Unraveling the Runaway Electron Distribution Emitted by Lightning and Laboratory Discharges

大气物理学中最令人着迷的发现之一是雷暴可能起到粒子加速器的作用，产生强烈的高能辐射，这些辐射有不同的形式，例如：从下降的闪电通道发出的X射线闪光，在卫星高度观测到的伽马射线爆发，称为地面伽马射线闪电(TGFs)，以及以闪电终止的一分钟长的伽马射线光芒。所有这些现象都是所谓的逃逸电子韧致辐射的不同表现，尽管与空气分子碰撞，这些逃逸电子仍被加速到高能。从理论上讲，电子加速到失控能量(通过所谓的热失控电子机制)可能发生在存在强电场的闪电通道的尖端，但近年来，有关详细物理及其含义的一些问题一直困扰着研究人员。虽然逃逸电子在大气电学中的关键作用已经被认识到，但到目前为止，逃逸电子是否影响闪电的传播和实验室放电仍然没有定论。目前还不清楚闪电先导发出的逃逸电子是否可以为TGFs播种。在某些情况下，X射线辐射是在地面上观测到的，并与下行闪电先导通道尖端的逃逸电子相关联。但在其他一些情况下，探测到的是强大的地面TGFs。什么样的闪电放电会产生能量更高的伽马射线(而不是X射线)，以及这种机制是否涉及热失控电子的产生，仍然是一个谜。项目的主要目标是用一种强有力的方法来解决这一知识差距，以推断闪电和实验室放电发出的逃逸电子的通量和光谱能量分布。这是一种分三步进行的方法，涉及：(1)测量实验室短放电的X射线辐射，这种放电有效地产生逃逸电子，具有可重复性，并且电学性质可受控制；(2)开发可伸缩的蒙特卡罗模拟程序，可揭示在步骤(1)中收集的丰富数据集驱动/验证逃逸电子时的通量和光谱分布；(3)在朗缪尔实验室山顶设施进行X射线观测，利用步骤(1)-(2)中获得的知识推断天然闪电发出的逃逸电子的性质。此外，该项目还有一个教育目标-研究团队将开发一个新生级别的课堂模块，通过演示电子放电来教授空气中电击穿的基本概念。该项目由NSF物理和动态气象学计划和既定的激励竞争研究计划(EPSCoR)联合资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

One of the most fascinating discoveries in atmospheric physics is that thunderstorms may work as particle accelerators, producing intense fluxes of energetic radiation, which take different forms, such as: X-ray flashes emitted from the descending lightning channels, bursts of gamma rays observed at satellite altitudes known as terrestrial gamma-ray flashes (TGFs), and minute-long gamma-ray glows that terminate with a lightning bolt. All of these phenomena are different manifestations of bremsstrahlung emissions of the so-called runaway electrons, which are accelerated to high energies despite the collisions with air molecules. It is theoretically plausible that electron acceleration up to runaway energies (via the so-called thermal runaway electron mechanism) can happen at the tips of lightning channels where strong electric fields exist, but a number of questions about the detailed physics and its implications have puzzled researchers in recent years. Although the key role of runaway electrons in atmospheric electricity has been recognized, to date it remains unsettled whether runaway electrons influence the propagation of lightning and laboratory discharges. It also remains unclear if runaway electrons emitted by lightning leaders can seed TGFs. In some instances, X-ray emissions are observed on the ground and correlated to runaway electrons at the tips of descending lightning leader channels. But in some other occasions, powerful ground TGFs are detected instead. It remains a mystery what kind of lightning discharges produces the more energetic gamma emissions (instead of X-rays) and whether the mechanism involves thermal runaway electron generation. The main project goal is to address this knowledge gap with a robust methodology to infer the flux and spectral energy distribution of runaway electrons emitted by lightning and laboratory discharges. This is a 3-step approach that involves: (1) measuring X-ray emissions from short laboratory discharges that efficiently produce runaway electrons, are repeatable, and the electrical properties can be controlled; (2) developing scalable Monte Carlo simulation codes that can unveil the flux and spectral distribution of runaway electrons when driven/validated by the rich dataset collected in step (1); and (3) performing X-ray observations at the Langmuir laboratory mountain-top facility, leveraging the knowledge acquired from steps (1)-(2) to infer the properties of runaway electrons emitted by natural lightning. Additionally, the project has also an educational aim - the research team will develop a freshman-level classroom module to teach basic concepts of electrical breakdown in air via demonstrations with electrical discharges.This project is jointly funded by NSF Physical and Dynamic Meteorology program and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.