CONE STRUCTURE AND FOCUSING OF VLF AND LF ELECTROMAGNETIC-WAVES AT HIGH-ALTITUDES IN THE IONOSPHERE

The frequency and angle dependencies of the electric field radiated by an electric dipole E = E(0) cos omega t are studied through numerical calculations of \E\ in the VLF and LF frequency bands 0.02f(b) less t han or equal to F less than or equal to 0.5f(b) in a model ionosphere over an altitude region of 800-6000 km where the wave frequency and el ectron gyrofrequency varies between F similar to 4 - 500 kHz and f(b) similar or equal to (1.1 to 0.2) MHz respectively. It is found that th e amplitudes of the electric field have large maxima in four regions: close to the direction of the Earth magnetic field line B-0 (it is cal led the axis field E(0)), in the Storey E(St), reversed Storey E(RevSt ), and resonance E(Res) cones. The maximal values of E(0), E(Res), and E(Rev.St) are the most pronounced close to the lower hybrid frequency , F similar to F-L. The flux of the electric field is concentrated in very narrow regions, with the apex angles of the cones Delta beta simi lar or equal to (0.1 - 1) deg. The enhancement and focusing of the ele ctric field increases with altitude starting at Z > 800 km. At Z great er than or equal to 1000 up to 6000 km, the relative value of \E\, in comparison with its value at Z = 800 km is about (10(2) to 10(4)) time s larger. Thus the flux of VLF and LF electromagnetic waves generated at high altitudes in the Earth's ionosphere are trapped into very narr ow conical beams similar to laser beams.