Excitation of the Whistler Mode in the Ionosphere by Leakage From VLF Guided‐Wave Modes

Abstract

The penetration of VLF waves into the ionosphere is calculated from the mode representation of the fields in the earth‐ionosphere waveguide. This calculation is more appropriate for evaluation of leakage of VLF energy into the ionosphere at large distances from the transmitter than is the calculation based on a ray theory representation of the fields. The ratio of the amplitude of the field excited in the ionosphere to that observed on the earth is found to agree with the observations of the LOFTI I experiment.In carrying out the analysis of the leakage of energy from the waveguide modes, it is necessary to reevaluate the modal propagation constants, employing a self‐consistent calculation requiring that the wave polarization of a modal field be preserved upon reflection at the waveguide boundaries. Upon imposing these conditions, it is found that two sets of modes result, having different polarizations. These sets of modes have been designated as quasi‐TE and quasi‐TM.by Budden, but both sets tire no1 usually used in VLF field strength calculations. Under nighttime conditions, both types of modes will be of importance, while in daytime only the quasi‐TM modes will be excited by a vertical monopole antenna on the earth. These results predict a nonreciprocal attenuation over a north‐south propagation path.