Lightning is one of Earth's most energetic events, but there's much more to it than just a flashing fork and the rumble of thunder. Lightning strikes have been known to generate gamma rays, and now a team of Japanese researchers has found that those bursts can create photonuclear reactions in the atmosphere, resulting in the production – and annihilation – of antimatter.

Bursts of gamma rays from lightning were first detected in 1992, thanks to NASA's Compton Gamma-ray Observatory. Since then, these Terrestrial Gamma-ray Flashes (TGF) have been studied intently, and the new research out of Kyoto University has found an unexpected cause of some of the signals.

"We already knew that thunderclouds and lightning emit gamma rays, and hypothesized that they would react in some way with the nuclei of environmental elements in the atmosphere," says Teruaki Enoto, lead researcher on the project. "In winter, Japan's western coastal area is ideal for observing powerful lightning and thunderstorms. So, in 2015 we started building a series of small gamma-ray detectors, and placed them in various locations along the coast."

During a thunderstorm on February 6 this year, the team captured some intriguing data. Four detectors installed in the city of Kashiwazaki picked up a large gamma ray reading immediately after a nearby lightning strike, and when the researchers analyzed it they found three distinct bursts, each one lasting longer than the one before. The first gamma ray burst, which lasted less than a millisecond, was pretty self-explanatory.

"We could tell that the first burst was from the lightning strike," says Enoto. "Through our analysis and calculations, we eventually determined the origins of the second and third emissions as well."

A diagram of how lightning triggers photonuclear reactions in the atmosphere, to produce the three distinct gamma ray signals detected by the Kyoto University team Kyoto University/Teruaki Enoto

Both of the later signals were caused by photonuclear reactions, when the gamma rays knock neutrons out of the nucleus of nitrogen atoms in the atmosphere. That has two effects: first, those loose neutrons are reabsorbed by other particles in the air, giving off a gamma ray afterglow that lasts a few dozen milliseconds.

The third emission lasted for a full minute, and its cause was a little weirder. The nitrogen atoms that have lost their neutrons become unstable and break down, releasing positrons into the air. Positrons are the antimatter equivalent of electrons, and when the two of them touch they annihilate each other. These annihilation events produce the extended gamma ray bursts.

"We have this idea that antimatter is something that only exists in science fiction," says Enoto. "Who knew that it could be passing right above our heads on a stormy day?"

With 10 gamma ray detectors dotted along the Japanese coast, the researchers hope to gather more data on the phenomenon.

The research was published in the journal Nature.

Source: Kyoto University via Science Daily