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Illuminating Earth's mysterious gamma ray flashes

Dark lightning Electron avalanches could be generating some of the highest energy radiation bursts ever discovered on Earth.

The study in Geophysical Research Letters, suggests bursts known as terrestrial gamma ray flashes (TGF), are possibly produced by dark lightning generated by an avalanche of electrons.

TGFs were first detected by chance by NASA's Earth-orbiting Compton gamma ray telescope in 1994.

Compton was searching for gamma ray bursts from exploding stars in deep space, when it unexpectedly began detecting very strong bursts of high energy x-rays and gamma rays, coming from Earth.

"These bursts last about a thousandth of a second, so they're very short and they're very bright," says the study's lead author Professor Joseph Dwyer of the Florida Institute of Technology.

"In fact they're so bright, that they temporarily blind spacecraft."

The flashes were originally thought to be coming from the top of Earth's atmosphere, but spacecraft measurements and energy modelling show they're coming from altitudes below 20 kilometres.

"People now know they're coming from deeper down, from thunderstorms at about the same altitudes where aircraft fly," says Dwyer.

"We've been struggling to figure out how thunderstorms could generate these flashes."

Dwyer and colleagues have been using computer models and simulations to try and solve the problem.

The authors say, there are two possible ways in which thunderstorms are producing these powerful events.

Scientists already know high energy x-rays can be generated by normal lightning, close to the ground. So it's possible normal lightning is also producing x-rays and gamma rays at higher altitudes inside thunderstorms.

"The second idea, is that there's this sort of exotic type of discharge, which we've coined dark lightning," says Dwyer.

"It produces a lot of high energy electrons and their anti-matter counterparts called positrons. This generates lots of gamma rays, but not much visible light, which is why we call it dark lightning.

"We're pretty sure that in order to make gamma rays inside thunderstorms we need high energy electrons, accelerated by strong electric fields to almost the speed of light," says Dwyer.

"When these electrons hit air atoms, they make gamma-rays, so we're pretty sure that's what's going on. But for that to work, we need lots of high energy electrons to be made very quickly, and figuring out how that happens is challenging."

Electron avalanche

Dwyer thinks, part of the process involves an electron avalanche.

"You start with just a couple of high energy particles from other sources like cosmic rays," says Dwyer.

The electron avalanche builds up quickly, generating more and more high energy particles, in an ever increasing feedback loop, eventually generating a terrestrial gamma ray flash.

On average each thunderstorm produces at least one TGF.

"Passengers flying through a thunderstorm when a TGF occurs would receive a significant dose of radiation, comparable to a full body CT scan," says Dwyer.

He says it amounts to a fair fraction of your lifetime radiation budget.

"There's probably a half-dozen ways a thunderstorm can hurt you if you're in a plane inside one, and now we've found another way," says Dwyer.

"The good news is this would be a very rare event. Pilots already know to stay away from thunderstorms, they're dangerous places even without gamma rays."