A physical picture of the upper atmosphere cannot be obtained without determining whether vertical distribution depends on mixing or diffusion or on a chemical or photochemical equilibrium. It is necessary to determine how dissociation and recombination of molecular and atomic oxygen and nitrogen are distributed with height. The structure of the atmosphere deduced from density measurements is related to the variation of the mean molecular mass depending on diffusion effects. In addition, it is necessary to know how the heat budget is affected by conduction.