We report measurements of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma‐ray Burst Monitor that was produced during a negative cloud‐to‐ground (CG) lightning leader. This is the first report of a downward directed TGF occurring during a CG flash but detected by a space‐based instrument. The gamma ray photons are produced 3 ms preceding a return stroke (−146 kA) and are essentially simultaneous with an isolated low frequency radio pulse. Based on timing, the pulse is estimated to initiate at approximately 6 km altitude, and its polarity indicates downward moving negative charge, the opposite of regular satellite‐detected upward TGFs. A likely scenario is that the runaway electrons accelerate into the upper, positively charged end of the leader in a high field region, with the reverse positron beam generating upward gamma rays detectable from space. A search for similar waveform features indicates that this type of downward CG‐TGF may occur prior to 1% of high peak current CG strokes. Extrapolating gives a global rate of 5–10% of previously known TGFs and potentially a significant fraction of global TGFs.

中文翻译：

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云到地面的闪电领袖制作的卫星检测到的地面伽玛射线闪光灯

我们报告了费米伽马射线爆破监测仪检测到的地面伽马射线闪光（TGF）的测量结果，该监测器是在负云对地（CG）闪电领袖的引导下产生的。这是关于CG闪光期间发生向下定向的TGF的第一份报告，但被天基仪器检测到。在返回行程之前3 ms产生伽马射线光子（-146 kA），并且基本上与隔离的低频无线电脉冲同时发生。根据定时，估计脉冲将在大约6 km的高度启动，其极性表示向下移动的负电荷，这与常规卫星检测到的向上的TGF相反。一种可能的情况是，失控的电子在高场区域内加速进入前导体的上部带正电的一端，而反向正电子束会产生从太空可探测到的向上伽马射线。对类似波形特征的搜索表明，这种类型的向下CG-TGF可能发生在高峰值电流CG冲程的1％之前。外推法得出的全球转化率是先前已知的TGF的5-10％，可能占全球TGF的很大一部分。