Photonuclear reactions triggered by lightning discharge

Lightning, particularly the very energetic -ray flashes, can theoretically generate radioactive isotopes through the interaction of relativistic electrons with atoms and molecules in the air. Some weak observational evidence for this was recently claimed. Teruaki Enoto and collaborators report observations of a coastal thunderstorm in Japan on 6 February 2017, in which they see a clear signature of positron annihilation associated with -ray flashes, combined with -rays arising in the de-excitation of nuclei excited by neutron capture. They conclude that the positrons arise from the decay of neutrons after the lightning.