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

空气的热失控分解：地球大气中产生的高能辐射的可能来源

• 批准号: 0741589
• 负责人: Victor Pasko
• 金额: $ 25.35万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-04-01 至 2012-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0741589&HistoricalAwards=false
• 关键词: Thermal; Runaway; Breakdown; Air; Possible

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.

主要研究者将从理论上研究空气的热失控击穿以及从X射线到硬X射线/低能伽马射线的相关高能辐射。 最近从空间和地面平台上探测到了源自地球大气层的高能辐射，并开展了大量研究工作，以调查其来源。X射线和伽马射线的发射被认为是来自于高能电子和原子粒子之间通过韧致辐射的相互作用。能够解释这些高能电子在空气中产生的唯一现有机制是电子逃逸。这种现象是电子与原子粒子相互作用的概率降低的结果，电子的能量范围从约100 eV到约1 MeV。 对于高能电子，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更多，并继续加速到非常高的能量（1 MeV）。到目前为止，已经提出了两种失控击穿的情况：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（由极大电场加速到高能量的冷电子）引发的失控击穿。解释观测到的X射线和伽马射线发射的最初研究工作集中在相对论逃逸电子雪崩和由此产生的空气击穿的研究上。然而，最近的研究表明，相对论逃逸电子理论在解释观测的一些特征，如X射线和伽马射线发射的光谱以及X射线爆发与闪电先导的步进过程之间的相关性方面存在困难。发生热失控击穿的一个临界条件是在相对较长的时间段内在有限空间上维持一个非常大的场。这种情况是一个主要的障碍，应用热失控击穿解释所观察到的高能发射在早期的研究工作。然而，最近的进展，在了解流光放电预测可能实现这种情况下，在这种类型的放电。研究的目的是研究由热逃逸电子引发的空气的逃逸击穿，并量化相关的能量发射。这项研究的智力价值在于需要发展一种理论来解释源自地球大气层的高能辐射。为了推进目前的理解热失控击穿的空气和其引起的能量排放，三个突出的科学问题将得到解决：（1）什么是电场配置在闪电先导步进过程？(2)热失控击穿是如何开始和发展的？产生的逃逸电子的能量分布是什么？(3)空气热失控分解产生的排放物是什么？这些排放物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什么联系？解决这些问题的重要性，强调需要了解的作用，热失控击穿的空气中观察到的X射线和伽马射线的排放，并在形成和传播的闪电。这项研究对于解释最近的闪电放电观测和实验，以及建立地球大气高能辐射的确切物理机制具有重要意义。研究的更广泛影响包括：（1）发展跨学科研究计划，包括与传统和失控击穿理论以及电子-原子相互作用产生的高能辐射有关的研究，（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）以参考出版物和会议报告的形式传播成果。