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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.
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