The earth's atmosphere

This book is mainly concerned with the effect on radio wave propagation of the ionised regions of the earth's atmosphere. Near the ground the air is almost unionised and its electrical conductivity is negligibly small, because the ionising radiations have all been absorbed at greater heights. When any part of it is in equilibrium, its state is controlled by the earth's gravitational field so that it is a horizontally stratified system. Although it is never in complete equilibrium, gravity has a powerful controlling effect up to about 1000 km from the ground.

The molecules of the neutral atmosphere have an electric polarisability which means that the refractive index for radio waves is very slightly greater than unity, about 1.00026, near the ground. The water vapour also affects the refractive index. Thus the neutral air can very slightly refract radio waves. This can lead to important effects in radio propagation. For example in stable meteorological conditions a duct can form near the surface of the sea, acting as a wave guide in which high frequency radio waves can propagate to great distances (see Booker and Walkinshaw, 1946; Brekhovskikh, 1960; Budden, 1961b; Wait, 1962). Spatial irregularities of the refractive index of the air can cause scintillation of radio signals, and also scattering which can be used to achieve radio propagation beyond the horizon (Booker and Gordon, 1950). These effects are beyond the scope of this book.