Research On The Propagation Characteristics Of The VLF Electromagnetic Wave In Magnetized Plasma Based On FDTD

Very low frequency(VLF)electromagnetic waves have small propagation loss,slow attenuation,stable phase and amplitude in the Earth-Ionospheric waveguide cavity.VLF is widely used in VLF communication and navigation,ionosphere heating,global lightning positioning and other fields.VLF electromagnetic waves are sensitive to ionospheric disturbances.When the ionospheric parameters change,their phase and polarization will change.In this paper,the propagation model of VLF electromagnetic waves in magnetized plasma is implemented based on the Finite Difference Time Domain(FDTD)method.The following studies are performed.Firstly,the 2D-FDTD propagation model in spherical coordinates is studied,and the Uniaxial Perfectly Matched Layer(UPML)is introduced to the reflection problem at the traditional truncated boundary.The Maxwell equation at the UPML boundary is deduced and implemented.The 2D-FDTD model with UPML boundary effectively solves the reflection problem existing in the models with PEC boundary.To further optimize the model,a phase compensation algorithm is used to solve the numerical dispersion problem caused by the finite difference of the FDTD method.The numerical dispersion is greatly reduced without changing the grid size.After that,the single-frequency signal was used to study the phase and polarization characteristics of the VLF electromagnetic wave propagating in the magnetized plasma ionosphere.In the vertical direction,at low height,the phase of the electromagnetic wave remains basically linear,and when the ionization degree gets high at high altitude,the influence of the ionosphere on the electromagnetic wave causes the phase to deviate from the linear change.The lower the frequency of the electromagnetic radiation,the lower the height of the phase anomaly will occur in its propagation.Parameters in the ionosphere,electron density,and collision frequency also have different influences on the phase of the electromagnetic wave.When the electron density increases or the collision frequency decreases,the ionosphere will have more influence on the phase.This will make the phase anomaly occur at a lower altitude.At a certain position below the ionospheric bottom height,the polarization of the electromagnetic wave does not change with time,showing a stable elliptical polarization.Electromagnetic waves approach the line polarization in the horizontal direction,and the polarization elliptical amplitude does not change.The polarization ellipse in the vertical direction changes regularly at low height of the waveguide,but changes more violently than in the horizontal direction.After reaching a certain height,the attenuation of the electromagnetic wave by the ionosphere causes the size of the polarization ellipse to decrease,and the polarization ellipse will not be closed.According to the axial ratio,in the vertical direction,the electromagnetic wave gradually changes from a linearly polarized wave to a circularly polarized wave at the time of emission.An abnormality starts at about 70 km,the axial ratio will not change periodically and lose the trend of increasing.Finally,the VLF propagation model was corrected and compared using the measured data of the artificial source lightning pulse.According to the measured data,the ionospheric parameters are simulated.The first and second reflections of the lightning pulse in the simulation data are in good agreement with the measured data.According to the observation of the ionosphere inversion,the difference in ionospheric reflection height between day and night is approximately 23.2 km.The effect of ionospheric parameters on the reflection height was further analyzed using broadband lightning pulse signals.Simulation results showed that the increase in electron density or the reduction in the collision frequency resulted in a decrease in the reflection height.