  Lightning and thundercloud are the most dramatic natural particle
accelerators on the Earth. Relativistic electrons accelerated by electric
fields therein emit bremsstrahlung gamma rays, which have been detected at
ground observations, by airborne detectors, and as terrestrial gamma-ray
flashes (TGFs) from space. The energy of the gamma rays is sufficiently high to
potentially invoke atmospheric photonuclear reactions 14N(gamma, n)13N, which
would produce neutrons and eventually positrons via beta-plus decay of
generated unstable radioactive isotopes, especially 13N. However, no clear
observational evidence for the reaction has been reported to date. Here we
report the first detection of neutron and positron signals from lightning with
a ground observation. During a thunderstorm on 6 February 2017 in Japan, a
TGF-like intense flash (within 1 ms) was detected at our monitoring sites
0.5-1.7 km away from the lightning. The subsequent initial burst quickly
subsided with an exponential decay constant of 40-60 ms, followed by a
prolonged line emission at about 0.511 megaelectronvolt (MeV), lasting for a
minute. The observed decay timescale and spectral cutoff at about 10 MeV of the
initial emission are well explained with de-excitation gamma rays from the
nuclei excited by neutron capture. The centre energy of the prolonged line
emission corresponds to the electron-positron annihilation, and hence is the
conclusive indication of positrons produced after the lightning. Our detection
of neutrons and positrons is unequivocal evidence that natural lightning
triggers photonuclear reactions. No other natural event on the Earth is known
to trigger photonuclear reactions. This discovery places lightning as only the
second known natural channel on the Earth after the atmospheric cosmic-ray
interaction, in which isotopes, such as 13C, 14C, and 15N, are produced.
