Research On The Calculation Method And Propagation Along Ground Surface Of Lightning Electromagnetic Field

Research on the propagation of the lightning radiation electromagnetic field under complicated condition in large scale is of great significance, for accuracy knowledge of lightning phenomena and mechanism, as well as the prewarning and protection of lightning. Supported by the National Nature Science Fund (60601013), this thesis is focused on lightning radiation electromagnetic field propagating along ground under the cases of lossy ground, ionosphere, earth curvature, horizontally-stratified and vertically-stratified soil, building and mountain taken into account respectively. Based on the proposed method and formula, which is validated through the comparison with corresponding results from measurement and other literature, the role of various factors playing in the propagation of lightning radiation field is investigated and summarized. The main achievements are as follows:(1) the calculation formula for the radiation field from electric dipole over lossy ground as well as that for lightning radiation magnetic field on lossy ground surface is deduced. Based on the proposed formula and the calculation formula of the lightning radiation field above perfect ground, the effect of soil resistivity and the height of lightning channel and observation point are all discussed. The modified method is presented for the inversion of lightning current from lightning magnetic field.(2) by using multi-layer image method and conductor globe image method, the calculation formula for the lightning radiation electromagnetic field with the ionosphere and earth curvature taken into account is deduced respectively to account for the effects of the ionosphere height and earth curvature. Measured result is also provided to illustrate the effect of ionosphere.(3) based on Hertz potential, the formula for the calculation of lightning radiation magnetic field above horizontally-stratified soil is deduced using method of recursion.The usually two-layer model is considered and the effect of the upper and down soil with same or different is discussed in great detial.(4) based on the calculation of the radiation field of electric dipole above two-vertical-layer soil, the expression for the radiation field of electric dipole propagating along three-vertical-layer mixed path is obtained through recursion. The difference between the lightning radiaiton field above vertically-stratified soil and that above perfect ground is analysed.(5) time-domain integral equation method applicable to the electromagnetic scattering from surface element above perfect ground is presented, based on which the effect of building on the lightning magnetic field sensor placed on its top or nearby is investigated. The improved approach to reduce the effect of building on surrounding magnetic field is given according to the curve of magnetic field versus building height and width.(6) with the total electric field inside the dielectric body taken as unkonwn variables, the frequency-domain volumn integral equation method is presented to analyze the scattering of lossy dielectric body. Point matching method is used to solve the equation. By using the proposed method, the effect of mountain height and the distance from lighning point and observation point to the mountain are investigated and significant tesult is obtained.