On the relationship between the lightning electromagnetic field and the channel-base current based on the TL model

A set of approximation expressions relating the lightning channel-base current and the lightning electric and magnetic fields on earth surface are proposed respectively in the near-zone and far-zone, by employing the transmission line (TL) model. The derived expressions show that, the electric and magnetic fields waveforms can be expressed approximately by the channel-base current waveform with different factors at a certain distances, whether in the near-zone or far-zone. The factors can be expressed in terms of the return stroke wavefront speed v, the speed of light c, and the horizontal distance r between the return-stroke channel and the observation point.