Electromagnetic radiation from a dipole source in a homogeneous magnetoplasma

Abstract

An electric Hertzian dipole is immersed in a cold homogeneous magnetoplasma and it is required to calculate the electromagnetic field at a moderate or great distance. Known methods of doing this are reviewed and extended. They all, in effect, express the field as an integral representing an angular spectrum of plane waves or of waves with conical wavefronts. The integral is evaluated by the method of steepest descents and extensions of it. Results are then presented of some calculations for various plasmas containing one or more species of positive ion. A study is made of the dependence of the radiated field, and of its Poynting vector, on direction and on frequency, when the source dipole is parallel to the superimposed magnetic field. There are three conditions where signals of large or very large amplitude can occur, namely ( a ) enhancement for directions very close to the direction of the superimposed magnetic field, ( b ) resonance cones, in which the signal is large for directions where the refractive index is very large, and ( c ) Storey cones and reversed Storey cones, which may be thought of as conical caustic surfaces where two rays have moved to coalescence and give constructive interference. These three features occur only in certain limited frequency ranges. The classification of these results is complicated and necessitates discussion of the transition frequencies of the plasma. For a plasma with more than one species of positive ion the phenomenon of crossover occurs, and its effect on the three types of signal enhancement is discussed.