FDTD Analysis Of VLF Wave Propagation In Ground-ionospheric Waveguide

In the very low frequency(VLF: 3-30KHz)band,the ground and the ionosphere can be regarded as good conductors,and electromagnetic waves propagate in the groundionosphere waveguide.The ionosphere is a weakly ionized plasma whose medium parameters change with frequency.In the geomagnetic field,the plasma is an anisotropic medium,and the dielectric constant can be described by a tensor.Analytical methods or numerical methods can be used to analyze electromagnetic wave propagation in the groundionospheric waveguide.Analytical methods were mostly used in the previous research.In order to study the characteristics of ionospheric radio wave propagation at the day-night interface,numerical methods such as finite difference time domain(FDTD)calculation of ionospheric-related radio wave propagation problems have attracted people's attention.In this paper,considering the characteristics of the ionosphere as an anisotropic dispersion medium,using the shift operator finite difference time domain(SO-FDTD)method to study the propagation of VLF waves in the ground-ionospheric waveguide,the main work is as follows:1.Research on FDTD method of ionospheric radio wave propagation characteristics analysis.The ionosphere is an anisotropically dispersive non-uniform medium.In this paper,the SOFDTD method is used to calculate its radio wave propagation characteristics.Since the computational domain is anisotropically dispersive and lossy medium,this paper adopts MIPML to absorb outward traveling waves.In the calculation,the ionosphere is stratified vertically,and the corresponding regional media parameters are determined according to the change law of electron concentration and collision frequency and the size of the external magnetic field.The calculation example shows that the method in this paper can meet the requirements of ionospheric radio wave propagation characteristics,and the selected absorption boundary has a good absorption effect.2.Use the FDTD method to calculate the propagation characteristics in a flat layered groundionospheric waveguide.The calculation results of the analytical method are compared with those of the FDTD method to illustrate the feasibility of the algorithm and program in this paper in calculating the propagation characteristics of electromagnetic waves in the VLF band in the analysis of the ground-ionospheric waveguide.The propagation characteristics of VLF waves in the plane ground-ionospheric waveguide model during the day and night are simulated.3.Calculated the propagation characteristics in the ground-ionospheric waveguide considering the curvature of the earth and the different segmentation parameters.In view of the many complicated situations in the actual propagation of VLF waves,this article considers the possibility of multiple media in the propagation path to establish a segmented ground-ionospheric waveguide model,and takes the curvature of the earth into account to establish a curved ground-ionospheric waveguide model Considering that the equivalent height of the ionosphere will change when the day and night alternates,the waveguide model under the alternate day and night is established,and the simulation calculation and analysis are carried out.