A Satellite-Detected Terrestrial Gamma Ray Flash Produced by a Cloud-to-Ground Lightning Leader

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.

Duke Scholars

Author Yunjiao Pu  

Author Steven A. Cummer Electrical and Computer Engineering