The optical and radiation field signatures produced by lightning return strokes

The optical signals radiated by Florida lightning in the 0.4- to 1.1-μm wavelength interval have been recorded in correlation with wide-band electric field signatures. The initial light signal from a return stroke tends to be linear for about 15 μs and then rises more slowly to a peak that is delayed by about 60 μs from the electric field peak. The transition between the fast linear portion and the slower rise may be due to the return stroke entering the cloud base. A small percentage of the records indicate that two different branches of the same stepped leader can initiate separate return strokes. The light pulses from cloud discharges tend to be smaller and more slowly varying than those from return strokes. The total optical power radiated by first strokes in the 5- to 35-km range has a mean and standard deviation of 2.3±1.8×109 W at peak. Normal subsequent strokes produce 4.8±3.6×108 W at peak, and subsequent strokes preceded by a dart-stepped leader produce 5.4±2.2×108 W. The characteristic widths of 23 subsequent stroke signals range from 103 to 235 μs, with a mean and standard deviation of 158±33 μs. Analyses of the initial linear slopes of the light signals suggest that the space- and time-averaged radiance of first strokes is about 1.0±0.9×106 W/m during the bright phase and that normal and dart-stepped subsequent strokes produce about 2.5±1.8×105 W/m and 4.3±3.1×105 W/m, respectively. Further analyses suggest that the dependence of the average radiance on the peak electric field, and probably the peak current, is neither linear nor quadratic.