On the zero crossing of distant electromagnetic fields radiated by lightning

We discuss the reasons why some return stroke models do not reproduce one of the characteristic features of the electromagnetic fields radiated by lightning, namely the far-field inversion of polarity with a zero crossing occurring in the tens of microseconds range. A property of time-domain radiation requires that far fields predicted by lightning return stroke channel models exhibit a zero crossing as long as the duration of the return stroke current and the height of the channel are finite. However, many of the available models predict the zero crossing to occur at times that fall well beyond those observed experimentally. Three mechanisms responsible for the time of occurrence of the reversal of polarity in the far fields are identified, namely (1) the current attenuation along the channel, (2) the width of the return stroke current and (3) the return stroke speed. An analysis of the MTLL and MTLE return-stroke models shows that the higher the attenuation of the current along the channel, the earlier the polarity reversal of the vertical electric field. Also, for a given value of the attenuation factor, higher propagation speeds correspond to earlier polarity reversal times. For the TCS model, in which the only adjustable parameter is the return-stroke speed, we show that the far-field zero crossing occurs considerably later than the values predicted by both the MTLE and the MTLL models. This is shown to be essentially due to the fact that the decrease of the current wave along the channel according to the TCS model is less pronounced than the current attenuation predicted by the MTLE and MTLL models.