TABLE OF CONTENTS Rajeev Thottappillil
Electromagnetic fields from lightning can couple to electrical systems and produce transient overvoltages, which can cause power and telecommunication outages and destruction of electronics. Therefore calculation of the electric and magnetic fields from different lightning processes has practical applications. In this Chapter expressions for electric and magnetic fields are derived for some simplified charge and current configurations applicable to lightning. In general, lightning currents and charges vary with time. First, simple expressions for nontime-varying cases are presented, then electric and magnetic field expressions from time-varying lightning sources are given.
Measurements indicate that thunderclouds have a bipolar structure with a lower negative charge region and an upper positive charge region. Therefore as far as the slowly varying fields at ground level are concerned, the thundercloud can be approximately modelled as a dipole. The charge structure of thunderclouds has been dealt with in Chapter 1. For simplicity we can assume a spherically symmetric distribution of charges for negatively and positively charged regions of the thundercloud, even though the charge distribution is more extended horizontally than vertically. The charge centres are separated by a distance h. The earth is assumed to be flat and a perfect conductor. We will now see how the static electric field at ground level will look as a function of distance from the thundercloud. The geometry of the problem is shown in Figure 5.1. The influence of ground on the fields is taken into...
