A time-domain formula for the horizontal electric field at the earth surface in the vicinity of lightning

This paper presents a time-domain formula for the calculation of the horizontal electric field in the vicinity of the lightning channel. The formula is based on the incident azimuthal magnetic field, so that it does not depend on the assumptions considered in the calculation of the magnetic field, such as return stroke model or propagation effects. The application of the formula with the magnetic field calculated from perfectly conducting earth provided results that agreed very well with results from Sommerfeld integrals, even in the vicinity of the lightning channel, where the Cooray-Rubinstein formula does not hold. The deviations of the results of the formula with respect to the Sommerfeld integrals are on the order of a few percent, which is the same order of magnitude as the deviations involved in the numerical evaluation of Sommerfeld integrals.