ral variations. It has a profound effect on the characteristics of radio waves propagated within or through it. By means of wave refraction, reflection or scattering it permits trans- 9.2 Formation of the ionosphere and its mission over paths that would not otherwise be possible, but at the morphology same time it screens some regions that could be illuminated in its absence (see Figure 9.1) There is widespread interest in the characteristics of the ionosphere The ability of the ionosphere to refract, reflect or scatter rays by scientists all over the world. Several excellent general survey depends on their frequency and elevation angle. Ionospheric refrac- books have been published describing the principal known features tion is reduced at the higher frequencies and for the higher elevation (Davies, 1990; Ratcliffe, 1970) and other more specialised books angles, so that provided the frequency is sufficiently great rays 1 in concerned with aeronomy, and including the ionosphere and mag- netosphere, are of great value to the research worker (Rishbeth and Garriott, 1969; Davies, 1969; Ratcliffe, 1972). Several journals in the English language are devoted entirely, or to a major extent, to papers describing investigations into the state of the ionosphere and of radio propagation in the ionosphere. Height 20/10 3 10 20/50 The formation of the ionosphere is a complicated process involv- ing the ionising influences of solar radiation and solar and cosmic particles on an atmosphere of complex structure. The rates of ion Scattering and free-electron production depend on the flux density of the irregularities incident radiation or particles, as well as on the ionisation efficiency, which is a function of the ionising wavelength (or particle energies) South North and the chemical composition of the atmosphere. There are two heights where electron production by the ionisation of molecular nitrogen and atomic and molecular oxygen is a maximum. One Height occurs at about 100km and is due to incident X-rays with wave- lengths 