SAN FRANCISCO — The most energetic particles in the electromagnetic spectrum could pose a danger to commercial airline passengers.

About every 3000 hours of flying time, a plane is hit with a bolt of lightning. Recently, spacecraft have found gamma rays can be created by thunder storms, and according to new research presented at the American Geophysical Union annual meeting this week, the rays could be intense enough to cause radiation sickness.

"Everywhere we look, we're seeing x-rays and gamma rays flying out of thunderstorms and lightning," said Joseph Dwyer, a physicist at the Florida Institute of Technology and lead author of the study. "The gamma rays coming out of thunderstorms are so intense we can measure these 600 kilometers away and so bright that it almost blinds the spacecraft."

Finding particles flying around at what physicsts call "ultrarelativistic speeds,," i.e. very close to the speed of light, came as a shocker to physicists. Gamma rays had previously been associated with only the most extreme environments in the universe, like supernovae. Now, scientists believe that about 50 terrestrial gamma ray flashes occur per day on Earth.

It turns out that these highly-energetic particles can be created by thunderstorms at altitudes that airplanes regularly fly. While planes generally avoid thunderclouds, sometimes they get surprised or can't avoid them. And it's those situations that worry Dwyer.

"We just don't know enough. The consequences are bad enough that people could potentially get hurt from this," he said. "This is a call for more research. We really need to find out where we are and how big these things are. Could people be hit by these things and get sick? And how would you know?"

His team and others across the country have measured the number of gamma rays that reach satellite observatories, which has allowed them to back into how many particles are created in the thunderstorms.

What remains unclear is just how large and concentrated the source of the gamma rays is. If it's large — hundreds of meters across — then airplane passengers are probably safe. But if the gamma rays are created in a small area, they could be delivering doses of radiation in one millisecond that are many times beyond what the government sees as safe.

"If the source is a little bit smaller — and there are some arguments you can make that the source should be smaller — then the dose someone would get inside an aircraft, through a quarter inch of aluminum, is getting to the point where we'd be worried."

Dwyer's data shows that as the size of the gamma ray source shrinks under 100 meters across, the dose of radiation accelerates to levels that could cause very serious radiation sickness — and even death.

Dwyer is a leader in the emerging field of terrestrial gamma rays who has refined scientists' notions of how lightning works. Back in 2003, while experimenting with triggering lightning by launching specialized rockets into clouds, his team accidentally triggered a gamma ray flash.

"His stuff is fabulous," Dave Sentman, a physicist at the University of Alaska-Fairbanks, who also studies terrestrial gamma rays.

Sentman noted that Dwyer, trained as a nuclear physicist, has a different perspective on the atmospheric physics than most of their colleagues, which has allowed him to gain insight into the high-energy particles

"He has a different way of thinking," Sentman said.

Image: flickr/syne

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