Modeling terrestrial gamma ray flashes produced by relativistic feedback discharges
Modeling terrestrial gamma ray flashes produced by relativistic feedback discharges
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模拟相对论反馈放电产生的地面伽马射线闪光
DOI：
10.1002/jgra.50232
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
J. Dwyer
中科院分区：
文献类型：
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作者：
Ningyu Liu;J. Dwyer
本文报道了由相对论反馈放电产生的地面伽玛射线闪光（TGFs）的模拟研究。地球伽马射线闪光是由航天器观测到的来自地球大气层的强烈高能辐射。它们是由高能电子（称为逃逸电子）与空气原子的韧致辐射相互作用产生的。产生大通量逃逸电子以制造TGF的有效物理机制是相对论反馈放电，其中种子逃逸电子由正电子和X射线产生，它们是放电本身的产物。一旦相对论性反馈放电变得自我维持，相对论性电子雪崩的数量就会呈指数级增长，通过雷暴云内部的同一高场区域传播，直到电场被放电产生的电离部分放电。模拟结果表明，相对论反馈放电产生的TGF脉冲的持续时间从几十微秒到几毫秒不等，涵盖了迄今为止观察到的所有TGF脉冲的持续时间。此外，当雷暴云中有足够大的电位差时，可以形成称为相对论反馈流光的自传播放电，其传播方式与传统的正流光一样。对于相对论反馈流注，正电子和X射线产生逃逸电子的正反馈机制与传统正流注的光电离作用相似。对相对论反馈流光的模拟结果表明，该流光可以产生一系列不同持续时间的TGF脉冲。相对论性流光可以最初以脉冲方式传播，并且在稍后阶段转变成连续传播模式。反馈拖缆在连续传播过程中可以产生毫秒长的TGF脉冲。然而，流光的连续传播往往是不稳定的，因为它不像传统的正流光那样膨胀。它的头部电场继续增加，可以达到传统击穿阈值电场的一半，这导致紧随头部的通道中的离子密度为3- 10× 1014 m −3。流光头部高场区的空间宽度约为100 m，流光速度约为5×105 m/s。因此，传统的正拖缆可以从雷暴云水凝物或相对论反馈拖缆头中毫米尺寸的增强电导率的不均匀性开始，并且正拖缆可以进一步导致先导的形成。此外，相对论反馈流注可以在几十毫秒内导致几十库仑千米的电荷矩变化，这表明相对论反馈放电过程可能是雷暴云电荷动力学的重要组成部分。
This paper reports a modeling study of terrestrial gamma ray flashes (TGFs) produced by relativistic feedback discharges. Terrestrial gamma ray flashes are intense energetic radiation originating from the Earth's atmosphere that has been observed by spacecraft. They are produced by bremsstrahlung interactions of energetic electrons, known as runaway electrons, with air atoms. An efficient physical mechanism for producing large fluxes of the runaway electrons to make the TGFs is the relativistic feedback discharge, where seed runaway electrons are generated by positrons and X‐rays, products of the discharge itself. Once the relativistic feedback discharge becomes self‐sustaining, an exponentially increasing number of relativistic electron avalanches propagate through the same high‐field region inside the thundercloud until the electric field is partially discharged by the ionization created by the discharge. The modeling results indicate that the durations of the TGF pulses produced by the relativistic feedback discharge vary from tens of microseconds to several milliseconds, encompassing all durations of the TGFs observed so far. In addition, when a sufficiently large potential difference is available in thunderclouds, a self‐propagating discharge known as the relativistic feedback streamer can be formed, which propagates like a conventional positive streamer. For the relativistic feedback streamer, the positive feedback mechanism of runaway electron production by the positrons and X‐rays plays a similar role as the photoionization for the conventional positive streamer. The simulation results of the relativistic feedback streamer show that a sequence of TGF pulses with varying durations can be produced by the streamer. The relativistic streamer may initially propagate with a pulsed manner and turn into a continuous propagation mode at a later stage. Milliseconds long TGF pulses can be produced by the feedback streamer during its continuous propagation. However, the continuous propagation of the streamer tends to be unstable, because it does not expand like the conventional positive streamer. Its head electric field continues to increase and can reach half of the conventional breakdown threshold field, which results in an ion density of 3– 10×1014m−3 in the channel immediately following the head. The spatial width of the high field region in the streamer head is about 100 m and the streamer speed is about 5×105 m/s. As a result, conventional positive streamers can be initiated from thundercloud hydrometeors or inhomogeneities of enhanced conductivities of millimeter sizes in the relativistic feedback streamer head and the positive streamers may further result in the formation of leaders. In addition, a relativistic feedback streamer can result in a charge moment change of several tens of coulomb‐kilometers in a few tens of milliseconds, indicating that the relativistic feedback discharge process could be an important component for thundercloud charge dynamics.