Energetic radiation associated with thunderstorm activities have been detected by on-ground, airborne, rocket-triggered lightning experiments, as well as in-orbit detectors as terrestrial gamma-ray flashes (TGFs). The high energy emissions, including bremsstrahlung gamma rays with energy extending up to 20 MeV, indicate powerful electron accelerations inside thunderclouds or along lightning discharge paths. We focus on unique characteristics of winter thunderstorms such as lower cloud bases and powerful activities, and started a multipoint mapping observation campaign since 2015. In the 2016-2017 winter season, we developed low cost and small size detectors dedicated to our observation. The data acquisition system records energy and timing of individual gamma-ray photons by 4-ch and 50 MHz sampling electrical boards (9.5 cm x 9.5 cm), coupled with BGO scintillator crystals. The systems were installed in portable water-proof boxes. We operated 10 detectors in two areas (Ishikawa and Niigata) along the coast of Japan Sea from October 2016 to April 2017. During this period, detectors in Ishikawa detected in total 10 long-duration gamma-ray bursts lasting for several minutes associated with passage of a thundercloud (known as gamma-ray glows). We also detected a short-duration gamma-ray burst lasting for a few hundred milliseconds associated with a lightning discharge from four independent detectors placed ~500 m apart simultaneously, on February 6th 2017 at Niigata. This event includes (1) strong gamma-ray flash (< 1 ms) like “downward TGF”, (2) de-excitation gamma-ray emissions of neutron capture from atmospheric nitrogen-14 (~50 ms), and (3) 511 keV annihilation gamma rays lasting for 1 min. This is the first conclusive detection of atmospheric photonuclear reactions (e.g. 14N + gamma -> 13N + n) induced by gamma rays from a lightning discharge, namely a downward TGF. Lightning can produce radioactive isotopes nitrogen-13 and oxygen-15, as well as stable and quasi-stable isotopesnitrogen-15, carbon-13 and carbon-14 (Enoto, Wada et al. Nature 2017).
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- Astrophysique à Haute Energie