Refractive index of spherical waves in magnetoplasma

In geospace electromagnetic waves from a radio sounder travel in the ionosphere or the magnetosphere and, if reflected in the plasma, return to the receiver. The received signals provide the information of the sounded media. By analyzing the measured data physical parameters of the medium like density and movement can be obtained. The waves originate from a source of small dimensions relative to the distances involved and the resulting waves have spherical wave fronts. For ground-based sounding, when the wave reaches the ionosphere the radius of curvature is so large that the wave in a given direction can be treated as one of two plane waves, either L mode or R mode, and the error introduced with this approximation is generally believed to be negligibly small. For sounding in the topside ionosphere and the magnetosphere, however, the transmitter and receiver on board a rocket or satellite are located within an anisotropic medium, and the question arises how well the excited field is still approximated by a single plane wave in a given direction. An attempt is made in this paper to derive the properties of the excited electromagnetic field including the wave mode, polarization, and the spatial distribution of the power density when the current source is embedded in anisotropic plasma.