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(mu)s. 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.