The Gamma-ray Observation of Winter Thunderclouds collaboration has detected 70 gamma-ray glows, a high-energy phenomenon associated with thunderstorms, from October 2016 to March 2020 in Kanazawa and Komatsu, Ishikawa Prefecture, a central part of Japan facing the Sea of Japan. Based on surface and 500 hPa analyses, numerical prediction models, and surface and satellite observations, we classify their synoptic meteorological conditions into mainly three types. Most of the glow-detection cases were in west or west-southwest winds around the detection sites. Over half of the cases took place when a convex structure of surface pressure, often associated with a trough at 500 hPa, was formed along the coast of the Sea of Japan. Besides, we extract non-detection cases during winter thunderstorms in Kanazawa to compare with the glow-detection cases. While some of the non-detection cases have similar meteorological conditions as the glow-detection cases, most of the non-detection cases exhibited higher temperatures at the surface and 850 hPa, and higher − 10 $$^{\circ }$$ ∘ C altitudes, which indicates that electrification occurs at higher altitudes than the glow-detection cases. Therefore, gamma rays might have been produced but were attenuated before reaching the ground and undetectable at sea level.
Electric field of thunderclouds modifies components and energy spectra of the cosmic‐ray air shower. In particular, thunderstorms accelerate charged particles, resulting in an enhancement of gamma‐ray fluxes on the ground, known as a gamma‐ray glow. This phenomenon has been observed in recent years by the Gamma‐Ray Observation of Winter THunderclouds collaboration from winter thunderstorms in the Hokuriku area of Japan. The present work examines the ambient conditions required to produce spectral features of the previously detected gamma‐ray glows, by using Monte Carlo simulations of particle interactions in the atmosphere. We focus on three parameters, the strength and length of the electric field, and the length of a null‐field attenuation region below the electrified region. The average spectrum of observed gamma‐ray glows in winter thunderstorms of Japan requires an electric field intensity close to 0.31 MV/m, slightly exceeding the Relativistic Runaway Electron Avalanche threshold of 0.284 MV/m. The vertical size of the electric field region should be comparable to 1 km. The estimated attenuation region size is 300–500 m, necessary to reduce the low‐energy photon flux of the average gamma‐ray glows. There is still a wide range of acceptable parameter sets with degeneracy to make a similar spectrum.
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