Determination of the Wave Front of Ligktning Stroke Currents from Field Measurements

The very nature of the return stroke is that of a traveling wave. It is felt, therefore, that the interpretation of field measurements from this phase of the stroke will be facilitated by expressions based upon traveling waves rather than upon electric moments. Expressions of this nature have been derived and compared with corresponding expressions derived from Lejay´s moment equation. The wave expressions are more rigorously correct and have been used to indicate the limitations of the moment expression. The field measurements due to the return stroke, when these measurements are made remote from the stroke, are essentially responsive to current effects only and not to charges. The return stroke has been visualized as beginning from the earth. The velocity of the highly luminous head of the upward plasma channel is assumed to start at a low value and to increase exponentially with time until union occurs with the downward plasma channel. At union the velocity of the upward plasma channel is of the order of 12% of that of light. Then for purposes of calculation it is assumed to be constant thereafter. The current in the upward plasma channel at any instant is constant throughout its length behind the head as it proceeds upward and like the velocity also increases exponentially with time until union occurs, after which it is constant with time and distance.