10. The Return Stroke – How to Model It

To calculate the electromagnetic fields generated by return strokes, it is necessary to know the spatial and temporal variation of the return-stroke current along the channel. Unfortunately, only the return-stroke velocity and the current generated at the channel base can be measured directly, and the way in which the return-stroke current varies along the channel must be extracted indirectly. For example, the magnitude of a current and its wave shape at different heights can in principle be extracted by studying the optical radiation. Unfortunately, the exact relationship between the return-stroke-generated optical radiation and the return-stroke current parameters are not known, and therefore only qualitative inferences can be made on the return-stroke current. The information thus obtained indicates that the return-stroke peak current decreases with height while the rise time of the current increases.