Mode Conversion of ELF Radio Waves Propagation in an Inhomogeneous Oceanic Lithosphere Waveguide

In this paper, we investigate the mode conversion of ELF radio waves propagation in an inhomogeneous oceanic lithosphere waveguide with an abrupt change of thickness over the propagation range. By applying a mode-matching approach at the junction between two uniform waveguides, we derive a system of linear equations for the mode conversion coefficients. The closed-form expressions for the mode conversion coefficients and phase deviations are obtained for both transverse magnetic (TM) and transverse electric (TE) polarized modes by solving these equations. Numerical results for the mode conversion coefficients and phase deviation are given. Computations show that as the frequency and waveguide thickness difference increase, the mode conversion coefficient and phase deviation increase. Under the same conditions, the conversion of the incident fundamental mode to the higher order modes remains relatively low, with the higher the order, the lower the conversion. Besides, an increase in the conductivity of the medium at the lower boundary of the waveguide will result in a decrease in the mode conversion coefficient.