusion coefficients and their upper limits based on application of the similarity theory