电离层扰动对VLF电磁波的传播影响

发布时间：2018-08-25 18:10

【摘要】：甚低频(VLF)电磁波在地-电离层波导中传播具有衰减小、稳定性高、传播距离远等优点。研究VLF电磁波在电离层中的传播机理与仿真计算,对进一步开展VLF通信、VLF监测电离层有重要的理论价值和实际意义。针对VLF电磁波从地面到卫星高度的传播,特别是电离层扰动对VLF传播特性的影响,本课题进行了如下仿真研究。首先基于麦克斯韦方程推导了全波分析法下的VLF传播模型。通过对色散方程解得的虚部为负的折射系数取反,解决了数值不稳定问题。并将辐射源看做由电流密度表示的能量薄层,使得传播计算能够引入任意形式的辐射源。进行傅里叶变换,将计算引入到波矢量k域,得到全向辐射传播模型。同时,结合电离层底部参数的解析模型与IRI模型、NRLMSISE-00大气模型计算引入电离层参数,利用IGRF模型引入地磁场参数,建立了从地面到卫星高度的VLF传播全波分析模型。其次,在单向辐射模式下,针对辐射源参数、地磁场参数、电离层参数以及纬度对VLF电磁波传输特性的影响进行了系统的仿真分析。仿真计算发现,在地-电离层波导当中能量几乎不衰减,在大约60-100km范围能量衰减迅速,电磁波进入E、F层后能量衰减缓慢。夜间传播衰减明显低于白天。随着频率的升高,反射吸收层上的传播衰减增大,反射高度升高。传播衰减并不随地磁倾角线性变化,VLF电磁波的上行传播在南半球衰减小于北半球,赤道附近衰减最大。然后,在全向辐射模式下,同样针对辐射源参数、地磁场参数、电离层参数及纬度进行了全面的仿真计算和分析。并将全波法计算结果和DEMETER卫星观测数据对比,验证了全向辐射模型的正确性。仿真结果发现,在地-电离层波导当中电磁波以波跳形式传播,能量呈现南北对称分布。电磁波到达电离层底部的反射吸收层上时,大部分被反射、吸收,只有较少的部分穿透底部电离层,因此底部电离层上的传播衰减最显著。穿透底部电离层的电磁波将顺着磁力线方向在电离层中传播,传播衰减很小,且受地磁场影响能量呈现南北不对称分布。最后,考虑到多种空间天气事件都会对电离层电子密度、中性粒子密度、电子温度造成扰动,创新性地将这三种电离层扰动分别引入传播计算,分析了VLF传输特性的变化。仿真结果发现,这三种扰动主要影响VLF在穿透反射吸收层时的衰减,加强性扰动使得衰减增大。
[Abstract]:The propagation of very low frequency (VLF) electromagnetic wave in the ground-ionospheric waveguide has the advantages of low attenuation, high stability and long propagation distance. It is of great theoretical and practical significance to study the propagation mechanism and simulation of VLF electromagnetic wave in ionosphere. Aiming at the propagation of VLF electromagnetic wave from ground to satellite, especially the influence of ionospheric disturbance on the propagation characteristics of VLF, the following simulation research is carried out in this paper. Firstly, the VLF propagation model based on Maxwell equation is derived. The numerical instability problem is solved by inverting the refraction coefficient with negative imaginary part of the solution of the dispersion equation. The radiation source is regarded as a thin layer of energy represented by the current density, so that the propagation calculation can be introduced into the radiation source of any form. Fourier transform is carried out and the calculation is introduced into the wave vector k domain to obtain the omnidirectional radiation propagation model. At the same time, combined with the analytical model of the bottom parameters of the ionosphere and the IRI model, the ionospheric parameters are introduced into the atmospheric model NRLMSISE-00. Using the IGRF model to introduce the geomagnetic field parameters, the full-wave analysis model of VLF propagation from the ground to the satellite is established. Secondly, the effects of radiation source parameters, geomagnetic field parameters, ionospheric parameters and latitudes on the transmission characteristics of VLF electromagnetic waves are analyzed systematically in unidirectional radiation mode. The simulation results show that the energy in the ground-ionospheric waveguide almost does not attenuate. The energy decay is rapid in the range of 60-100km, and the energy decay is slow after the electromagnetic wave enters the EOF layer. The attenuation of nocturnal propagation is lower than that of daytime. With the increase of the frequency, the propagation attenuation and the reflection height on the reflection absorption layer increase. Propagation attenuation does not vary linearly with magnetic dip angle. The attenuation of VLF electromagnetic wave in the southern hemisphere is less than that in the northern hemisphere, and the attenuation near the equator is the largest. Then, in the omnidirectional radiation mode, the emitter parameters, geomagnetic field parameters, ionospheric parameters and latitude are also simulated and analyzed. The validity of the omnidirectional radiation model is verified by comparing the full wave method with the DEMETER satellite observation data. The simulation results show that the electromagnetic wave propagates in the ground-ionospheric waveguide in the form of wave hopping, and the energy is distributed symmetrically in the north and south. When the electromagnetic wave reaches the reflection and absorption layer at the bottom of the ionosphere, most of the waves are reflected and absorbed, and only a few of them penetrate the bottom ionosphere, so the propagation attenuation on the bottom ionosphere is the most significant. The electromagnetic wave passing through the bottom ionosphere will propagate along the direction of the magnetic line in the ionosphere, and the propagation attenuation will be very small, and the energy affected by the geomagnetic field will present an asymmetric distribution from the north to the south. Finally, considering that the ionospheric electron density, neutral particle density and electron temperature will be disturbed by various space weather events, the three ionospheric disturbances are innovatively introduced into the propagation calculation, and the variation of VLF transmission characteristics is analyzed. The simulation results show that these three kinds of disturbances mainly affect the attenuation of VLF when it penetrates the reflective absorption layer, and the enhanced disturbance increases the attenuation.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TN011

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