Energetic Radiation from Lightning Leaders: Effects and Origins

无论是闪电最初被触发并通过自由大气传播的确切机制，还是这些过程与最近出现的与活跃雷暴相关的高能辐射观测的关系，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来研究闪电相关的电荷流和阶跃前导放电的高能电子发射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”，即从施加的电场中获得的能量超过了在碰撞中失去的能量，从而将电子加速到非常高的能量（1兆电子伏）。迄今为止，已经提出了两种最初“播种”这种失控击穿过程的情景：(1)宇宙射线，(2)由极高电场引起的“热”失控。理论和实验研究表明，拖缆和引线放电动力学耦合对热失控过程至关重要，而热失控过程现在又被认为与闪电发射的x射线或地面伽马射线闪光的产生密不可分。最近已经从地面、空中和卫星平台上获得了雷暴高能辐射发射和相关电活动的目标观测，但是这些以前不相关的现象之间关系的详细性质以前没有被量化，因为这个项目开始做。这项活动的智力价值在于努力对观测到的源自地球大气层的高能辐射的位置、时间和具体性质作出统一的理论合理的解释。需要量化闪电发出的辐射强度，了解其与闪电通道形成和传播的关系，并延伸到对飞机过境强雷暴活动区域的机组人员和乘客的不利健康影响的可能性，从而强调了相关问题的重要性。通过跨学科研究项目的发展，通过研究生和本科生的教育和培训，以及通过使用具有启发性的图形方法传播结果，这一努力将产生更广泛的影响。

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.