Propagation Effects on Distant Electromagnetic Fields Radiated by Narrow Bipolar Events

In this article, we studied the propagation effect of finite ground conductivity and earth's curvature on the far vertical electric fields at ground level radiated by narrow bipolar events (NBEs), within several hundreds of kilometers from the discharge channel. Two different NBE current moments with risetimes of 5 and 8 μs are adopted, and two discharge heights of 5 and 15 km are considered, respectively. Simulation results show that the effect of earth's curvature should be considered when the propagation distance is larger than 200 km, and the propagation effect is closely related the NBE discharge current waveform, the discharge height, the ground conductivity, and the observation distance. With the decrease in ground conductivity, the field attenuation obviously increases. Besides, the attenuation is more when the source contains more high frequency component. It is also found that the propagation effect is more obvious when the discharge height is lower, especially for ground conductivity lower than 0.01 S/m. The reduction of NBE's far field peak caused by the propagation effect can reach about 42%, 60%, and 70% at the propagation distance of 100, 200, and 300 km, respectively. Furthermore, the current moment of NBE will be underestimated by about 5-35% from the observed far field without considering the propagation effect, within 300 km from NBE channel. However, the propagation effect on the estimation of NBE charge moment can be approximately ignored.