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 μ s 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.

中文翻译：

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窄双极事件辐射对远距离电磁场的传播效应

在本文中，我们研究了有限的地面电导率和地球曲率对距离放电通道数百公里之内的窄双极事件（NBE）辐射的地面水平远垂直电场的传播影响。两种不同的NBE当前时刻，其上升时间分别为5和8μ 采用s，分别考虑两个放电高度5 km和15 km。仿真结果表明，当传播距离大于200 km时，应考虑地球曲率的影响，且传播效果与NBE放电电流波形，放电高度，地面电导率和观测距离密切相关。随着地面电导率的降低，场衰减明显增加。此外，当信号源包含更多的高频分量时，衰减会更大。还发现，当放电高度较低时，尤其是对于接地电导率低于0.01 S / m时，传播效果更加明显。在传播距离为100时，由传播效应引起的NBE远场峰的减少可以达到大约42％，60％和70％，分别为200公里和300公里。此外，在不考虑传播效应的情况下，距离NBE通道300公里以内，NBE的当前矩将比观测到的远场低约5–35％。然而，传播对估计NBE电荷矩的影响几乎可以忽略不计。