Scientists have known for a long time that the large electric fields and currents produced during thunder storms can also generate X-rays and gamma rays in the vicinity of clouds. But in the early 1990s a rare type of lightning event was discovered that can produce extremely bright, energetic gamma rays – known as terrestrial gamma ray flashes. So far, however, it has been difficult to determine specific details about this phenomenon such where the radiation originates from and its energy range.

Now researchers involved with the Italian Space Agency's AGILE mission have been able to home in on terrestrial gamma ray flashes. Launched in 2007, this satellite is dedicated to observing gamma rays originating from terrestrial and cosmic sources. Its silicon-based gamma ray detector has recently been fitted with new software that enables the satellite to take snapshots of the radiation at sub-millisecond time intervals.

Flash strikes

The AGILE team led by Marco Tavani gathered data collected from 130 terrestrial gamma ray flash events occurring in the past two and a half years. Reporting its findings in Proceedings of the National Academy of Sciences, Tavini's team noted radiation emerging in all directions from the upper atmosphere, covering a wide range of energies. In some cases gamma rays were up to 100 MeV – more than twice as energetic as previous measurements.

100 MeV is absolutely amazing considering that these things are made deep in our atmosphere by thunderstorms Joseph Dwyer, Florida Institute of Technology

"100 MeV is absolutely amazing considering that these things are made deep in our atmosphere by thunderstorms," says Joseph Dwyer, a lightning physicist at the Florida Institute of Technology, who was not involved in this research. "For comparison, 100 MeV is even impressive coming from big explosions on the Sun."

Tavani tells that his team became interested in these gamma rays because they can reveal details about the processes occurring in thunderclouds. "People have sent probes directly into clouds but it is very difficult to catch this kind of strong lightning in action," he says.

Runaway electrons

While the specific details of lightning initiation are not yet known, scientists do know that it requires large potential differences to be established within thunderclouds. This can transform the clouds into particle accelerators where electrons can be rapidly accelerated, producing an avalanche of particles as they liberate more electrons along the way. These "runaway electrons" are believed to be the source of gamma rays, and they may be involved in producing the initial spark for bolts of lightning.

Tavani's team intends to continue observing gamma rays emitted during thunderstorms. The researchers are planning a programme for surveying the upper atmosphere in more detail, which will involve sending an aeroplane equipped with gamma ray detectors into the vicinity of a thunder cloud. "We will have to be careful and I think flying over the clouds will be best because you don't get the dangerous discharge," Tavani says.

In addition, Tavani is concerned with the potential threat the gamma rays may cause to aviation. In the next few months his team will be publishing a separate study that reviews the impacts of gamma rays on electrical equipment.