The Influence of Channel Geometry on VLF/VHF Radiations from Lightning Return Strokes

The change in propagation direction of the lightning current causedby the channel geometry (branch or tortuosity) may introduce subsidiary peaks into lightning return stroke waveformsand also alter its VHF radiation. By using the transmission\|line\|mode the influence of channel geometry on VLF/VHF radiation from lightning return strokes is presented. It is indicated that the strength of the subsidiary peak depends on the length and orientation of the channel geometry filaments. Among those heavily vertically\|oriented filaments the large one (with the length of 100 m in scale) tends to produce subsidiary peak comparative to return strokes in strength and the small one (with the length of 1 m in scale or less) can only produce peaks weaker than 10% of return strokes, while the horizontally\|oriented filament seems to produce no distinguishable subsidiary peaks. The average lengths of all channel branch or turtuosity filaments(ALF) are among the important channel geometry parameters and the intensity of the subsidiary peak and the interval time between successive peaks strongly relate to this parameter. In frequency domain the channel geometry can add to by a factor in maximum the high frequency energy of lightning return stroke radiation at the wavelength approximate 8 times ALF. However, this high frequency component is a factor weaker than the measured VHF radiation strength during lightning return strokes, strongly indicating that the channel branch or tortuosity is not the main source of VHF radiation from lightning return strokes.