Abstract
In 2015, the Gamma-Ray Observation of Winter Thunderstorms (GROWTH) collaboration launched a mapping observation campaign for high-energy atmospheric phenomena related to thunderstorms and lightning discharges. This campaign has developed a detection network of gamma rays with up to 10 radiation monitors installed in the cities of Kanazawa and Komatsu, Ishikawa Prefecture, Japan, where low-charge-center winter thunderstorms frequently occur. During four winter seasons from October 2016 to April 2020, a total of 70 gamma-ray glows, i.e., minute-lasting bursts of gamma rays originating from thunderclouds, were detected. Their average duration is 58.9 s. Among the detected events, 77% were observed at night. The gamma-ray glows can be classified into temporally symmetric, temporally asymmetric, and lightning-terminated types based on their count-rate histories. An averaged energy spectrum of the gamma-ray glows is well fitted with a power-law function with an exponential cutoff, whose photon index, cutoff energy, and flux are $0.613\ifmmode\pm\else\textpm\fi{}0.009$ MeV, $4.68\ifmmode\pm\else\textpm\fi{}0.04$ MeV, and $(1.013\ifmmode\pm\else\textpm\fi{}0.003)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ erg ${\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ (0.2--20.0 MeV), respectively. The present paper provides a catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas.
Highlights
Strong electric fields inside thunderclouds have recently been recognized as a particle accelerator in nature
The present paper provides a catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas
Since the launch of that campaign, we have reported the discovery of photonuclear reactions triggered by a downward terrestrial gamma-ray flashes (TGFs) [34], an observation of a glow termination with radiofrequency lightning mapping and electric field measurements [6], the simultaneous detection of a glow termination and a downward TGF [24], and so on
Summary
Strong electric fields inside thunderclouds have recently been recognized as a particle accelerator in nature. Ray glows, called thunderstorm ground enhancements (TGEs) when detected at the ground, are minute-lasting high-energy phenomena in the atmosphere caused by strong electric fields [1,2,3,4]. Electrons accelerated to a relativistic energy in an electric field emit bremsstrahlung photons by colliding with atmospheric atoms. The energy spectrum of gamma-ray glows typically extends up to tens of MeV [5,6], and their observed duration ranges from seconds to tens of minutes [5,7,8,9]. Since the first detection by an F-106 aircraft [1,10], gamma-ray glows have been detected via experiments
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