Abstract
Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown. Recent discoveries, such as sprites, jets, terrestrial gamma ray flashes, and fast positive breakdown, highlight the diversity of complex phenomena that thunderstorms can produce, and point to the possibility for electrical breakdown/discharge mechanisms beyond dielectric breakdown theory based mainly on laboratory experiments. Here we present one such confounding discovery, termed fast negative breakdown, that does not fit with our current understanding of dielectric breakdown. Our adaptation of radio astronomy imaging techniques to study extremely transient lightning-associated events confirms that electrical breakdown in thunderstorms can begin with oppositely-directed fast breakdown of negative polarity, similar and in addition to fast positive breakdown expected from conventional dielectric theory and recent observations. The discovery of fast negative breakdown calls for an addendum to the physical description of electrical discharges in air.
Highlights
Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown
The observations include three-dimensional lightning mapping array (LMA) data[5,26] from the 10-station Kennedy Space Center Lightning Mapping Array (KSCLMA), very high frequency (VHF) waveforms recorded by the New Mexico Tech Broadband Interferometer[27] (INTF), and waveforms recorded by a flat-plate Fast Antenna (FA) that measured the change in vertical electric field at the ground with a 100-μs decay constant
Though dart leaders[40,41,42] and K-processes[43,44] both constitute negative-polarity breakdown events that can reach speeds in excess of 107 m s−1, they occur along a path preconditioned by preceding discharges, whereas fast negative breakdown appears to occur in virgin air
Summary
Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown. Rison et al.[8] recently discovered that NBEs are generated by a newly recognized process, termed fast positive breakdown, and concluded that many or possibly all lightning flashes are initiated by fast positive breakdown This finding is consistent with a lightning initiation theory proposed several decades ago[9,10,11], which hypothesized that lightning is initiated by a succession of positive streamers, resolving the conundrum that measured thunderstorm electric fields are well below the dielectric breakdown threshold[12,13,14,15]. The Florida storms were found to be prolific generators[25] of high-power 30–50 dBW (1–100 kW) NBEs, some of which were caused by fast breakdown that propagated in the opposite direction of located fast positive events, which were downward-directed In both cases, and in a number of different storms, the discharges in question occurred between the mid-level negative and upper positive charge regions of the storms (e.g., Fig. 1), typical of intra-cloud lightning.
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