Current and electromagnetic field associated with lightning-return strokesto tall towers

In this paper, an analysis of electric and magnetic fields radiated by ligh tning first and subsequent return strokes to tall towers is presented. The contributions of the various components of the fields, namely, static, indu ction, and radiation for the electric field, and induction and radiation fo r the magnetic field are illustrated and discussed. It is shown in particul ar that the presence of a tower tends, in general, to increase substantiall y the electric and magnetic field peaks and their derivatives. This increas e is mainly caused by the presence of two oppositely propagating current wa vefronts originating from the tower top and by the very high-propagation ve locity of current pulses within the tower (practically at the speed of ligh t), and depends essentially on the wavefront steepness of the channel-base current. Because of the last factor, the increase of the field magnitudes i s found to be significantly higher for subsequent return strokes, which are characterized by much faster risetimes compared to first return strokes. F urthermore, the presented results are shown to be consistent with recent ex perimental observations of current in lightning strokes to the Toronto CN T ower and of the associated electric and magnetic fields measured 2 km away. These findings partially explain the fact that subsequent return strokes c haracterized by lower current peaks but higher front steepnesses and return stroke speeds may result in higher field peaks. The results obtained in th is study have important implications in electromagnetic compatibility. It i s found that lightning strikes to tall metallic objects (towers, rods, etc. ) lead to increased electromagnetic field disturbances. Also, subsequent re turn strokes are to be considered an even more important source of electrom agnetic interferences than first return strokes. Indeed, electromagnetic fi elds from subsequent strokes are characterized by faster fronts and additio nally, they may reach greater peaks than first strokes. Lastly, findings of this study emphasize the difficulty of extracting reliable lightning retur n stroke current information from remote electromagnetic field measurements using oversimplified formulae.