Terrestial gamma ray flash (TGF) produced in thunder storms were originally detected during experimental observations from aircraft targeting sprite and jet formation. Observations and data gathering continue in order to determine if any hazard exists for commercial air passengers.
https://arxiv.org/pdf/1509.00997.pdf
I think I’ve heard of this before. There seems to be some consensus that they’re caused by what they call a “relativistic runaway electron avalanche”. Electrons can gain enough momentum to strike atomic nuclei, producing a sort of nuclear reaction. And there’s cosmic rays that also spawn antimatter particles, which annihilate to produce gamma rays.