Positive and Negative Lightning Leaders Imaged in UV and Visible Ranges

We examined three descending positive leaders at distances of 5-11 km and three descending negative leaders at distances of 6-7 km, all simultaneously imaged by high-speed framing cameras operating in the visible and UV ranges. UV images (290-370 nm) of the positive leaders each exhibited a strong embellishment at the lower channel end, which was not observed in the corresponding visible images (480-800 nm). In contrast, none of the negative leaders exhibited channel embellishment in the UV range and their morphology in UV was similar to that in the visible. Additionally, no embellishment was seen in four negative leaders imaged in UV only. The observed UV embellishment, which is likely to be the streamer zone at the positive-leader tip, appeared to undergo expansion-contraction cycles. We attributed the lack of detectable streamer-zone emission in the UV range in negative leaders to a much lower streamer generation rate compared to positive leaders.