The plasma parameters in the Earth's ionosphere display a marked variation with altitude, latitude, longitude, universal time, season, solar cycle, and magnetic activity. This variation results not only from the coupling, time delays, and feedback mechanisms that operate in the ionosphere–thermosphere system, but also from the ionosphere's coupling to the other regions in the solar–terrestrial system, including the Sun, the interplanetary medium, the magnetosphere, and the mesosphere. The primary source of plasma and energy for the ionosphere is solar EWV. IV. and X-ray radiation; but magnetospheric electric fields and particle precipitation also have a significant effect on the ionosphere. The strength and form of the magnetospheric effect are primarily determined by the solar wind dynamic pressure and the orientation of the interplanetary magnetic field (IMF), i.e., by the state of the interplanetary medium. Also, tides and gravity waves that propagate up from the mesosphere directly affect the neutral densities in the lower thermosphere, and their variation then affects the plasma densities. The different external driving mechanisms, coupled with the radiative, chemical, dynamical, and electrodynamical processes that operate in the ionosphere, act to determine the global distributions of the plasma densities, temperatures, and drifts.

As noted in Section 2.3, the ionosphere is composed of different regions and, therefore, it is instructive to show the regions in which the different external processes operate. Figure 11.1 indicates the altitudes where the various external processes are most effective. Solar radiation leads to ion–electron production and heating via photoelectron energy degradation, with EUV wavelengths dominating in the lower thermosphere (E and F1 regions) and UV and X-ray wavelengths dominating in the mesosphere (D region). These processes occur over the entire sunlit side of the Earth.