资源环境科技发展态势分析平台

We report measurements of a terrestrial gamma‐ray flash (TGF) detected by the Fermi Gamma‐ray Burst Monitor that was produced during a negative 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.