FDTD Simulations of LEMP Propagation in the Earth-Ionosphere Waveguide Using Different Lightning Models

In this article, vertical electric fields (including skywaves) produced by lightning return strokes at distances ranging from 100 to 958 km have been computed using the finite-difference time-domain (FDTD) method in the 2-D spherical coordinate system. The return stroke is represented by the original transmission-line (TL) model (with no current decay with height and abrupt current termination at the channel top), the modified TL model with linear current decay with height (MTLL model), the modified TL model with exponential current decay with height (MTLE model), or the Hertzian dipole (HD) model. All the TL-type models include propagation delay, while in the HD model the current changes with time, but does so simultaneously in all channel sections; that is, without any propagation delay. The HD model predicts considerably higher fields than the TL-type models, if the same channel length is employed. The HD model with an unrealistically small channel length of 0.7 km yields electric field peaks similar to those produced by TL-type models with a 7-km channel. Ground waves and skywaves computed using the HD model have faster rise and decay times (shorter pulsewidths) than those computed using a TL-type model. Electric fields produced by a longer risetime current are less attenuated due to propagation over lossy ground when a TL-type model is used, while they are more attenuated if the HD model is employed. The opposite trends are related to the fact that the radiation field peak is proportional to the product of the current and propagation speed for a TL-type model, while for the HD model, it is proportional to the product of the time derivative of current and channel length. In the TL model, abrupt termination of significant current at the channel top results in the so-called mirror image (abrupt radiation field polarity change). This artifact does not occur in the framework of the MTLL model and is negligible in the MTLE model with λ = 2 km and H = 7 km.