Thermal Runaway Breakdown of Air: A Possible Origin of Energetic Radiation Generated in the Earth's Atmosphere

首席研究员将从理论上研究空气的热失控分解和相关的从x射线到硬x射线/低能伽马射线的高能辐射。最近从空间和地面平台都探测到源自地球大气层的高能辐射，并进行了大量的研究工作，以调查其来源。x射线和伽马射线的发射被认为是由高能电子和原子粒子通过轫致辐射相互作用产生的。目前唯一能够解释空气中这些高能电子产生的机制是电子失控。这种现象是由于能量在100 eV到1 MeV之间的电子与原子粒子相互作用的概率减小的结果。对于高能电子来说，电子从外部电场中获得的能量可能比它在碰撞中失去的能量要多，并继续加速到非常高的能量（1兆电子伏）。到目前为止，已经提出了失控击穿的两种情况：由宇宙射线产生的相对论性失控电子引发的失控击穿，以及由热失控电子（冷电子被极大的电场加速到高能量）引发的失控击穿。为了解释观测到的x射线和伽马射线发射，最初的研究工作集中在相对论性失控电子雪崩和由此导致的空气分解的研究上。然而，最近的研究表明，相对论失控电子理论难以解释观测的某些特征，例如，x射线和伽马射线发射的光谱以及x射线爆发与闪电前导步进过程之间的相关性。热失控击穿发生的一个临界条件是在一个有限的空间内，在一个相对较长的时间内持续一个极大的场。这个条件是早期研究工作中应用热失控击穿解释观测到的高能发射的主要障碍。然而，最近对流光放电的理解进展预测了在这种类型的放电中可能实现这种情况。研究热失控电子引发的空气失控击穿现象，并对其能量辐射进行量化。这项研究的智力价值在于需要发展一种理论来解释起源于地球大气层的高能辐射。为了提高对空气热失控击穿及其诱导的高能辐射的现有理解，将解决三个突出的科学问题：(1)雷锋步进过程中的电场结构是什么？(2)热失控击穿是如何发生和发展的？逃逸电子的能量分布是怎样的？(3)空气热失控分解的辐射是什么？这些辐射与最近发现的与雷暴活动有关的x射线和伽马射线闪光有何联系？解决这些问题的重要性被强调了，因为需要了解空气的热失控分解在观测到的x射线和伽马射线发射的产生以及闪电的形成和传播中的作用。这项研究对于解释最近的雷电放电观测和实验，以及建立源自地球大气层的高能辐射的确切物理机制具有重要意义。本研究的更广泛影响包括：(1)跨学科研究项目的发展，涉及传统和失控击穿理论的研究，以及电子-原子相互作用中产生的高能辐射，(2)研究生的教育和培训，(3)博士后学者的支持和进一步培训，(4)本科生在夏季期间参与研究活动，(5)以评审出版物和会议报告的形式传播成果。

The Principal Investigator will theoretically investigate thermal runaway breakdown of air and the associated energetic radiation from X-rays to hard X-rays/lower-energy gamma rays. Energetic radiation originating in the Earth's atmosphere has been recently detected from both space and ground based platforms, and intensive research efforts have been carried out in order to investigate its origin. The X-ray and gamma ray emissions are believed to originate from interactions between high-energy electrons and atomic particles through bremsstrahlung. The only existing mechanism capable of explaining the generation of these high-energy electrons in air is electron runaway. This phenomenon is a result of a decreasing probability of electron interactions with atomic particles for electrons with energies in the range from about 100 eV to about 1 MeV. For a high-energy electron, it is possible that the electron gains more energy from an external electric field than it loses to collisions, and continues to accelerate to very high (1 MeV) energies. Thus far two scenarios of runaway breakdown have been proposed: runaway breakdown initiated by relativistic runaway electrons generated by cosmic rays, and initiated by thermal runaway electrons (cold electrons accelerated to high energies by extremely large electric fields). The initial research work to explain the observed X-ray and gamma ray emissions focused on studies of relativistic runaway electron avalanche and the resulting breakdown of air. However, recent studies show that relativistic runaway electron theory has difficulty in explaining some features of observations, for example, the spectrum of X-ray and gamma ray emissions and the correlation between the X-ray bursts with the stepping processes of lightning leaders. A critical condition for the occurrence of thermal runaway breakdown is an extremely large field sustained over a finite space during a relatively extended time period. This condition was a major obstacle for applying the thermal runaway breakdown to interpretation of the observed energetic emissions during early research work. However, recent progress in understanding of streamer discharges predicts possible realization of this condition during this type of discharge. It is the purpose of the research to investigate the runaway breakdown of air initiated by thermal runaway electrons and quantify the associated energetic emissions. Intellectual merit of the research is defined by the need to develop a theory to explain the energetic radiation originating in the Earth's atmosphere. To advance current understanding of thermal runaway breakdown of air and its induced energetic emissions, three outstanding scientific questions will be addressed: (1) What is the electric field configuration during a lightning leader stepping process? (2) How does the thermal runaway breakdown initiate and develop? What is the energy distribution of the resulting runaway electrons? (3) What are the emissions from the thermal runaway breakdown of air, and how do these emissions link to the recently-discovered X-ray and gamma-ray flashes associated with thunderstorm activity? The importance of addressing these questions is underscored by the need to understand the role of thermal runaway breakdown of air in the production of observed X-ray and gamma-ray emissions, and in the formation and propagation of lightning. This study is important for interpreting recent observations and experiments on lightning discharges, and for establishing of the exact physical mechanism of the energetic radiation originating from the Earth's atmosphere. Broader impacts of the research include: (1) the development of interdisciplinary research program involving studies relevant to both conventional and runaway breakdown theories, and energetic radiation generated in electron-atom interactions, (2) the education and training of a graduate student, (3) the support and further training of a post-doctoral scholar, (4) the involvement of undergraduate students in research activities during summertime periods, (5) the dissemination of results in the form of refereed publications and conference presentations.