On the influence of elevated strike objects on directly measured and indirectly estimated lightning currents

The knowledge of lightning current parameters comes essentially from direct measurements obtained using instrumented towers or triggered lightning, and from measurements of lightning electromagnetic fields assuming empirical or theoretical relations between fields and currents (indirect estimation). In this paper, the effects of an elevated strike object on both the direct and indirect estimation of the current parameters is examined. We first give the expressions which allow for what we call the 'decontamination' of measured lightning currents from the tower reflections, assuming the tower as a perfectly conducting strike object. We show that even though the reflections might not be discernible in experimental recordings for tower heights as those used in Berger ct al's and Garbagnati and Lo Piparo's investigations, the peak current might be significantly;affected. Then, we extend the Transmission Line (TL) return-stroke model of Uman and McLain, by taking into account the presence of an elevated strike object and the multiple reflections at the bottom and at the top of it. In general, radiated fields calculated considering and neglecting the presence of the strike object differ. It is shown that relatively short strike objects, such as:triggered lightning launching structures, may not appreciably affect the calculated field peak values and overall wave shapes, but could affect the field derivatives. This has to be taken into account when current parameters are inferred from measured fields or field derivatives. Our extension of the TIL model is consistent with the differences in the return-stroke velocities found by other authors when inferring such a parameter from measured fields and currents or from measured current and field derivatives. Our results suggest that all statistics obtained so far on lightning currents may have been 'affected' by the presence of the towers and may have also an impact on the lightning locating system industry.