Runaway Discharges and Their Roles in Atmospheric Processes

人们正在对失控击穿及其在闪电的产生和传播中的作用进行理论、观测和实验研究。尽管近年来在理论方面取得了很大的进展，但对失控破坏机制仍存在很大的分歧。目前，该理论正处于一个大变动时期，新观点层出不穷，争论也很激烈。这本书的学术价值在于它对这场辩论的贡献。研究失控击穿的最佳观测方法是测量放电产生的x射线，这种放电是由失控电子与空气中的中性分子碰撞时产生的。x射线辐射可以比失控的电子本身传播更大的距离，并允许远程观察失控的击穿过程。在美国国家科学基金会/核磁共振基金的资助下，我们的团队在佛罗里达州布兰丁营的佛罗里达大学/佛罗里达理工学院国际闪电研究与测试中心（ICLRT）完成了雷暴高能辐射阵列（TERA）。TERA的设计目的是对自然和火箭引发的雷暴闪电的x射线和伽马射线发射进行详细测量。该项目支持对目前获得的大量x射线数据的分析，利用已经为MRI奖完成的工作，这是一项不资助科学研究的仪器拨款。这项工作使理论有充分的观察基础。对闪电发出的x射线的观测继续为闪电现象提供新的见解。例如，他们已经证明了在云对地闪电的第一次回击之前出现的阶跃前导和在同一闪电中随后的回击之前出现的飞镖前导之间存在更深层次的联系。这推进了以前从光学和现场测量中推断出来的知识。利用ICLRT的仪器，我们的小组最近发现，实验室火花也会产生x射线爆发，这与从自然闪电和触发闪电中观察到的x射线发射非常相似。这一发现使人们第一次能够在实验室里研究x射线在击穿过程中是如何发射的。作为当前项目的一部分，这项实验室工作的继续为闪电先导的步进过程提供了重要的线索，因为在这些步进的形成过程中发现了x射线。模拟击穿过程的蒙特卡罗模拟和有限差分代码用于生成模拟的x射线和伽马射线能谱，然后将其与x射线和伽马射线能量观测结果进行比较。这一比较为目前的失控击穿理论提供了具体的检验。该理论还为实验提供了指导，帮助解释实验，并阐明了涉及击穿的重要数量。这项研究通过研究以前很少探索的闪电特性具有更广泛的影响，使该领域取得重大进展成为可能。在更广泛的社会中，对失控的崩溃存在着大量的困惑和许多误解；其结果是，失控崩溃经常被用来解释一些可能与机制无关的观察结果。本研究将为社区提供可用于失控崩溃的定量测量方法。这项研究直接有利于雷暴、闪电和实验室火花的研究，并推动了等离子体物理新领域的探索。通过提高我们对雷暴和闪电的物理过程的理解，这样的研究可能最终导致改进闪电预测和闪电安全。

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.