Image caption Electrons racing up electric field lines give rise to light, then particles, then light

A space telescope has accidentally spotted thunderstorms on Earth producing beams of antimatter.

Such storms have long been known to give rise to fleeting sparks of light called terrestrial gamma-ray flashes.

But results from the Fermi telescope show they also give out streams of electrons and their antimatter counterparts, positrons.

The surprise result was presented by researchers at the American Astronomical Society meeting in the US.

It deepens a mystery about terrestrial gamma-ray flashes, or TGFs - sparks of light that are estimated to occur 500 times a day in thunderstorms on Earth. They are a complex interplay of light and matter whose origin is poorly understood.

Thunderstorms are known to create tremendously high electric fields - evidenced by lightning strikes.

Electrons in storm regions are accelerated by the fields, reaching speeds near that of light and emitting high-energy light rays - gamma rays - as they are deflected by atoms and molecules they encounter.

These flashes are intense - for a thousandth of a second, they can produce as many charged particles from one flash as are passing through the entire Earth's atmosphere from all other processes.

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The Fermi space telescope is designed to capture gamma rays from all corners of the cosmos, and sports specific detectors for short bursts of gamma rays that both distant objects and TGFs can produce.

I think this is one of the most exciting discoveries in the geosciences in quite a long time Steven Cummer, Duke University

"One of the great things about the Gamma-ray Burst Monitor is that it detects flashes of gamma rays all across the cosmic scale," explained Julie McEnery, Fermi project scientist at Nasa.

"We see gamma-ray bursts, one of the most distant phenomena we know about in the Universe, we see bursts from soft gamma-ray repeaters in our galaxy, flashes of gamma rays from solar flares, our solar neighbourhood - and now we're also seeing gamma rays from thunderstorms right here on Earth," she told BBC News.

Since Fermi launched in mid-2008, the Gamma-ray Burst Monitor (GBM) has spotted 130 TGFs, picking up on the gamma rays in low Earth orbit as storms came within its scope.

But within that gamma-ray data lies an even more interesting result described at the meeting by Dr McEnery and her collaborators Michael Briggs of the University of Alabama Huntsville and Joseph Dwyer of the Florida Institute of Technology.

"We expected to see TGFs; they had been seen by the GBM's predecessor," Dr McEnery explained.

"But what absolutely intrigues us is the discovery that TGFs produce not just gamma rays but also produce positrons, the antimatter equivalent to electrons."

When gamma rays pass near the nuclei of atoms, they can turn their energy into two particles: an electron-positron pair.

Because electrons and positrons are charged, they align along the Earth's magnetic field lines and can travel vast distances, gathered into tightly focused beams of matter and antimatter heading in opposite directions.

Image caption Gamma rays (purple) can turn into focused matter/antimatter beams (yellow)

The dance of light and matter continues when positrons encounter electrons again; they recombine and produce a flash of light of a precise and characteristic colour.

It is this colour of light, picked up by the Fermi's GBM, that is a giveaway that antimatter has been produced.

The magnetic field can transport the particles vast distances before this characteristic flash, and one of the Fermi detections was from a storm that was happening completely beyond the horizon.

The results will be published in the journal Geophysical Research Letters.

Steven Cummer, an atmospheric electricity researcher from Duke University in North Carolina, called the find "truly amazing".

"I think this is one of the most exciting discoveries in the geosciences in quite a long time - the idea that any planet has thunderstorms that can create antimatter and then launch it into space in narrow beams that can be detected by orbiting spacecraft to me sounds like something straight out of science fiction," he said.

"It has some very important implications for our understanding of lightning itself. We don't really understand a lot of the detail about how lightning works. It's a little bit premature to say what the implications of this are going to be going forward, but I'm very confident this is an important piece of the puzzle."