Lightning can accelerate some electrons to almost the speed of light, and the electrons can then produce γ-rays. [Leonid] Babich proposed that when one of these γ-rays hits the nucleus of a nitrogen atom in the atmosphere, the collision can dislodge a neutron. After briefly bouncing around, most of the neutrons get absorbed by another nitrogen nucleus. This adds energy to the receiving nucleus and puts it in an excited state. As the receiving nucleus relaxes to its original state, it emits another γ-ray — contributing to the giveaway γ-ray glow.

Meanwhile, the nitrogen nucleus that has lost one neutron is extremely unstable. It decays radioactively over the next minute or so; in so doing, it emits a positron, which almost immediately annihilates with an electron, producing two 511-keV photons. This was the third signal, Enoto says. He suspects that his detectors were able to see it only because the briefly radioactive cloud was low, and moving towards the detectors. This combination of circumstances might help to explain why the photonuclear signature has been seen so rarely. Enoto says that his team has observed a few similar events, but that the one described in the paper is the only clear-cut event so far.

Babich also predicted that not all of the neutrons dislodged from nitrogen by a γ-ray are absorbed. Some of them instead will trigger the transmutation of another nitrogen nucleus into carbon-14, a radioactive isotope that has two more neutrons than ordinary carbon. This isotope can be absorbed by organisms; it then decays at a predictable rate long after the organism's death, which makes it a useful clock for archaeologists.

The main source of the carbon-14 in the atmosphere has generally been considered to be cosmic rays. In principle, lightning could also contribute to the supply. But it is not clear yet how much of the isotope is produced in this way, says Enoto, in part because it's possible that not all bolts initiate photonuclear reactions.