turbopause

The turbopause, also called the homopause, marks the altitude in an atmosphere below which turbulent mixing dominates. Mathematically, it is defined as the point where the coefficient of Eddy diffusion is equal to the coefficient of molecular diffusion. Because the molecular diffusion coefficient is dependent on both the composition of the diffusing molecule and the background gas, the turbopause altitude will be different for the different molecular species in an atmosphere. For example, in hydrogen (H2) dominated atmospheres, the helium (He) and methane (CH4) turbopause altitudes are different as a result of the different binary diffusion coefficients of helium and methane in molecular hydrogen.

The region below the turbopause is known as the homosphere, where the atmosphere is well mixed for chemical species which have long mean residence times. Highly reactive chemicals tend to have variable concentration throughout the atmosphere, while unreactive species have more homogeneous concentrations. The region above the turbopause is the heterosphere, where molecular diffusion dominates and the chemical composition of the atmosphere varies according to chemical species and their atomic weight.