[M-IS13] Atmospheric electricity: Application of technology for reducing disaster risks

[MIS13-07] Comparison of gamma-ray glow and radar observations detected by Citizen Science "Thundercloud Project" during the 5 years from FY 2018

*Miwa Tsurumi^1,2, Teruaki Enoto^1,3, Yuko Ikkatai⁴, Naoki Tsuji^1,3, Gabriel Diniz¹, Taro Shinoda⁵, Ting Wu⁶, Daohong Wang ⁶, Toru Takagaki⁷, Syoko Miyake⁸, Kazuhiro Nakazawa⁵, Takeshi Morimoto⁹, Yoshitaka Nakamura¹⁰, Masashi Kamogawa¹¹, Harufumi Tsuchiya¹² (1.Kyoto University, 2.Aoyama Gakuin University, 3.RIKEN, 4.Kanazawa Universty, 5.Nagoya University, 6.Gifu University, 7.TAC, 8.National Institute of Technology, Ibaraki College, 9.Kindai University, 10.Kobe City College of Technology, 11.University of Shizuoka, 12.Japan Atomic Energy Agency)

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

In a developed thundercloud, electrons are accelerated to relativistic speeds by a strong electric field, and bremsstrahlung in the gamma-ray band is emitted. This phenomenon is called "gamma-ray glow," and gamma rays in the MeV range have been observed on the ground for tens of seconds to a minute at the same time as the thundercloud passes. Since gamma rays in the atmosphere attenuate within several hundred meters, it is necessary to observe clouds at low altitudes. In winter, ground-based observations of gamma-ray glow events are actively conducted along the Japan sea coast area, where low-level clouds are generated when cold, dry air from the Siberian air mass passes through the warm Tsushima Current. This previous study revealed that electrons are accelerated by the electric field in thundercloud and emit gamma rays, and that this electric field region is generated by charged particles such as ice crystals and graupels in thundercloud. However, the limited number of observation points makes it difficult to track moving thundercloud, which hinders the measurement of the temporal and spatial distribution of the gamma-ray emission region. To solve these problems, the "Thundercloud Project" was started using a citizen science framework. With the help of citizen supporters in the Hokuriku region, lightweight and compact radiation detectors called "Cogamo" were installed around Kanazawa City in winter. Combining these detectors with automatic data transmission by Sakura IoT, a large-scale observation network was constructed. By increasing the scale and density of the observation network at multiple locations, the project aims to track thunderclouds and unravel the mysteries of thundercloud gamma-ray radiation, including the onset, duration, and termination of electron acceleration within thunderclouds. The project also aims to identify the thunderclouds in which electron acceleration occurs and to determine if the accelerated electrons trigger lightning discharges.
We searched for gamma-ray glow events in the five years of data observed by the Thundercloud Project from FY 2018 to FY 2022. As a result, we found at least about 40 events. Using X-band meteorological radar operated by the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT), We analyzed the thunderclouds that passed over our detectors. We examined the development of thunderclouds and the spatial distribution of particles such as raindrops, ice crystals, and graupels that make up the clouds. As a result, we found high-reflectivity region above the detector at the time of the event, and areas of wet graupel region that are thought to be responsible for the positive charge in the lower layer of the electron acceleration region. The charge separation mechanism has shown that graupel is positively charged above -10°C and negatively charged below -10°C in typical thundercloud. Using this mechanism, we estimated the charge structure in the thundercloud and found that the electric field region where electron acceleration occurred is thought to have been located around 2 km above the ground.
The temporal variation of the vertical distribution showed that the altitude of the graupel region decreased from 5 minutes before to 5 minutes after the detection.
This suggests that thundercloud gamma rays are detected when the electron acceleration region approaches the ground during the "waning phase" of thunderclouds.
We will report the results of a systematic analysis of five years of gamma-ray glow data and radar analysis.