Observation of an Energetic Radiation Burst from Mountain-Top Thunderclouds

Abstract

During thunderstorms on 20 September 2008, a simultaneous detection of $\gamma$ rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 sec, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung $\gamma$ rays arriving from a source which is 60–130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds.