Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders
Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders
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DOI：
10.1029/2005ja011350
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发表时间：
2006-02
影响因子：
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通讯作者：
G. Moss;V. Pasko;Ningyu Liu;G. Veronis
中科院分区：
文献类型：
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作者：
G. Moss;V. Pasko;Ningyu Liu;G. Veronis
[1]流光是薄的非导电等离子体，其可以在接近地面压力的相对短（几厘米）的间隙中引发火花放电，并且还已知其充当闪电先导的流光区域的构建块。这些流光在地面压力，1/N缩放与大气空气密度N后，似乎是完全类似于那些记录使用望远镜成像仪在瞬态发光事件（TLEs）称为精灵，发生在海拔范围40-90公里以上的地球大气雷暴。人们还认为，在一些其他类型的TLEs中观察到的附加等离子体结构，从雷暴云的顶部发出，被称为蓝色喷流和巨大的喷流，直接与闪电领导者的流光区的过程。在很宽的外加电场范围内，通常会观察到拖缆的加速、膨胀和分支。最近的分析表明，非常高的电场幅度<$10 Ek，其中Ek是传统的击穿阈值场定义的电离和离解的附着系数在空气中的平等，产生周围的流光的尖端在其分支之前的阶段。本文描述了一个Monte Carlo模型的制定，它能够描述在空气中的电子动力学，包括热失控现象，在任意强度的外电场的影响下。Monte Carlo模拟结果表明，10 Ek场能够加速一小部分低能（几eV）流光尖端电子到12 -8 keV的能量。在闪电先导的流光区域中可获得的总电势差为数十MV的量级的情况下，提出了在负阶跃先导发展的高度瞬变负电晕闪光阶段期间，从先导头部附近的流光尖端喷射的能量为2-8 keV的电子可以进一步加速到数百keV的能量，并且可能加速到数十MeV的能量，这取决于先导头电位的具体大小。有人建议，这些高能电子可能是负责（通过“韧致辐射”过程）从地面和卫星观测到的硬X射线的产生之前的闪电放电或与闪电放电的情况下，当领导人的过程并没有达到高潮的回击。对于携带100 A电流的闪电先导，在先导的横截面积上积分的102 -8 keV热失控电子的初始通量估计为1018 s-1，加速到相对论能量的电子的数量取决于先导发展的负电晕闪光阶段期间先导流光区中的特定场幅度和配置。这些热逃逸电子可以提供相对论种子电子的替代来源，而这些种子电子以前被认为需要银河宇宙射线。负电晕闪光和相关高能辐射的持续时间估计在101 μs到101 ms之间，主要取决于先导流光区的压力大小。
[1] Streamers are thin filamentary plasmas that can initiate spark discharges in relatively short (several centimeters) gaps at near ground pressures and are also known to act as the building blocks of streamer zones of lightning leaders. These streamers at ground pressure, after 1/N scaling with atmospheric air density N, appear to be fully analogous to those documented using telescopic imagers in transient luminous events (TLEs) termed sprites, which occur in the altitude range 40–90 km in the Earth's atmosphere above thunderstorms. It is also believed that the filamentary plasma structures observed in some other types of TLEs, which emanate from the tops of thunderclouds and are termed blue jets and gigantic jets, are directly linked to the processes in streamer zones of lightning leaders. Acceleration, expansion, and branching of streamers are commonly observed for a wide range of applied electric fields. Recent analysis of photoionization effects on the propagation of streamers indicates that very high electric field magnitudes ∼10 Ek, where Ek is the conventional breakdown threshold field defined by the equality of the ionization and dissociative attachment coefficients in air, are generated around the tips of streamers at the stage immediately preceding their branching. This paper describes the formulation of a Monte Carlo model, which is capable of describing electron dynamics in air, including the thermal runaway phenomena, under the influence of an external electric field of an arbitrary strength. Monte Carlo modeling results indicate that the ∼10 Ek fields are able to accelerate a fraction of low-energy (several eV) streamer tip electrons to energies of ∼2–8 keV. With total potential differences on the order of tens of MV available in streamer zones of lightning leaders, it is proposed that during a highly transient negative corona flash stage of the development of negative stepped leader, electrons with energies 2–8 keV ejected from streamer tips near the leader head can be further accelerated to energies of hundreds of keV and possibly to several tens of MeV, depending on the particular magnitude of the leader head potential. It is proposed that these energetic electrons may be responsible (through the “bremsstrahlung” process) for the generation of hard X rays observed from ground and satellites preceding lightning discharges or with no association with lightning discharges in cases when the leader process does not culminate in a return stroke. For a lightning leader carrying a current of 100 A, an initial flux of ∼2–8 keV thermal runaway electrons integrated over the cross-sectional area of the leader is estimated to be ∼1018 s−1, with the number of electrons accelerated to relativistic energies depending on the particular field magnitude and configuration in the leader streamer zone during the negative corona flash stage of the leader development. These thermal runaway electrons could provide an alternate source of relativistic seed electrons which were previously thought to require galactic cosmic rays. The duration of the negative corona flash and associated energetic radiation is estimated to be in the range from ∼1 μs to ∼1 ms depending mostly on the pressure-dependent size of the leader streamer zone.