VLF (Very low frequency; 3-30 kHz) wave propagation in the Earth-Ionosphere waveguide can be simply analyzed by the method developed in this paper. The finite-difference time-domain (FDTD) method is used with a conductivity tensor to model the Ionosphere, and the applicability is limited in monotonic cases. The proposed method is compared with the wideband one, and it is concluded that the developed method can be used as the VLF wave simulator without errors. Moreover, the conformal FDTD is introduced to consider the curvature of the Earth's surface, and we have clarified the difference between the two-dimensional modeling in the Cartesian coordinates and the cylindrical ones.
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