Observations of radiation enhancements due to thunderstorms at an altitude of 4300 m

Tsuchiya, Harufumi ; Hibino, Kinya*; Kawata, Kazumasa*; Onishi, Munehiro*

Recent observations related to thunderstorms show that there are three types of radiation enhancements based on their duration; terrestrial gamma-ray flashes (TGFs) with duration of 1 ms or less, short bursts (or TGF afterglows) with duration of a few tens of ms to a few hundred ms, and long bursts (or gamma-ray glows) with duration of a few minutes to a few tens of minutes. Such a thunderstorm-related radiation consists of not only bremsstrahlung emissions due to accelerated electrons, but also neutrons and/or the subsequent prompt gamma rays. It is currently debated how those radiation enhancements with different durations and/or different particles are generated in lightning and thunderclouds. A high-mountain observatory, located at an altitude of 4300 m (Yangbajing, Tibet), has neutron monitors with an area of 32 m. Neutron monitors are mainly sensitive to hadrons of neutron and protons, but has a small detection efficiency of gamma rays and other leptons. Therefore, neutron monitors can give us insights into production mechanism of radiation increases due to thunderstorms. During rainy seasons from 1998 to 2017, the Tibet neutron monitors detected around 140 significant enhancements of long bursts. Observed duration ranged from 5 minutes to 50 minutes, with the peak of around 10-20 minutes. In addition, electric-filed measurements, which started from 2010 at the site, showed that the observed increases correlate with electric-field variations caused by thunderclouds and/or lightning. Using the results observed by the Tibet neutron monitors and other meteorological information, we discuss in this presentation how the observed long bursts are produced and what kind of particles mainly contribute to them. This work is partly supported by JSPS/MEXT KAKENHI Grants No. 18H01236.