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This combination of circumstances might help to explain why the photonuclear signature has been seen so rarely.

In particular, the photonuclear reaction does not seem to match an event Dwyer observed in 2009 from a research aeroplane.

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.

Together, the three waves of γ-rays point to a photonuclear reaction first proposed2 a decade ago by Leonid Babich, a physicist at the Russian Federal Nuclear Center in Sarov.

The energy of the γ-rays is sufficiently high that they can trigger atmospheric photonuclear reactions10,15,16,17,18,19 that produce neutrons and eventually positrons via β+ decay of the unstable radioactive isotopes, most notably 13N, which is generated via 14N + γ → 13N + n, where γ denotes a photon and n a neutron.