Electromagnetic waves in the very-low-frequency band (3-30 kHz) propagate between the Earth's surface and the ionosphere. This study develops the application of the finite difference time domain (FDTD) method to a simplified Earth-ionosphere waveguide model. First of all, we model the ionosphere as a medium with anisotropic conductivity, which is called the equivalent conductivity model. The numerical results show an excellent agreement with results computed by FDTD treating the ionosphere as an anisotropic and dispersive medium. The curvature of the Earth and the ionosphere structure is then considered by using the conformal FDTD method. Two-dimensional analysis model with the azimuthal symmetry is compared with the three-dimensional analysis, showing relatively small errors. The Earth's conductivity is also taken into account by implementing the surface impedance method. Finally, a daily variation is calculated by using all methods developed above.%本研究では解析対象を単一周波数に限定することで広帯域解析手法のEJ法と同精度でありながら計算量削減を可能にした。また、地球の曲率、大地の導電率が伝搬に影響を及ぼすことを示した。電界強度日変化の計算は電離圏のプロファイルに依存し、しかも電離圏下部のプロファイルが不明であるため非常に困難な計算である。逆に日変化の観測データがあれば電離圏下部の状態を推定することが可能なはずである。
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