Tortuosity of Lightning Current Arc Channel and the Resultant Distortion of Luminosity Waveform

Light emissions in two optical bands are simultaneously observed from arc discharges produced by three laboratory impulse current generators, representing three current components (return stroke, $M$ component, and continuing current) of lightning, applied across an air gap between two round graphite electrodes in a light-tight environment. The results demonstrate that if a tortuosity of the discharge channel did occur, the luminosity waveforms would exhibit distortions. However, these luminosity distortions, often found in naturally-occurred lightning arcs, previously attributed to the spectroscopic wavelength transfer or the increase of neutral spectral lines, are hardly observed in a normal straight laboratory-generated lightning arc, confirming that the distortions in luminosity waveform should be attributed to the tortuosity of lightning channel. The distortions of luminosity waveforms, corresponding to three types of simulated lightning-current components, are featured with various fine structures, which have also been observed in other investigations for electromagnetic fields and acoustic signatures, if a tortuous channel is involved.