Characteristics of electromagnetic return-stroke models

Impedance and associated electromagnetic (EM) fields of an EM return-stroke model are investigated with the help of NEC-2. The impedance of a cylindrical vertical conductor increases with time. The impedance of a vertical conductor having additional distributed inductance is initially 700 to 1000 Omega for the current propagation velocity ranging from 170 to 130 m/mus. This is in good agreement with the apparent impedance of a lightning channel, estimated from lightning-current waveforms measured at the top and bottom of tall towers. A model considering a horizontally bent channel in a thundercloud succeeds in reproducing all of the established features of measured field waveforms except the hump of close magnetic field, while a vertically straight conductor fails to reproduce two of the features.