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 b y an annular electric current in the VLF frequency range. It is assume d that the duct possesses the form of a quasi-cylindrical density enha ncement which smoothly reduces with distance from the source. We have found that the field in the duct is represented with a good accuracy i n terms of eigenmodes whose propagation is supported in density enhanc ements. The radiation field in the background medium arises due to ree mission of these eigenmodes from the duct. We calculate the excitation coefficients of eigenmodes and study the characteristics of a radiati on field. We show that the presence of a duct with enhanced density ca n cause a considerable increase in the power radiated from the source. In particular, we study the distribution of radiated power over the s patial spectrum of waves excited in a surrounding magnetoplasma. It is concluded that under certain conditions an appreciable increase in th e power radiated into the long wavelength part of the spatial spectrum can be achieved. We give concrete estimations for the conditions appr opriate to the Earth's ionosphere. (C) 1997 Elsevier Science Ltd.