Expanding the modified transmission line model to account for reflections within the continuously growing lightning return stroke channel

Computation of the EM fields associated with lightning return-stroke (RS) currents of flashes to a tall structure is based on the modeling of the current distribution along both the tall structure and the lightning RS channel. For the determination of the current distribution, the existing modified transmission line model is adopted here and expanded. This advanced transmission line (ATL) model accounts not only for multiple current reflections within the tall structure, but also for those within the continuously growing RS channel. Current waves reflected from structural discontinuities within the struck object are refracted into the lightning RS channel. These refracted current waves travel in the lightning channel with almost the speed of light and therefore experience reflections at the RS channel front. To simulate the current distribution along the lightning path, the input parameters of first and subsequent return strokes are derived from measurement of lightning strokes to the CN Tower. In the paper, the azimuthal magnetic and the vertical electric field components associated with lightning strikes to the CN Tower are computed for a number of conditions: (a) different wavefront parameters characterizing the first and the subsequent RS current pulses; (b) two RS front velocities; and (c) several values of the channel current decay factor. Resultant field waveforms of first and subsequent return strokes and the ratios of their peak values are discussed and compared with actual lightning fields measured 2 km north of the CN Tower.