A Numerical Study of VLF Mode Structure and Polarization Below an Anisotropic Ionosphere

Abstract

The cylindrical waveguide problem with a diffuse, anisotropic upper boundary is rigorously formulated using a conformal transformation. This will allow us to demonstrate the adequacy of an ad hoc method used in a previous paper for simultaneously treating earth curvature and ionospheric effects. Weakly attenuated components of the mode spectrum associated with easterly propagation below a highly anisotropic ionosphere are developed numerically in the frequency range from 10–30 kHz. Also, the field strength distributions in the guide are presented. It is shown that modes of the earth detached variety, unlike the usual quasi‐TM or quasi‐TE modes, can contain comparable amounts of vertical and horizontal polarization. The numerical results also suggest that at the high end of the frequency spectrum considered, modes which are principally TE may be of importance in the mode sum.