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

[MIS13-10] Observation of high-energy radiation emitted from thunderclouds on the Tibetan plateau

*Harufumi Tsuchiya¹, Kinya Hibino², Kazumasa Kawata³, Munehiro Ohnishi³, Masato Takita³, Kazuoki Munakata⁴, Chihiro Kato⁴, Susumu Shimoda⁵ (1.Japan Atomic Energy Agency, 2.Kanagawa University, 3. ICRR, 4.Shinshu University,, 5.RIKEN)

Keywords:Thunderclouds, High-energy radiation, Tibetan plateau

Radiation associated with thunderclouds occurring on the ground and in high mountains (thundercloud radiation) has been observed. Much research has been conducted to understand its generation mechanism. Previous studies indicate that thundercloud radiation is likely bremsstrahlung emitted by electrons accelerated to high energy due to the strong electric field within thunderclouds. However, the duration and timing of its generation exhibit significant variations based on the altitude of the observation point. For instance, thundercloud radiation observed at the Kashiwazaki-Kariwa Nuclear Power Plant in the Hokuriku region and a university near Kanazawa City occurs exclusively in winter and lasts for approximately one minute. In contrast, thundercloud radiation observed at altitudes above 3000 m occurs predominantly during the summer rainy season and endures for about 10 minutes. The underlying mechanism of this difference remains unresolved. Therefore, we conducted an analysis of thundercloud radiation data collected by neutron monitors operational since 1998 on the Tibetan Plateau at an altitude of 4300 m. Our aim is to investigate the characteristics of thundercloud radiation and understand the cause of the observed differences in duration and timing.
Neutron monitors located on the Tibetan Plateau recorded 127 events of thundercloud radiation between 1998 and 2017. Among these, 95% of them exhibited durations ranging from 10 to 40 minutes and indicate a notable contrast in the longevity of thundercloud radiation on the Tibetan Plateau compared to that in the Hokuriku region. An analysis of the occurrence dates of thundercloud radiation revealed an association with the rainy season of May to September. Thunderclouds in the Hokuriku region have a life cycle of at most 10 minutes, and a few minutes during severe winter conditions. In summer, including on the Tibetan Plateau, the life cycle of thunderclouds ranges from 30 to 60 minutes. The difference in thundercloud life cycle may contribute to differences in the duration of thundercloud radiation. Additionally, there was a trend indicating a substantial occurrence of thundercloud radiation between 6:00 p.m. and 6:00 a.m., typically the nighttime when thunderclouds are diminishing. This observation suggests that the generation of thundercloud radiation may not necessitate a strong electric field as previously believed. In addition to diurnal and seasonal variations, there were indications of periodic fluctuations in the yearly occurrence frequency of thundercloud radiation detected at the Tibetan plateau. From the comparison between these fluctuations in thundercloud radiation and the yearly variations in the number of sunspots, it was speculated that the occurrence of thundercloud radiation may be influenced by solar activity.