Introduction

The earlier chapters have discussed the propagation of a plane progressive radio wave in a homogeneous plasma. It is now necessary to study propagation in a medium that varies in space, and this is the main subject of the rest of this book. The most important case is a medium that is plane stratified, and the later chapters are largely concerned with the earth's ionosphere that is assumed to be horizontally stratified. Because of the earth's curvature the stratification is not exactly plane, but for many purposes this curvature can be neglected. In cases of curved stratification it is often possible to neglect the curvature and treat the medium as locally plane stratified. Some cases where the earth's curvature is allowed for are discussed in §§ 10.4, 18.8.

The theory of this chapter is given in terms of the earth's ionosphere. A Cartesian coordinate system x, y, z is used with x, y horizontal and z vertically upwards. These coordinates are not in general the same as the x, y, z of chs. 2–4. The composition of the plasma, that is the electron and ion concentrations Ne, Ni are assumed to be functions of z only. For the study of radio propagation the frequency is usually great enough for the effect of the ions to be ignored. When radio waves are reflected in the ionosphere the important effects occur in a range of height that is small enough for the spatial variation of the earth's magnetic induction B to be ignored.