Numerical Research Of Ionospheric Langmuir Turbulence Using The Finite-Difference Time-Domain Method

The ionospheric F layer is a natural plasma region with large scale,where the plasma is inhomogeneous and time-varying.In recent years,attention has been paid to the study of wave propagation characteristics among ionospheric plasma by heating experiments.The finite-difference time-domain(FDTD)method can directly solve the complicated electromagnetic problems in time domain and is widely applied in the field of electromagnetic simulation.The numerical simulation of the ionospheric heating process using the FDTD algorithm can intuitively show the changes in the electromagnetic field and plasma density,and it is useful for understanding the complex interaction between the ionosphere and electromagnetic(EM)wave.In this paper,numerical simulation of Langmuir turbulence during ionospheric modification is carried out using FDTD algorithm.Main research contents are as follows:1.This paper presents the implementation of FDTD simulation for ionospheric Langmuir turbulence.The numerical model for EM wave propagation in the ionosphere is eatablished by combining the Maxwell's equations with the plasma fluid equations.The dispersion relation in the ionospheric plasma is briefly analyzed.Then,the specific formation of the FDTD algorithm is presented.The parameters used in the simulation are from the ionospheric heating experiment in high latitude area.The heating effect of EM wave on the ionosphere may excite Langmuir turbulence.Distribution of the EM field varying with time and altitude as well as mode conversion during the process is discussed according to numerical results,which proves the feasibility and validity of the FDTD scheme.2.Connection boundary condition in plasma is proposed.Considering the propagation characteristics of EM wave in the dispersive medium,this paper proposes a modified connection boundary condition based on Huygens principle,in the case of the plasma background.A numerical test is carried out to validate the performance of the method by analyzing the relative error in the total field and the leakage into the scattered field region.3.This paper studies a non-uniform nested grid for FDTD simulation.The ionospheric profile,EM wave and Langmuir turbulence involve vastly different length scales.Under the restriction of Courant-Friedrich-Lewy(CFL)stability condition,uniform grid with small spatial step size will seriously affect computational efficiency.In order to reduce calculation amount and computing time,the non-uniform nested grid method of FDTD algorithm is studied.Taking the numerical model of ionospheric heating as an example,the principle of grid partition and the schemes for field values transformation between coarse grid and dense grid are discussed in detail.Simulation results of a non-uniform nested grid and a dense,equidistant grid in the case with and without a cavity in the ion density are compared,which proves the proposed grid is practicable.Furthermore,computing time of the two methods are compared.The result shows that the non-uniform nested grid can greatly improve the computational efficiency.At last,FDTD numerical simulation of ionospheric Langmuir turbulence is carried out using the connection boundary condition and non-uniform nested grid proposed in this paper at the same time.Distributions of electric field at different times are in agreement with the case that the incident source is in vacuum and the grids are uniform and fine.