Propagation Of Lightning Electromagnetic Fields On Complex Ground

Lightning electromagnetic field, especially the radiation field, can propagate hundreds of kilometers along the surface. The propagation of the lightning electromagnetic field in the wide area will produce dispersion and reflection due to ground conductivity and rough ground surface. This could influence the lightning waveform measurement in the lightning location system. As a result, the error will be brought in inversing parameters, such as the peak and rise time of the lightning current.This paper focuses on lightning-generated electromagnetic field propagating along the complicated surface conditions. Based on MTLL lightning return stroke model, a finite-difference time-domain method (FDTD) in 2D cylindrical coordinate is put forward to analyze the electromagnetic field over different stratified ground. Compared with other different approximate algorithms, the accuracy and the reliability of the model are discussed. The main achievements are as follows:(1) Compiling the FDTD calculation programs of the lightning return-strokes electromagnetic field, it can simulate the lightning return-strokes electromagnetic field of the complicated surface conditions.(2) The usually two-horizontally-stratified ground model is discussed in great detail. Electromagnetic interference effect will enhance some high frequency components when the surface ground conductivity is less than the bottom and the certain frequency component higher than 1 Mhz because of small complex refractive index on the surface. Therefore, the attenuation factor will be greater than unit in the approximate algorithm, and electromagnetic field appears vibration in high frequency.(3) Compared with the two Wait's approximates and the Cooray approximate above a horizontal stratified ground and examined the accuracy using FDTD technique at closer distances. The results show that the two Wait's approximates can simulate the propagation of the return stroke electromagnetic field along the lossy ground with a reasonably good accuracy at distances of 50m to lkm from the lightning channel. However, the Cooray approximate can be used to predict the lightning electromagnetic field with a better accuracy only when the upper layer has a larger conductivity than the lower one.(4) Using FDTD method to simulate the two-vertically-stratified ground model where two mediums have the same horizontal size, we find that the peak of lightning electromagnetic field is larger than the result of the single lossy ground at distance of 1km from the lightning channel, while the slope of rise time is opposite.(5) Results of the Wait's approximate algorithm over two-vertically-stratified ground are analyzed in detail and compared with the FDTD method. There is only one correction conclusion can be obtained when Wait's approximate algorithm was applied. A slight difference between the results the actual situation could be found. Consequently, this approximate algorithm only can simulate the amplitude trend roughly of the vertical electric field.(6)The effect of the rough ocean surface on the derivation of the electric field is much more obvious than on the vertical electric field, and the horizontal field can be ignored because it is tiny. The high frequency components (>10MHz) in the electromagnetic fields attenuate rapidly when they propagate over rough ocean surface while the attenuation of high-frequency caused by ocean waves is mainly concentrated in about 30km away from the lightning channel.