A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.

[1] Atmospheric electric field change (sferic) waveforms were detected at Los Alamos Sferic Array stations in association with terrestrial gamma-ray flashes (TGFs). Five TGF sferic waveforms detected at sufficiently close range were all found to be consistent with a positive-polarity intracloud (+IC) discharge process which transported electrons upward. The amplitudes of the events were among the top 5% of IC discharge flashes. Altitudes obtained from ionosphere reflections for two of the closer events were found to be 13.6 km and 11.5 km. These altitudes are lower than expected if one assumes that the sferic was near the source of the gamma-rays. One of the sferics was an energetic narrow bipolar event which occurred near the inferred onset of a flash, suggesting that the preceding TGF may correspond to the actual onset.

(2014), Continuous broadband digital interferometry of lightning using a generalized cross-correlation algorithm, J. Geophys. Res. Atmos., 119, 3134-3165, doi:10.1002 MHz signals received at three orthogonally located antennas are continuously digitized over multisecond intervals, as opposed to sequences of short-duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When postprocessed using cross correlation, the measurements provide angular uncertainties less than 1 • and time resolution better than 1 μs. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000-80,000 radiation sources are located for a typical lightning flash. In this study, two-dimensional interferometer observations of a classic bilevel intracloud flash are presented and combined with three-dimensional Lightning Mapping Array observations to produce a quasi 3-D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3-D progression speed of negative leaders associated with intracloud K-leaders.

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.