[M-IS24] Atmospheric electricity: High energy phenomena

[MIS24-02] Observations and analyses of gamma-ray glows in the fiscal year 2022 by the citizen science “Thundercloud Project”

*Miwa Tsurumi^1,2, Teruaki Enoto^2,3, Yuko Ikkatai⁴, Ting Wu⁵, Daohong Wang⁵, Taro Shinoda⁶, Kazuhiro Nakazawa⁶, Naoki Tsuji^2,3, Diniz Gabriel², Jun Kataoka⁷, Masashi Kamogawa⁸, Toru Takagaki⁹, Shoko Miyake¹⁰, Dai Tomioka¹⁰, Takeshi Morimoto¹¹, Yoshitaka Nakamura¹², Harufumi Tsuchiya¹³ (1.Aoyama Gakuin University, 2.Kyoto University, 3.RIKEN, 4.Kanazawa University, 5.Gifu University, 6.Nagoya University, 7.Waseda University, 8.University of Shizuoka, 9.TAC, 10.National Institute of Technology, Ibaraki College, 11.Kindai University, 12.Kobe City College of Technology, 13.Japan Atomic Energy Agency)

Keywords:gamma-ray glow, citizen science, mapping radiation observation

Electrons are accelerated and amplified in the strong electric field region in a developed thundercloud. The accelerated electrons collide with the atmosphere and emit bremsstrahlung. One such high-energy phenomenon is "gamma-ray glows." During the passage of a thundercloud, gamma rays up to 10 MeV or more are emitted to the ground for several minutes to several tens of seconds. In winter, the Japan Sea coast is ideal for observing gamma-ray glows on the ground because low-cloud base thunderclouds develop, and gamma rays generated in thunderclouds are less likely to be attenuated by the atmosphere before reaching the ground.
GROWTH experiment launched in 2006 has already observed about 70 gamma-ray glow events on the ground experiment in the winter season (Wada et al., 2021). However, there are many unresolved questions, such as their duration, the size of the electron acceleration region, and the relationship between accelerated electrons and the initiation of a lightning flash.
To solve this question, it is necessary to increase the number of observation points and track gamma-ray emitting thunderclouds. Therefore, we launched the Citizen Science "Thundercloud Project" in 2018 to conduct radiation mapping observations at multiple locations. In this project, we developed compact, lightweight radiation detectors called "Cogamo" and installed them in the garden of citizen supporters living along the coast of the Sea of Japan. The Cogamo detector is equipped with a 5 cm x 5 cm x 15 cm CsI (Tl) scintillator and a Photon Counter MPPC (Hamamatsu Photonics) and measures the radiation of 0.2-10 MeV. An experiment using an accelerator is also planned to calibrate the high-energy side. It is also equipped with an environmental sensor and GPS antenna. All data is recorded on a microSD card. Some data is also transmitted to the server through the Sakura io module, and an automatic alert system introduced in FY2021 enables real-time monitoring from a remote location.
In FY2022, 74 Cogamo detectors, which increased from 58 detectors in FY2021, were installed along the Japan Sea coast area, mainly in Kanazawa City, and have been observed since November. As of February 2023, at least 9 gamma-ray glow events have been observed in Kanazawa City. 5 events have been detected by 2 or more of Cogamo detectors. The largest count-rate event was observed by 4 detectors at around 14:00 JST on December 18. One of them, ID 29, recorded 37.6 counts/s in the 3-8 MeV, about 40 times higher rate then the background. Lightning was also reported in Kanazawa City on the same day of all gamma-ray glow events, and well-developed thunderclouds were passing over the Cogamo detectors during the events when the XRAIN radar data was available. At this presentation we will report on the operation of the “Thundercloud Project” in FY2022 and the analysis of the gamma-ray glow events including observation before FY2021.