Gamma-ray observations at the coastal area of Japan Sea in winter seasons
July 22, 2019
July 02, 2021
Since 2006, The Gamma Ray Observation of Winter Thunderclouds (GROWTH) collaboration has operated radiation measurement networks at the coastal area of Japan sea. The area is famous for its frequent occurrence of winter thunderstorms. We aims at elucidating how particles in lightning and thunderclouds are accelerated to relativistic energies. More than 10-years observations reveal that there are two types of radiation bursts associated with winter thunderstorms. One is a "long burst" or a "gamma-ray glow" lasting for a few tens of seconds to a few minutes. The other is a "short burst" in association with lightning. In order to expand the observational network, we have developed a small type of a radiation detector. In this paper, we focus on recent several findings obtained by the new detectors.One is a combination of a short burst and a long one, showing simultaneous detection of prompt gamma rays extending to $\sim$10 MeV and the 511-keV annihilation ones. These gamma-ray signals demonstrate the occurrence of photonuclear reactions in lightning.
Another is an event that a gamma-ray glow suddenly ceased just prior to lightning, suggesting a relation between lightning and the two types of radiation bursts. On the basis of these results, we discuss the production mechanism of gamma rays related to thunderstorms and the lightning initiation problem.
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