It is over a century since Marconi's famous radio transmission across the Atlantic Ocean, an experiment closely followed by Kennelly and Heaviside's suggestions that an ionized layer in the Earth's upper atmosphere had made it possible. From the first, the ionosphere has been put to use, supporting an increasing range of applications from point-to-point communication and broadcasting, to direction-finding, navigation, and over-the-horizon radar. After 75 years of active research, the ionosphere can hardly be considered one of the mysteries of the Universe, but in fact some scientific problems and technical difficulties do remain. Many of them concern the high-latitude regions, which are particularly subject to disturbances arising initially on the sun.

Since radio propagation depends so strongly on the behavior of the ionosphere, we have tried to bring the two topics together into a single monograph about the polar regions. The early chapters (1–4) provide introductions to the ionosphere in general, to the influence of the magnetosphere, to the principles of radio propagation, and to the major techniques of ionospheric observation. Chapters 5–7 describe the various phenomena of the ionosphere that are peculiar to the high latitudes. The final chapters (8–9) present the results of high-latitude propagation experiments, many of which have been published only in reports that were not widely disseminated at the time or have indeed remained unpublished. Short summaries are included at the end of each chapter to aid readers in getting a quick overview of the material in the chapter.