Lightning bolts have been recorded triggering nuclear reactions in the skies above Japan, causing radiation and antimatter to rain down upon the Earth.

But although it may sound like the plot of a science fiction movie, scientists say that there is no cause for concern as there is no health risk from the radiation, which is no more harmful than the background radiation our planet already experiences.

The findings have been made by a team of researchers from Kyoto University in Japan and published in scientific journal Nature.

“Since the radioactive isotopes are short-lived, spatially restricted and [comprise a] relatively small amount compared to usual background radiative environments, I think there is no health risk from this phenomena,” the team’s astrophysicist Teruaki Enoto told ScienceAlert.

It has been thought since the 1980s that nuclear fission reactions take place in the atmosphere during thunderstorms, caused when high-energy electrons produce gamma rays.

The findings also mark only the second time that scientists have seen radioactive isotopes being naturally produced in the atmosphere – the first time being in the form of particles produced by cosmic rays from the sun and other stars.

“The photonuclear reaction in the atmosphere has been theoretically expected [to be] triggered by such high energy radiation,” added Professor Enoto, according to ScienceAlert.

“Several groups have accumulated signatures of this [phenomenon], such as signals of either neutrons or positrons, which are the products of this reaction.”

Scientists have used observatories and satellites to detect these neutrons, positrons and other particles since the 1980s, but until now it was difficult to confirm they were the product of nuclear reactions.

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During a thunderstorm in February this year, Professor Enoto and his team detected “intense radiation” from a pair of lightning strikes just off the coast, using equiupment installed at the Kashiwazaki-Kariwa nuclear power station in Niigata.

It was followed by a prolonged gamma-ray line with the same energy signature scientists would expect to see from the positrons and electrons following a nuclear reaction.

“This line is a conclusive indication of electron-positron annihilation and represents unequivocal evidence that photonuclear reactions can be triggered by thunderstorms,” said experimental physicist Leonid Babich from the Russian Federal Nuclear Centre, reports ScienceAlert.

According to the paper in Nature, this result could “provide a previously unknown channel for generating isotopes of carbon, nitrogen and oxygen naturally on Earth”.

And at the very least, it proves that there’s more going on in thunderstorms than scientists originally thought.

“Usually people think lightning can interact with electrons in atoms,” Professor Enoto explained to ScienceAlert.

“The photonuclear reactions indicate that lightning also interacts even with nuclei if gamma rays have sufficiently high energy to knock out neutrons from the nuclei.”

Whether or not these reactions take place in all thunderstorms is still something scientists need to determine.

“This is still an open question,” Professor Enoto added.

“We have proved the existence of photonuclear reactions at least from one lightning discharge… in order to answer [the] question, we need more statistical and quantitative studies.