Chapter 23 - Influence de la Repartition du Courant Dans L'Eclair Sur Son Rayonnement a Grande Distance

Abstract

In considering the relation between the current in a lightning discharge and the magnetic field recorded at a short distance, using the classical laws of the dipole field one finds a considerable difference in amplitude as well as form. It is possible to explain these differences by the mechanism of the lightning discharge, which creates a non-uniform current distribution along the channel. The dipole laws, however, suppose a constant distribution. In the present study the author tries to predict the influence of the non-uniform distribution on the field at great distance (but less than 1000 km). It is shown, in particular, that the influence on the spectrum of the field is such that it is not solely a function of the derivative of the current but is the sum of three spectra, produced respectively by the three terms describing the non-uniform distribution. The coefficients of these terms depend on the local characteristics of the discharge. Calculating the field at a distance using a non-uniform current distribution, one obtains spectra with the experimentally recorded wave forms of the received field, which are not explained by the classical (uniform current distribution) theory which results in a bell-shaped spectrum having a maximum around 8 or 10 kc/s, according to the rise time of the current.