Scientists in Japan have discovered that thunderstorms act as large-scale particle accelerators.
Harafumi Tsuchita of Japan's RIKEN research institute and collegues installed a directional gamma ray detector at a nuclear power plant.
Recently this picked up a 40 second burst of high energy gamma rays with a frequency 40 million times higher than visible light.
By looking at the spectrum of these gamma rays, it looks as though they are being emitted by electrons that are accelerated to nearly the speed of light by the extreme voltages that precede a lightning bolt. When these fast-moving electrons abruptly decelerate following a collision with an atom, the excess energy is released as gamma rays.
The researchers realised that the source of the rays was a thunderstorm because the detector is directional and was pointing directly at the storm when the gamma ray burst was picked up.
Shorter bursts from thunderstorms have also accidently been detected in the past by space-based telescopes which were built to survey space for high-energy X-ray and gamma ray sources. But this is the first time such a long burst has been detected and tied directly to a thunderstorm.