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Lightning and thundercloud are the most dramatic natural particleaccelerators on the Earth. Relativistic electrons accelerated by electricfields therein emit bremsstrahlung gamma rays, which have been detected atground observations, by airborne detectors, and as terrestrial gamma-rayflashes (TGFs) from space. The energy of the gamma rays is sufficiently high topotentially invoke atmospheric photonuclear reactions 14N(gamma, n)13N, whichwould produce neutrons and eventually positrons via beta-plus decay ofgenerated unstable radioactive isotopes, especially 13N. However, no clearobservational evidence for the reaction has been reported to date. Here wereport the first detection of neutron and positron signals from lightning witha ground observation. During a thunderstorm on 6 February 2017 in Japan, aTGF-like intense flash (within 1 ms) was detected at our monitoring sites0.5-1.7 km away from the lightning. The subsequent initial burst quicklysubsided with an exponential decay constant of 40-60 ms, followed by aprolonged line emission at about 0.511 megaelectronvolt (MeV), lasting for aminute. The observed decay timescale and spectral cutoff at about 10 MeV of theinitial emission are well explained with de-excitation gamma rays from thenuclei excited by neutron capture. The centre energy of the prolonged lineemission corresponds to the electron-positron annihilation, and hence is theconclusive indication of positrons produced after the lightning. Our detectionof neutrons and positrons is unequivocal evidence that natural lightningtriggers photonuclear reactions. No other natural event on the Earth is knownto trigger photonuclear reactions. This discovery places lightning as only thesecond known natural channel on the Earth after the atmospheric cosmic-rayinteraction, in which isotopes, such as 13C, 14C, and 15N, are produced.

Photonuclear Reactions in Lightning Discovered from Detection of Positrons and Neutrons