Diffusion and Thermodiffusion of Atmospheric Neutral Gases: A Review

Abstract

The current state of knowledge on diffusion and thermodiffusion of neutral gases that govern the composition of the greater part of the Earth’s thermosphere is reviewed. Hydrodynamic equations determining diffusion velocities of neutral gases of the multicomponent atmosphere are considered in the first-order approximation of the velocity distribution functions of the Chapman–Enskog method using the first- and second-order approximations of the Sonine polynomial expansion to transport multicomponent coefficients of neutral gases. The interrelations among various definitions for the multicomponent diffusivities are given. The thermodiffusion factors and binary diffusion coefficients of N2, O2, O, He, H, Ar, N, NO, and H2 used in atmospheric studies to calculate the diffusion velocities and number densities of these atmospheric gases are discussed. The recommended temperature dependencies of the binary diffusion coefficients and the recommended average multicomponent diffusion correction and thermodiffusion factors of the atmospheric neutral gases under consideration are presented.