On The Electromagnetic Fields Radiated By Lightning Strike To Tall Object

The tall tower is the main target of lightning for its easier formation of upward-leader. The transient effect of the tower makes significant difference between the electromagnetic field radiated by lightning strike to the tower and the flat ground. there have been numerous studies of electromagnetic radiation by lightning strike to the tall tower.However, there has no studies of the impact of soil’s frequency dependence on the electromagnetic radiation for strike to the tall tower. Besides, the study of transient electric field at mesospheric altitudes is on the basis of assumption of strike to flat ground regardless of the existence of the tall tower. Hence it is necessary to study the impact of the frequency dependence of soil and the existence of the tower at mesospheric altitudes on electromagnetic field.In this paper, we select the frequency-dependence model of the soil with the best correlation coefficient by comparison of six different frequency-dependence models, and apply it to the computation of electromagnetic field and determine the scope of application of the frequency-dependence model of the soil, on the basis of the analysis, we revise the FCCFs (Field-to-Current Conversion Factors) with the consideration of the frequency-dependence model of the soil. In addition, we study the effect of the tower of the characteristics on the change of the transient electromagnetic field, analyze the impact of the tower comparing with the case striking the flat ground. The results obtained are showed as belowFirstly, we compare six different frequency-dependence models, and select the frequency-dependence model of the soil with the best correlation coefficient by Kramers-Kronig test, and then we study the propagation effect of frequency dependent soil (FDS) on the far vertical electric fields radiated by subsequent lightning strike to tall objects with heights from 50 m to 300 m. It is found that the field propagation attenuation along FDS is obviously less than that case where the parameters are assumed to be constant (low frequency conductivity at 100 Hz, LFC), and with the decrease of LFC, the corresponding field attenuation increases. When LFC is equal to or larger than 0.01 S/m, the effect of FDS can be ignored. However, when LFC is 0.0001 S/m, the field peak for FDS may reach as much as 2.5 times of that earth with LFC for strike to a 300-m-tall object. According to the analysis result, we revise FCCFs, after a revision, inversion deviations are within 10%, significantly improving the accuracy.Secondly, we study the effect of the lightning strike to tall towers ranging from 50 m to 300 m height on transient electric field at mesospheric altitudes. For subsequent lighting current wave with rise-time (RT) ranging from 0.2 μs to 2 μs, it is found that the presence of the tall tower tends to increase substantially the electric field peak values and their derivatives with more high frequency components due to transient process of lightning current wave in the tall tower. The less RT and taller towers result in a much stronger field peak. For example, the lightning current wave with a RT of 0.2 μs, striking a 300-tall-tower, produces the radiation field pulse with a magnitude about several times larger than that for strike to flat ground at 80 km altitude. However, with the increase of RT, the effect of strike to tall towers on the far field becomes less.