Tech Physicists Further Explain Genesis of Lightning

SOCORRO, N.M. February 29, 2016 – A group of New Mexico Tech researchers, led by Professors Paul Krehbiel and William Rison, have published a ground-breaking paper that explains how lightning begins in storm clouds.

The team of scientists used data collected by a high-speed broadband interferometer and a three-dimensional Lightning Mapping Array, developed by the Langmuir Laboratory group at Tech. This research was funded by more than $1 million from DARPA and the National Science Foundation.

The research was published in Nature Communications on February 16, 2016. In addition to Rison and Krehbiel, other authors are Michael Stock, Harald Edens, Ronald Thomas, and Mark Stanley, all of Tech, Xuan-Min Shao of Los Alamos National Laboratory, and Yang Zhang of the Chinese Academy of Meteorological Sciences. The full paper can be viewed at http://www.nature.com/ncomms/2016/160215/ncomms10721/full/ncomms10721.html.

A long-standing question in lightning studies has been how lightning is initiated inside storms, given the absence of physical conductors. In recent years, the issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown of the air, or involves relativistic runaway electron processes.

Initially, the study focused on an elusive type of compact, high power discharges known as a narrow bipolar events, or NBEs. Observations of  lightning in a small storm close to Tech's mountain-top observatory showed that NBEs are caused by a newly-recognized type of discharge called fast positive breakdown, and revealed how the breakdown initiated intracloud lightning discharges in the storm.

“Instead of propagating upward in the storm, as would be expected of runaway electron avalanches, the breakdown propagated downward, showing that it was positive rather than negative,” Rison said. “Once we figured that out, we started looking at other flashes in the storm.  To our great surprise we found that fast positive breakdown occurred with a wide range of strengths and was the initiating event of a number of other intracloud discharges as well.”

A “eureka moment” came when they found cloud-to-ground (CG) flashes that began with fast upward positive breakdown, said Rison, rather than downward-directed breakdown of intracloud (IC) flashes.  “This fit in perfectly with the fact that CG discharges are initiated below the storm's main negative charge region, while IC flashes are initiated above the negative charge.”

“From there, we went on to investigate other types of lightning and lightning in other storms,” Krehbiel said. “Particularly interesting was the finding that brief, millisecond long discharges commonly seen in storms by the LMA were initiated in the same way as IC flashes.  We call these discharges 'precursor' events, because they often occur a few seconds before a full-fledged IC flash.  Precursors are attempted breakdown that doesn’t succeed in becoming an IC flash, and constitute the most sensitive detector of the initation process.”

Krehbiel said the genesis of a lightning flash appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region.

“The streamers are produced by corona from frozen ice crystals, according to studies dating back to the 1950s and 1960s by Tech’s Bill Winn and others. The streamers self-intensify the electric forces back at their starting point, giving rise to negative breakdown and the ensuing flash.” The paper concludes that essentially all in-cloud lightning is initiated by fast positive breakdown.

“The speed of the breakdown is unusually fast, on the order of one-tenth to one-third the speed of light, previously unheard of in air that hasn’t been pre-ionized by prior breakdown,” said Krehbiel.  “This was a major concern of one of the paper's reviewers, but has shown that Nature is the best laboratory for understanding large-scale breakdown processes.”

The Tech scientists were supported by an NSF grant titled “Lightning and Thunderstorm Studies,” conducted by Krehbiel, Rison and Thomas, and by two grants from the Defense Advanced Research Projects Agency (DARPA) NIMBUS program: One awarded to Ken Eack and the Langmuir Laboratory group and the other to Krehbiel and colleagues through Duke University. Eack’s DARPA grant was titled “Triggered Lightning: Its Effects on Thunderstorms and Use as a Means of Lightning Protection.”

DARPA launched Project NIMBUS in 2010 specifically targeted to further understanding of basic lightning processes and lightning protection issues.  Eack said Tech was selected as one of about 10 nationwide collaborative groups to participate in the four-year program. Overall, the Langmuir researchers received $2.9 million from June 2010 through December 2014 for detailed studies of triggered and natural lightning, and of lightning-induced “transient luminous events” (sprites and related phenomena) above active thunderstorms.

The NSF funding was for $745,000 and concludes its fourth and final year in July.

“In both sets of studies, the highest-ranking objective out a 'top ten' list was understanding the question of lightning initiation, followed closely by determining the mechanism of narrow bipolar events,” Krehbiel said. “While there is still work to be done, the Nature Communications study basically answers both of those questions.”

The overall funding supported development and operation of the broadband interferometer by doctoral students Michael Stock and Jeff LaPierre, and the post-doctoral studies of Harald Edens with the Langmuir LMA.

– NMT –