Observation of an energetic radiation burst from mountain-top thunderclouds.

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.