Particle acceleration in thunderstorms

Evidence for particle acceleration by the electric field in a thundercloud was first reported by Baksan group in 1985 and recently by the EAS-TOP group at the Utah ICRC (1999). In this paper, we will present further evidence for particle acceleration in thunderheads, using the large area (64 m¡ ) proportional counter and scintillator array located at Mount Norikura Cosmic Ray Observatory at an altitude of 2770m. Atmospheric conditions at Mount Norikura in the Japanese Alps during the 4 8 August 2000, were very unstable in the afternoon. Thunderstorms developed, followed by rainfall lasting several hours. We obtained evidence that, in association with these thunderstorms, particles (probably electrons) were accelerated to energies higher than 40 MeV (perhaps higher than 100 MeV) in the atmosphere somewhere above the detector. We propose that the effect is the result of the acceleration of high energy knock-on electrons or decay electrons of muons.