Although Hurricane Patricia was one of the most powerful storms ever recorded, that didn't stop the National Oceanic and Atmospheric Administration (NOAA) from flying a scientific aircraft right through it. Now, the researchers have reported their findings, including the detection of a beam of antimatter being blasted towards the ground, accompanied by flashes of x-rays and gamma rays.

Scientists discovered terrestrial gamma-ray flashes (TGFs) in 1994, when orbiting instruments designed to detect deep space gamma ray bursts noticed signals coming from Earth. These were later linked to storms, and after thousands of subsequent observations have come to be seen as normal parts of lightning strikes.

The mechanisms behind these emissions are still shrouded in mystery, but the basic story goes that, first, the strong electric fields in thunderstorms cause electrons to accelerate to almost the speed of light. As these high-energy electrons scatter off other atoms in the air, they accelerate other electrons, quickly creating an avalanche of what are known as "relativistic" electrons.

All of these collisions also give off gamma rays, and when enough of them are happening at once, they can build to create an extremely bright TGF. But there's another side effect: the creation of antimatter. When the gamma rays collide with the nucleus of atoms in the air, they create an electron and its antimatter equivalent, the positron, and send them screeching off in opposite directions.

Antimatter signatures have been spotted in storms in the past, but a particular phenomenon known as a reverse positron beam, where antimatter particles are sent downwards, had only been predicted by models of TGFs.

At least, until NOAA's Hurricane Hunter aircraft paid Patricia a visit in 2015. On board was an instrument named the Airborne Detector for Energetic Lightning Emissions (ADELE), which is designed to measure the x-rays and gamma rays created during TGFs. Sure enough, the team spotted the telltale signature of a reverse positron beam as the plane flew through the eyewall, the most intense region of the storm.

"This is the first confirmation of that theoretical prediction, and it shows that TGFs are piercing the atmosphere from top to bottom with high-energy radiation," says David Smith, an author of the study. "This event could have been detected from space, like almost all the other reported TGFs, as an upward beam caused by an avalanche of electrons. We saw it from below because of a beam of antimatter (positrons) sent in the opposite direction."

In future, the researchers say instruments may not need to be flown into the storms to make these detections.

"We detected it at an altitude of 2.5 km (1.6 mi), and I estimated our detectors could have seen it down to 1.5 km (0.9 mi)," says Smith. "That's the altitude of Denver, so there are a lot of places where you could in theory see them if you had an instrument in the right place at the right time during a thunderstorm."

The research was published in the Journal of Geophysical Research: Atmospheres.

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