These two sessions, which cover the topics of E-region models, electric fields, and electric currents, underscore a major difficulty necessarily encountered in studies of the E region. This difficulty arises from the fact that the E region of the ionosphere is just one part of a very large system, all of whose component parts are strongly coupled electrodynamically and hydrodynamically. Before describing a general approach to overcome this difficulty, let me briefly discuss some of the important coupling processes in the overall system.
As the solar wind flows past the earth, it exerts a tangential stress at the magnetopause boundary. Information of this stress is propagated by hydromagnetic waves to the polar cap ionosphere, where an electric potential distribution is established that is consistent with the plasma motions required by the applied tangential stress. The resultant electric field pattern covers not only the polar cap region, but extends to the auroral zone and, to some extent, to middle and low latitudes. The electric fields imposed on the ionosphere equatorward of the polar cap are mapped into the closed field line regions of the magnetosphere, thus establishing a plasma convection pattern in the magnetosphere. The actual electric field pattern established is strongly affected by the magnetospheric plasma population, ionospheric conductivity, and neutral thermospheric winds. In fact, due to magnetospheric plasma effects, the electric field is largely shielded (on time scales of several hours or more) from the middle and low latitude regions. Nevertheless, relatively steady solar-wind-generated electric fields of a few mv m−1, along with much larger transient fields, probably are imposed at middle and low latitudes.