Excitation and Propagation of SLF/ELF Electromagnetic Waves in the Earth–Ionosphere Waveguide/Cavity

Abstract

In this chapter, the region of interest is a waveguide or cavity between the Earth's surface and an isotropic homogeneous ionosphere. The dipole (vertical electric dipole (VED), the vertical magnetic dipole (VMD), or the horizontal electric dipole (HED)) and the observation point are assumed to be located on or near the spherical surface of the Earth. The approximate all formulas are obtained for the electromagnetic field radiated by a VED and a VMD in the Earth-ionosphere waveguide or cavity. Based on the above results, the approximate formulas are derived readily for the electromagnetic field of an HED in the Earth-ionosphere waveguide or cavity by using the reciprocity theorem. Analyses and computations in SLF/ELF ranges are carried out specifically.

Mode Interferences of VLF Waves in the Presence of an Anisotropic Terrestrial Waveguide

“…At VLF radio frequencies, the surface impedance of the ground is much smaller than that of the ionosphere. This makes the change of the ground conductivity to have little effect on the phase velocity and attenuation rate of the waves [19]. In the following computations, the ground is assumed to be idealized to the sea surface.…”

“…However, it is also known from Figures 4(a) and (b) that the phase velocity of the fundamental mode, standing for the ground wave, is affected very little by earth geomagnetic inclination angles, while that of the high modes changes greatly by the angles. Specifically, the influence of the relative phase velocity resulted from the earth geomagnetic field is about in the range of 1-3% [19].…”

“…For fundamental modes, there exist directional actions in attenuation rate and phase velocity. For example, the attenuation rate propagating westward will be greater than that propagating eastward, if the relative phase velocity propagating eastward is also smaller than that propagating westward [19].…”

“…The computation is obtained by choosing a nighttime-todaytime propagation path with an abrupt effective height change of ionosphere by 20km across the discontinuity. Technically, the magnitudes of the first-second and second-first modeconversion coefficients will affect more than the higher modes [19]. It is seen that more summation terms would be required to guarantee accuracies of calculations at high operating frequencies.…”

“…The analytical formulas for mode-conversion coefficients have been derived in the circadian transitional period by Pan et al [19][20][21] applied in ELF/SLF electromagnetic wave propagation in earth-ionosphere waveguide due to sunrise and sunset. Apart from its simplicity, the proposed computations are also feasible for studying the influence of the anisotropic behavior from the ionosphere, including reflection characteristics of the transitional daytime and nighttime sections.…”