Correlated peak relative light intensity and peak current in triggered lightning subsequent return strokes

Abstract

We have examined the correlation between peak relative light intensity (LR) and stroke peak current (IR) for 39 subsequent return strokes in two triggered lightning flashes. One flash contained 19 strokes and the other 20 strokes for which direct measurements were available of the return stroke peak current at ground. Peak currents ranged from 1.6 to 21 kA. The measurements of peak relative light intensity were obtained from photographic streak recordings using calibrated film and microsecond resolution. Correlations, significant at better than the 0.1% level, were found for the following functional relationships: LR vs. IR, log LR vs. log IR, log LR vs. IR, and log LR vs. IR2. Although a relation between LR and IR is evident in these data, none of the analytical relations considered is clearly favored. We also examined the correlation between LR and the maximum rate of current rise (dI/dt)max but found less correlation than that between LR and IR. In addition we evaluated the peak relative intensity near ground for 22 dart leaders and found a mean ratio of peak dart leader to peak return stroke relative light intensity of 0.1 with a range of 0.02–0.23. Using two different methods, we estimated the peak current near ground in these dart leaders to range from 0.1 to 6 kA.