The use of near-antenna artificial density ducts for increasing the power of VLF radiation in space plasma

We investigate the radiation from an artificial density duct excited by an annular electric current in the VLF frequency range. It is assumed that the duct possesses the form of a quasi-cylindrical density enhancement which smoothly reduces with distance from the source. We have found that the field in the duct is represented with a good accuracy in terms of eigenmodes whose propagation is supported in density enhancements. The radiation field in the background medium arises due to reemission of these eigenmodes from the duct. We calculate the excitation coefficients of eigenmodes and study the characteristics of a radiation field. We show that the presence of a duct with enhanced density can cause a considerable increase in the power radiated from the source. In particular, we study the distribution of radiated power over the spatial spectrum of waves excited in a surrounding magnetoplasma. It is concluded that under certain conditions an appreciable increase in the power radiated into the long wavelength part of the spatial spectrum can be achieved. We give concrete estimations for the conditions appropriate to the Earth's ionosphere.