电离层-空气-海水介质中水下VLF电磁波传播的HIE-FDTD方法OA

The study of underwater very low frequency(VLF)electromagnetic wave propagation in ionosphere-air-seawater media is of great significance for cross-air-sea interface communication.For the fine-layered structure,the conventional finite-difference time-domain(FDTD)method was limited by the minimum grid size,making it difficult to increase the time step.In this paper,the hybrid implicit-explicit finite-difference time-domain(HIE-FDTD)method was proposed to predict the propagation characteristics of underwater VLF electromagnetic waves in ionosphere-air-seawater media.Iteration formulas of the HIE-FDTD method for dispersive media in the perfectly matched layer(PML)absorbing boundary were derived in cylindrical coordinates.A propagation model for electromagnetic waves radiated from an underwater VLF source in the three-layer ionosphere-air-seawater medium was constructed.The influence of ionospheric variations on underwater and above-water propagation was revealed.The results show that the HIE-FDTD method achieves comparable accuracy to the conventional FDTD method,with significantly improved efficiency.Underwater VLF waves radiated from a source attenuate severely and are almost unaffected by the ionosphere,while their propagation in the above-water air is significantly modulated by the ionosphere.Strong absorption by the daytime ionospheric D-layer leads to increased signal attenuation,whereas the disappearance of the D-layer at night significantly improves propagation conditions.During the day-night transition,gradual changes in ionospheric parameters cause mode coupling and conversion,resulting in significant oscillations and non-uniform attenuation of the field strength distribution.