Generation of Runaway Electrons Induced by Radon Decay Products in Thundercloud Electric Fields

Gamma ray dose-rate increases associated with winter thunderstorm activities have been observed in the coastal areas facing the Sea of Japan. In order to investigate the generation of energetic photons which originate in thunderstorm electric fields, we have calculated the behavior of electrons and photons in electric fields by the Monte Carlo method. We have carried out the calculations of the beta and gamma rays emitted by radon decay products, as a source of energetic electrons, in thundercloud electric fields. From the calculated results, it is confirmed that the energy deposition increased greatly and an enormous amount of the electron-ion pairs will be generated in the high field region. Since the radon decay products form a large part of the energetic electrons in the atmosphere, they can serve as the source of a considerable amount of electrons. Furthermore, we have developed a transport model of radon and its decay products for the Asian region based on a regional meteorological model and a material transport model to determine the mechanism for occurrences of lightning during the winter season in the coastal areas of the Sea of Japan. The analyzed concentration distribution clarifies that seasonal winds carry a large amount of radon from the Asian continent to the coastal area of the Sea of Japan. A large amount of radon and its decay products from the continent stays in that area between the ground and the altitude of 3 km msl. This result suggests that regions with strong electric fields in winter thunderclouds may be filled with a large concentration of radon. From these results, it suggests that beta and gamma rays emitted by radon progenies cause electromagnetic showers and the production of copious runaway electrons in the thundercloud electric fields.