WACCM-X Simulation of Tidal and Planetary Wave Variability in the Upper Atmosphere

This chapter describes the basic physical processes in the thermosphere, or Earth's neutral upper atmosphere, which need to be captured in a physics-based model. The thermosphere is the medium from which the ionosphere is created, and the neutral dynamics and composition is an important driver of the ionized component. The ionosphere, in turn, has important impacts on the neutral medium through ion drag and Joule heating. The frequent collisions of a gas close to thermal equilibrium enable the Maxwellian energy distribution of the individual particles to be replaced by the basic fluid properties of pressure, temperature, number density, and mass density. The continuity equation is also one of the most widely used and universal fluid concepts. Atmospheric models typically solve the equations in an Eulerian coordinate system fixed with respect to the Earth. The chapter talks about the Coriolis effect, neutral composition, and thermal expansion.