You've probably never seen it, but it's possible you've been exposed to it if you've ever flown through a thunderstorm. Dark lightning, flashes of gamma rays that occur at altitudes in which commercial aircraft fly, doesn't produce much light, but it does produce radiation.
New research presented Wednesday at a meeting of the European Geosciences Union in Vienna pinpoints the amount of radiation that dark lightning produces -- and how much pilots and passengers might be getting exposed to.
"The good news is that the doses are not super scary -- it could be worse," said lead research Joseph Dwyer, a physics professor at Florida Institute of Technology. "It's similar to going to the doctor's office and getting a CT scan."
The existence of dark lightning itself was discovered on a NASA spacecraft in 1994. In the electrical fields of a thunderstorm, electrons zoom close to the speed of light, colliding with atoms to emit the gamma rays.
In 2010, Dwyer and colleagues determined that dark lightning occurred at altitudes where airplanes commonly fly. That prompted the current work, which involved a physics-based model that can show exactly how the discharge happens.
The preliminary work showed how much radiation was being emitted, but the size of the space it was produced in was unclear. With the current model, Dwyer's team was able to pinpoint the exposure dose that someone on an airplane would likely receive.
"This work is very important because it gets you into the zone where you start to understand how often and how likely they are to happen," said University of California Santa Cruz physics professor David Smith, who has worked with Dwyer but was not involved in the modeling work.
The next step, Smith said, is to start determining how often the flashes occur. Because the bursts are so brief -- about 10-100 of microseconds -- they are usually undetected, although it's possible you could see a diffuse, purple light, Dwyer said.
"Unless you have gamma ray detectors on board, no one would think anything of it," Smith said.
The National Science Foundation is currently working on an armored plane that could fly through thunderstorms, Smith said. If an instrument were placed on board, researchers may begin to get a better idea of the frequency of the flashes. Currently, the bursts are thought to occur much less frequently than the lightning we see, but that could mean anywhere from 1/100th to 1/1000th as often, Smith said.
"It's a very rough number," Smith said. "The other question is, are there somewhat weaker ones that happen more often?"
Until those questions are answered, researchers say there's no need for pilots to change course, since avoiding thunderstorms is already part of the gig. Depending on future findings, though, frequent flyers may want to watch the weather when they fly.
"It's kind of cool that it's been 250 years since Benjamin Franklin's kite experiment, and we've realized there's a different kind of lightning going on that we never knew about," Dwyer said.