Energetic Radiation from Lightning Leaders: Effects and Origins

闪电最初被触发并在自由大气中传播的确切机制，以及这些过程与最近出现的与活跃雷暴有关的高能排放的关系，都没有得到很好的理解。这项研究将应用最先进的数值模拟方法，研究闪电相关的电荷流和阶梯先导放电发出的高能电子，以及相关的高能辐射(从X射线到硬X射线和地面伽马射线闪光)在地球大气层中的传播。唯一已知的能够解释相关高能电子产生的机制是“失控过程”，即从外加电场获得的能量超过碰撞所损失的能量，从而将电子加速到非常高的能量(1 MeV)。到目前为止，已经提出了两种方案来“播种”这种失控的崩溃过程：(1)宇宙射线，和(2)由极高的电场诱导的“热”失控。理论和实验研究表明，流光和先导放电动力学的耦合对热失控过程至关重要，而热失控过程现在被认为与闪电发射X射线或产生地面伽马射线闪光密不可分。最近从地面、机载和卫星平台获得了对雷暴和相关电气活动产生的高能辐射发射的有针对性的观测，但这些以前不相关的现象之间的关系的详细性质以前没有像本项目开始时那样被量化。这项活动的智力价值是努力为观测到的源自地球大气层的高能辐射的位置、时间和具体性质制定一个统一的理论上合理的解释。需要量化闪电发出的辐射强度并了解其与闪电通道的形成和传播的关系，从而强调了相关问题的重要性，并将其扩展到对过境雷暴活动强烈地区的机组人员和乘客的健康造成不利影响的可能性。这一努力的更广泛影响将来自于跨学科研究计划的发展，通过研究生和本科生的教育和培训，以及通过使用富有启发性的图形方法传播结果。

Neither the exact mechanisms through which lighting is initially triggered and propagates through the free atmosphere nor the relationship of these processes to recently emerging observations of high-energy emissions associated with active thunderstorms are well understood. This research will apply state-of-the-art numerical simulation methods to investigate the emission of energetic electrons from lightning-related charge-streamer and stepped-leader discharges and the propagation of the associated energetic radiation (from X-rays to hard X-rays and terrestrial gamma-ray flashes) through the Earth's atmosphere. The only known mechanism capable of explaining generation of associated high-energy electrons is the "runaway process" by which energy gained from an imposed electric field exceeds that lost to collisions, thus accelerating electrons to very high energies (1 MeV). To-date, two scenarios for initially "seeding" this runaway breakdown process have been proposed: (1) cosmic rays, and (2) "thermal" runaway induced by extremely high electric fields. Theoretical and experimental studies have shown the paramount importance of the coupling of streamer and leader discharge dynamics to the thermal runaway process, which is in-turn now thought to be inseparable from emission of X-rays by lightning or production of terrestrial gamma-ray flashes. Targeted observations of high-energy radiative emissions by thunderstorms and associated electrical activity have recently been obtained from ground-based, airborne and satellite platforms, but the detailed nature of relationships among these previously uncorrelated phenomena have not previously been quantified as this project sets out to do.The Intellectual Merit of this activity is defined by efforts to develop a unifying theoretically-sound explanation for the location, timing and specific nature of observed high-energy radiation originating in the Earth's atmosphere. The importance of related questions is emphasized by the need to quantify the intensity of radiation emitted from lightning and to understand its relationship to the formation and propagation of lightning channels, and extends to the possibility of adverse health impacts for crewmembers and passengers of aircraft transiting regions of strong thunderstorm activity. Broader impacts of this effort will come through development of an interdisciplinary research program, via graduate and undergraduate student education and training, and through dissemination of results using illuminating graphical methods.