The magnetosphere–ionosphere–atmosphere system at high latitudes is strongly coupled via electric fields, particle precipitation, field-aligned currents, heat flows, and frictional interactions, as shown schematically in Figure 12.1. Electric fields of magnetospheric origin induce a large-scale motion of the high-latitude ionosphere, which affects the electron density morphology. As the plasma drifts through the neutrals, the ion temperature is raised owing to ion–neutral frictional heating. The elevated ion temperature then alters the ion chemical reaction rates, topside plasma scale heights, and ion composition. Also, particle precipitation in the auroral oval acts to produce enhanced ionization rates and elevated electron temperatures, which affect the ion and electron densities and temperatures. These ionospheric changes, in turn, have a significant effect on the thermospheric structure, circulation, and composition. At F region altitudes, the neutral atmosphere tends to follow, but lags behind, the convecting ionospheric plasma. The resulting ion–neutral frictional heating induces vertical winds and O/N2 composition changes. These atmospheric changes then affect the ionospheric densities and temperatures.
The ionosphere–thermosphere system also has a significant effect on the magnetosphere. Precipitating auroral electrons produce conductivity enhancements, which can modify the convection electric field, large-scale current systems, and the electrodynamics of the magnetosphere–ionosphere system as a whole. Also, once the thermosphere is set into motion by convection electric fields, the large inertia of the neutral atmosphere will act to produce dynamo electric fields whenever the magnetosphere tries to change its electrodynamic state. Additional feedback mechanisms exist on polar cap and auroral field lines via a direct flow of plasma from the ionosphere to the magnetosphere.