Fine progression features of return stroke luminosity at the bottom of rocket-triggered lightning channels

We have performed a study on the luminosity waveforms of 13 return strokes (RSs) in 4 rocket triggered lightning flashes recorded by a high-speed optical imaging system LAPOS5 with a time resolution of 35 ns and a spatial resolution of about 2 m. It was found that as the return strokes propagate upward over the bottom 10 m to 26 m of the lightning channels, which is above their initiation heights, on average the RSs optical waveforms decrease their peaks by 41%, lengthen their 20-90% rise times and 100-80% decay times, and change the relative times of the fastest rising points by 0.38 μs, 0.51 μs, and 0.15 μs. We have also estimated the propagation speeds for all the return strokes by using their 20% peak light intensity points and fastest rising points as reference points for time difference measurements, respectively. It was found that the average RS speed obtained with the 20% peak light intensity points (1.11× 108 m/s) is about twice that obtained with the fastest rising points (0.67× 108 m/s). In addition, we found that for different RSs in an individual lightning flash, the RS with faster propagation speed tends to have a less distorted optical waveform at the higher height.