Photonuclear reactions triggered by lightning discharge

Enoto, Teruaki*; Wada, Yuki*; Furuta, Yoshihiro*; Nakazawa, Kazuhiro*; Yuasa, Takayuki*; Okuda, Kazufumi*; Makishima, Kazuo*; Sato, Mitsuteru*; Sato, Yosuke*; Nakano, Toshio*; Umemoto, Daigo*; Tsuchiya, Harufumi 

Relativistic electrons accelerated by electric fields of lightnings and thunderclouds emit bremsstrahlung rays, which have been detected at ground observations. The energy of the rays is sufficiently high to potentially invoke atmospheric photonuclear reactions N(, n)N, which would produce neutrons and eventually positrons via decay of generated unstable radioactive isotopes, especially N. 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, an intense -ray flash (1 ms) was detected at our monitoring sites. The subsequent initial burst quickly subsided with an exponential decay constant of 40-60 ms, followed by a prolonged line emission at 0.511MeV, lasting for a minute. The observed decay timescale and spectral cutoff at 10 MeV of the initial emission are well explained with de-excitation 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.