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

Abstract:

Observations of positive polarity dominated streamers preceding fast negative breakdown in lightning initiation is reported for the first time. 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 cannot be produced in virgin air without simultaneous positive streamers. Here, high bandwidth (>200 MHz) and fast time resolution (<0.5 microseconds) VHF interferometry is used to examine the detailed development of fast breakdown in the first several microseconds of the event. We find that positive streamer development always forms first, even in events that quickly become dominated by negative breakdown. In addition, we report and analyze a type of mixed fast breakdown, which begins and ends as fast positive breakdown but is interrupted by a burst of negative streamer emission in the middle. The overall trajectory of the positive breakdown during such mixed events indicates that it continues propagating even during the burst of strong negative breakdown. This finding strongly indicates that when negative streamers dominate the overall VHF emissions, both positive and negative streamers are propagating simultaneously from the initiation point. These observations of fast breakdown provide key new insight to our understanding of dielectric discharge in the thunderstorm and rendering crucially observation to substantiate the simulation results of dielectric discharges.