Fast negative breakdown in thunderstorms

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.

Indirectly Measured Ambient Electric Fields for Lightning Initiation in Fast Breakdown Regions

“…Requiring a reasonable width for the fast breakdown region and an ambient field not too far above thus points to a “zone of plausibility”, as shown in the right panel of Figure , with ambient field near and a fast breakdown region width of roughly 100–300 meters for these two NBEs of approximately 500 meters length. Interferometric measurements of fast breakdown reported by Rison et al () and Tilles et al () seem to suggest a narrow fast breakdown region with an aspect ratio , which according to this analysis would imply extremely high ambient electric fields. However, by estimating single‐point source locations over time windows of several microseconds that average many thousands (at least) of individual streamers, these measurements only provide the centroid of the VHF emission region.…”

Griffiths and Phelps Lightning Initiation Model, Revisited

“…The chosen speed, 3×10 7 m/s, is consistent with reported velocities of fast positive (Rison et al, ) and negative (Tilles et al, ) breakdown in thunderstorms. Rison et al () registered speeds typically in the range of 3–23×10 7 m/s for positive‐polarity events observed in a New Mexico storm, roughly 9–10 km above mean sea level.…”

“…In this work, for the sake of simplicity, we assume that the streamer system propagates at a constant velocity. This is a reasonable assumption according to the observations, which show a roughly linear relationship between height and time during the first 5–15 μs of propagation, when the sources are more intense and the vertical motion can be determined with great accuracy (see, e.g., Rison et al, , Figures 1–2; Tilles et al, , Figures 2–4). In reality, it is expected that the streamer system velocity depends on the electric field at the moving front E , in a similar manner that individual streamers do, but the functional form of v ( E ) is not known.…”

“…Figure shows detailed results for the first pass. The temporal dependence is added to the model by assuming that the streamer system is propagating at a constant velocity of 3×10 7 m/s, consistent with measured velocities of fast positive (Rison et al, ) and negative (Tilles et al, ) breakdown and sprite streamers (Stenbaek‐Nielsen et al, ; see section for further discussion). In this section, the velocity is merely used to illustrate how the streamer corona discharge properties change over time.…”

“…The contrasting difference suggests that long streamers (i.e., with great longitudinal extent in comparison to its diameter) tend to be in the faster end of the spectrum. The discharges reported by Rison et al (2016) and Tilles et al (2019) appear to be the fastest manifestation of streamer breakdown in the Earth's atmosphere; the underlying reason remains unknown.…”