Physicists have known for over a decade that 10 - 20 MeV gamma rays are produced in millisecond bursts during electrical storms. These bursts are believed to occur when high voltages in a thundercloud accelerate electrons to energies up to about 35 MeV. These electrons are slowed down by colliding with atoms in the air and as a result give off bremsstrahlung -- gamma rays that are created when an electron is deflected off its course by an atom.

Much longer bursts lasting up to several minutes have also been seen, but these events seem to be much rarer than their shorter counterparts. Physicists have yet to work out where in the sky the longer pulses are coming from – if they are indeed coming from the sky. The energy distribution of the pulses and whether the pulses contain any other types of radiation such as charged particles was also unclear.

Now Harafumi Tsuchiya of the Cosmic Ray Laboratory of Japan’s RIKEN research institute and colleagues have used a new bank of direction-sensitive detectors they installed at a nuclear power plant to detect a 40-s gamma ray burst during a very intense thunderstorm on 6 January, 2007. Their system at the Kashiwazaki-Kariwa plant on the coast of the Sea of Japan was designed to measure the energy distribution, composition and source of thundercloud pulses.

By analysing the energy distribution of the pulse, the team was able to say that the pulse was made of bremsstrahlung gamma rays. The directionality of their detectors allowed the team to confirm that the pulse came from the storm and because such gamma rays can only travel short distances in the atmosphere, the team could also conclude that the pulse was created a kilometre or less from the detectors.

Since the gamma rays arrived about a minute before the first lightning strike, Tsuchiya believes that the pulse was probably created while electrical energy was building up in the thundercloud, rather than when energy is being discharged as lightning. He adds that the process is likely to begin with a cosmic ray passing through the cloud and ionizing the air to produce electrons, which are accelerated to towards the bottom of the cloud, which has a positive charge. These electrons ionize other atoms on the way, creating a stream of high-energy electrons.

Tsuchiya says that bremsstrahlung at MeV energies would be focused into a beam that only illuminates a small area on the ground, which could explain why so few long-duration pulses have been seen. The team plan to verify this by placing many radiation detectors over a wider area.

David Smith, a physicist at the University of California at Santa Clara and an expert on atmospheric gamma ray pulses, agrees that the pulse was made in a thundercloud accelerator. “The spectrum looks just right for bremsstrahlung”, he says. According to Smith, the millions of volts required to produce MeV electrons could be sustained in a cloud for seconds or even minutes as long as the stream of electrons does not become so intense that it causes an avalanche-like electric breakdown of the type that could lead to lightning. Smith is now designing an airborne experiment to test whether lightning flashes are preceded by gamma-ray pulses.