Correlation between the optical signatures and current wave forms of long sparks: applications in lightning research

Broad-band optical output (500-850 nm) and channel current of 500 mm long sparks were measured. Experiment was conducted for several current rise times. The optical rise time and current rise time show a linear relationship. During the rising portion the optical signal closely follows the current wave shape. There is no detectable variation in the time separation between the starting point of the optical signal and that of the current signal, with varying current rise time or current amplitude. One can apply this result to use the optical signal, which is measurable, to estimate the information regarding the variation of current with altitude of long spark channels. One such case, that will be benefited from the results of this research, is the measurement of peak current along the channel of both artificial and natural lightning. For a 250 mm gap we performed the experiment by increasing the current amplitude between about 0.5 kA and about 3.5 kA, for the same current rise time. The current rise time and optical rise time remain approximately at a steady level with current amplitude, while the former is always slightly less than the latter. Hence, we suggest that the results obtained in this work may be extended to the measurements of the current along lightning channels where the peak currents are in the range of few tens of kiloamperes. For a given current pulse half-width, that of the optical pulse shows a linear variation with current amplitude. We also observed a linear relationship between the current amplitude and the optical amplitude in the range of current amplitudes relevant to this experiment. (C) 1998 Elsevier Science B.V. All rights reserved.