| Very low frequency(VLF) electromagnetic waves can propagation in the earth ionosphere waveguide, propagating attenuation small, penetrating strong, widely used in long distance communication, global lightning distribution and space weather events monitoring. VLF electromagnetic wave affected by ionospheric parameter and the propagation characteristics exhibited diurnal and seasonal variation. In order to systematically study VLF electromagnetic wave propagation characteristics with the variation of ionospheric parameters in the waveguide and properties of electromagnetic wave propagation in the ionosphere anisotropic media, the paper established VLF propagation model based on FDTD method in waveguides and conducted a series of simulation analysis.Firstly, VLF electromagnetic waves transmission model was constructed in the earth ionosphere waveguide, introducing the ionospheric conductivity model, derived the VLF electromagnetic waves iterative form of current density equation and Maxwell equations in anisotropic medium. The H-J method and E-J method were compared and the simulation results showed that the H-J method was simple, but long simulation time; E-J method was complex, accounting for more memory, but short simulation time, which was more suitable for practical applications.Secondly, the diurnal and seasonal variations and frequency characteristics of VLF electromagnetic waves were analyzed in this paper, according to different types of conductivity models, such as winter/summer, day/night and so on. The simulation results showed that the electric field intensity of the winter was slightly lower than that of the electric field in summer and the curve was more gentle than that in summer during night; The ionospheric parameters was same during night in summer and winter. During the day, the electric field intensity was lower than at night, indicating that the ionospheric parameters had a significant influence on the transmission characteristics of electromagnetic waves, which was consistent with the D layer of the ionosphere disappeared at night. At different distances, the signal attenuation caused by the direct wave and wave interference effects was different. The distance of the minimum value(nodal points) and maximum value(wave antinode) of the filed intensity curve from the source occurred significant change caused by the ionospheric height variations. The interference of electromagnetic waves of different frequencies was different. 5k Hz electromagnetic wave interference effect was not evident over distance and the intensity curve over distance was relatively flat. The terminator position had great influence on the field strength.In addition, the paper analysed the polarization characteristics and the intensity characteristic of VLF electromagnetic wave with height. The simulation results showed that the vertical and horizontal electric field intensity decreased quickly with height in the 80-110 km altitude, followed by a slight decrease. The distance between the two peaks in the curve was approximately equal to the signal wavelength. As the height increase, the polarization plane of the electric field appeared to rotate, which was consistent with the transmission characteristics of electromagnetic waves in the magnetized anisotropic media. At different heights of the polarization plane, the axial ratio of the polarization ellipse gradually increased with the height and the signal was changed into elliptical polarized wave from the linear polarization. The axial ratio decreased after the signal travel into the D layer, indicating the polarization characteristics of electromagnetic wave changed caused by the ionospheric anisotropy, which was consistent with the generation of the electric field in the horizontal Φ direction during propagation. |
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