Structures and dynamics of lightning discharges in atmosphere and ionosphere. High-energy γ-rays associated with particle acceleration in thunderclouds

Bursts of radiations up to 10 MeV range are sometimes observed with thunderstorms, which indicate generation of energetic particles in the thunderclouds. It has been known that those radiation bursts are classified into short and long ones based on their duration. In the present experiment to observe the long ones, two NaI scintillator systems covered with thin plastic scintillators, one to observe the radiations from the top direction and another from all directions, were used to catch the radiations while it was thundering in winter at Kashiwazaki-Kariwa, Niigata. The burst observed by the both systems on Jan. 7, 2007 continued about 40 seconds. The event was not coincident with any thunder discharge or lightning monitored. Result indicated that the radiations came from the top direction and they were photons but not charged particles. The result can be explained assuming that those photons are generated by the bremsstrahlung of runaway electrons which are accelerated and multiplied in the cloud. The starting of the runaway electrons are roughly estimated to be generated over 270 keV of which seeds can be either cosmic shower or Rn origin. The gamma rays from bremsstrahlung of the relativistic runaway electrons toward ground are emitted within the narrow cone due to the beaming effect. They are observed as bursts on the ground when the thundercloud runs overhead. (S. Funahashi)