Effect of local inhomogeneity of the ionosphere on the propagation of ULF radio waves

The Stratton-Chu integral equation is used to solve the electrodynamic problem concerning the propagation of ULF waves in an inhomogeneous spherical earth-ionosphere cavity. The effect of the local inhomogeneity of the ionosphere is taken into account in the framework of the Born approximation of perturbation theory. An analysis is made of perturbation variations of the vertical electric component of the field as the inhomogeneities travel along the source-observer line and across the propagation path. It is shown that a local inhomogeneity located above the source or observer can lead to a significant change in the field level in comparison with a homogeneous cavity.