Observations of high energy gamma-rays from winter thunderclouds

Recently, there is a possibility that thunderclouds and/or natural lightning may accelerate electrons to relativistic velocities in their electric fields. Some dose increases associated with winter thunderclouds have been reported along the coastal area of Japan Sea, however the mechanism is still unknown due to lack of detailed information on the types of radiation, short-term intensity variations, and the energy spectra. In order to study those phenomena in further detail, we started “GROWTH (Gamma-Ray Observation of Winter Thunderclouds) collaboration” since 2006. Our new system, installed at the coastal area of Japan Sea, has been successfully operated for two years. We observed two intense dose increases, with a duration around 1 minutes, associated with winter thunderclouds. The measurement clearly revealed that the increase is caused by high energy gamma-rays with a hard spectrum extending up to 10 MeV.