Lightning and thunderclouds are natural particle accelerators1. Avalanches of relativistic runaway electrons, which develop in electric fields within thunderclouds2,3, emit bremsstrahlung γ-rays. These γ-rays have been detected by ground-based observatories4,5,6,7,8,9, by airborne detectors10 and as terrestrial γ-ray flashes from space10,11,12,13,14. 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. However, this reaction has hitherto not been observed conclusively, despite increasing observational evidence of neutrons7,20,21 and positrons10,22 that are presumably derived from such reactions. Here we report ground-based observations of neutron and positron signals after lightning. During a thunderstorm on 6 February 2017 in Japan, a γ-ray flash with a duration of less than one millisecond was detected at our monitoring sites 0.5–1.7 kilometres away from the lightning. The subsequent γ-ray afterglow subsided quickly, with an exponential decay constant of 40–60 milliseconds, and was followed by prolonged line emission at about 0.511 megaelectronvolts, which lasted for a minute. The observed decay timescale and spectral cutoff at about 10 megaelectronvolts of the γ-ray afterglow are well explained by de-excitation γ-rays from nuclei excited by neutron capture. The centre energy of the prolonged line emission corresponds to electron–positron annihilation, providing conclusive evidence of positrons being produced after the lightning.
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We thank the members of the radiation safety group of the Kashiwazaki-Kariwa nuclear power station, TEPCO Inc., for providing observation sites, H. Miyahara, N. Kawanaka and H. Ohgaki for discussions, H. Sakurai, M. Niikura and the Sakurai group members at RIKEN Nishina Center for providing Bi4Ge3O12 scintillation crystals, T. Tamagawa for project support, G. Bowers, M. Kamogawa and D. Smith for suggestions on our interpretation, S. Otsuka and H. Kato for supporting the detector developments, and the RIKEN Advanced Center for Computing and Communication for use of the HOKUSAI GreatWave supercomputing system for Monte Carlo simulations. This research is supported by JSPS/MEXT KAKENHI grant numbers 15K05115, 15H03653 and 16H06006, by SPIRITS 2017 and Hakubi projects of Kyoto University, and by the joint research programme of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo. Our project is also supported by crowdfunding (‘Thundercloud Project’, using the academic crowdfunding platform ‘academist’), and we are grateful to Y. Shikano, Y. Araki, M. T. Hayashi, N. Matsumoto, T. Enoto, K. Hayashi, S. Koga, T. Hamaji, Y. Torisawa, S. Sawamura, J. Purser, S. Suehiro, S. Nakane, M. Konishi, H. Takami, T. Sawara and all of the backers of Thundercloud Project. We are grateful to M. Sakano of Wise Babel Ltd for linguistic help and to the ‘adachi design laboratory’ for supporting the crowdfunding acvitity. The background image in Fig. 1 was provided by the Geospatial Information Authority of Japan.