Runaway Discharges and Their Roles in Atmospheric Processes

Florida Institute Of Technology, Melbourne FL

Investigators

Abstract

Theoretical, observational and experimental investigations are being conducted on runaway breakdown and its role in the initiation and propagation of lightning. Despite the great theoretical progress made in recent years, there remain large disagreements about the runaway breakdown mechanism. Currently the theory is undergoing a period of great flux with many new ideas emerging and a large amount of debate. The intellectual merit of this work is its contributions to this debate. The best observational method for studying runaway breakdown is measuring x-ray emission from electrical discharges, produced by runaway electrons as they collide with neutral molecules in air. The x-ray emissions can travel over much larger distances than the runaway electrons themselves, and allow remote observations of the runaway breakdown process. With funding from a NSF/MRI grant, our group completed the Thunderstorm Energetic Radiation Array (TERA) at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. TERA is designed to make detailed measurements of x-ray and gamma-ray emission from natural and rocket-triggered lightning from thunderstorms. This project supports the analysis of the large sets of x-ray data now being acquired, leveraging the work already done for the MRI award, which is an instrumentation grant that does not fund scientific research. This work keeps the theory well grounded in observations. Observations of X-rays from lightning continue to provide new insight into lightning phenomena. For example, they have already demonstrated a deeper connection between stepped leaders that occur before the first return stroke of a cloud-to-ground lightning flash, and dart leaders that occur prior to subsequent return strokes in the same flash. This advances knowledge previously inferred from optical and field measurements. Using instruments from the ICLRT, our group recently discovered that laboratory sparks also produce x-ray bursts very similar to the x-ray emission observed from natural and triggered lightning. This discovery allows for the first time laboratory study of how x-rays are emitted during the breakdown process. The continuation of this laboratory work as part of this current project is providing important clues about the stepping process that occurs in lightning leaders, since x-rays are found to be produced during the formation of these steps. Monte Carlo simulations and finite difference codes simulating the breakdown process are used to produce simulated x-ray and gamma-ray spectra, which then are compared to x-ray and gamma-ray energy observations. This comparison provides concrete tests of the present runaway breakdown theory. The theory also provides guidance to the experiments, aiding in their interpretation and illuminating the important quantities involved in breakdown. This research has broader impacts through study of previously little-explored properties of lightning, making possible significant advancements in this field. There exists a large amount of confusion and many misconceptions about runaway breakdown in the broader community; with the result that runaway breakdown is frequently being used with little discretion for explaining a number of observations that probably have little to do with the mechanism. This research will provide quantitative measures of runaway breakdown that can be used by the community. This research directly benefits the study of thunderstorms, lightning and laboratory sparks and advances exploration of a new area of plasma physics. By improving our understanding of the physical processes involved in thunderstorms and lightning, such research may eventually result in improved lightning predictions and lightning safety.