  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.
