The authors investigate the ideal, nondriven multifluid equations of motion to identify consistent (i.e., truly stationary), mechanically static models for composition profiles within the thermosphere. These physically faithful functions are necessary to define the parametric core of future empirical atmospheric models and climatologies... Based on the strength of interspecies coupling, the thermosphere has three altitude regions: (1) the lower thermosphere (herein z < ~100 km), in which all species move together at the composite fluid velocity with an effective particle mass equal to the average particle mass of the composite fluid; (2) the upper thermosphere (herein z > ~200 km), in which the species flows are approximately uncoupled; and (3) a transition region in between, where the effective species particle mass and the effective species vertical flow interpolate between the solutions for the upper and lower thermosphere. We place this view in the context of current terminology within the community, i.e., a fully mixed (lower) region and an upper region in diffusive equilibrium (DE). The latter condition, DE, currently used in empirical composition models, does not represent a truly static composition profile in the presence of finite thermal diffusion. Rather, species by species hydrostatic balance is a consistent (i.e., stationary) static representation of vertical thermospheric composition profiles. (read more)