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，雷暴可以作为粒子加速器，产生不同形式的强烈高能辐射，例如：从下降的闪电通道发射的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光（TGFs），以及以闪电结束的一分钟长的伽马射线发光。所有这些现象都是所谓的失控电子的轫致辐射的不同表现，尽管与空气分子发生碰撞，这些电子仍被加速到高能量。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）可能发生在强电场存在的闪电通道尖端，但近年来，关于详细物理及其含义的一些问题一直困扰着研究人员。虽然失控电子在大气电中的关键作用已被认识到，但到目前为止，失控电子是否影响闪电和实验室放电的传播仍未得到解决。目前还不清楚闪电引子释放的失控电子是否能产生tgf。在某些情况下，在地面上观察到x射线的发射，并与下降雷锋通道尖端的失控电子有关。但在其他一些情况下，会检测到强大的地面tgf。什么样的闪电放电会产生更高能的伽马射线（而不是x射线），以及这种机制是否涉及热失控电子的产生，这仍然是一个谜。该项目的主要目标是通过一种强大的方法来推断闪电和实验室放电释放的失控电子的通量和光谱能量分布，从而解决这一知识差距。这是一种三步法，包括：(1)测量实验室短时间放电产生的x射线辐射，这种放电有效地产生失控电子，是可重复的，并且电学性质可以控制；(2)开发可扩展的蒙特卡罗模拟代码，揭示在第(1)步收集的丰富数据集驱动/验证时失控电子的通量和光谱分布；(3)在Langmuir实验室山顶设施进行x射线观测，利用从步骤(1)-(2)中获得的知识来推断自然闪电发射的失控电子的性质。此外，该项目还有一个教育目的——研究团队将开发一个新生水平的课堂模块，通过演示放电来教授空气中电气故障的基本概念。本项目由美国国家科学基金会物理与动态气象学计划和促进竞争研究的既定计划（EPSCoR）共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.