The polar ionosphere

During the International Magnetospheric Study period our understanding of phenomena in the polar ionosphere and the role of this region in the coupling between the ionopshere and the magnetosphere has progressed to a point where many self‐consistent pictures are emerging. In many cases the required relationships between electric currents, conductivity, and electric field can be demonstrated, and observed phenomena can be explained in terms of realistic variations in these parameters. The distribution and composition of plasma can be reconciled with realistic patterns of large‐scale plasma motion and ion production regions. The extent of our understanding is reviewed here. There are, however, gaps in our understanding of the electrodynamic and the plasma properties of the polar ionosphere. But the gaps show promise of being filled by our ability to properly formulate the problems and to provide the necessary measurements from present and future spacecraft missions.