Data from: Lightning Initiation from Fast Negative Breakdown is Led by Positive Polarity Dominated Streamers

Evidence of positive polarity dominated streamers preceding fast negative breakdown and of simultaneous positive and negative polarity streamer development in lightning initiation is reported. Observations of lightning initiation in the form of fast negative breakdown have remained a puzzle since their discovery, because simulations of lightning initiation have shown that negative streamers are not produced in virgin air without simultaneous positive streamers. Here, high bandwidth (>200 MHz) and fast time resolution (<0.5 microseconds) very high frequency (VHF) interferometry is used to examine the detailed development of fast breakdown in the first several microseconds of the event. We find downward motion of the VHF emissions for the first 1-2 microseconds, even in events that quickly become dominated by negative breakdown. This indicates that positive streamer development forms first or at least simultaneously with negative streamers. Further evidence for this picture comes from observations of mixed fast breakdown, which begins and ends as fast positive breakdown but is interrupted by a burst of upward-moving negative streamer emission. The overall trajectory of the positive breakdown during such mixed events indicates that the positive streamers continuously propagate downward even during the burst of strong negative breakdown. These observations indicate that even when negative streamers dominate the overall VHF emissions, both positive and negative streamers are likely propagating simultaneously from the initiation point. These findings on the structure and dynamics of fast breakdown provide key new insight to our understanding of lightning initiation.