Observation of cosmic radiation at the top of Mt. Fuji; High-energy radiation in association with thunderstorm activity

Torii, Tatsuo  ; Yasuda, Hiroshi*; Yajima, Kazuaki*; Sugita, Takeshi*; Tanabe, Sachiko*; Kimura, Yoshihisa*; Kamogawa, Masashi*

Fluctuations of energetic radiation that were seemed to be caused by a summer thunderstorm were observed at the top of Mt. Fuji. The largest of such fluctuations was gradual and lasted for about 20 minutes, and was found to be high-energy rays having a continuous energy spectrum up to 10 MeV or more. As for the feature of these fluctuations, it seems naturally that such fluctuations are caused by the bremsstrahlung photons generated by the energetic electrons produced continuously with an intense electric field in the thundercloud rather than originated in the process of lightning discharge.