San Francisco -- The energy released by a lightning bolt is so strong that it creates an intense flash of light and usually loud thunder. But recent data taken by spacecraft and a few crazy research pilots reveals that lightning also often emits an intense burst of X-rays and gamma rays. The bursts typically radiate outward from the tops of thunderstorms, around seven miles up—just about the cruising altitude of an airliner.
Okay, don’t panic. Although both types of radiation can penetrate a plane’s skin and strike passengers inside, the plane has to be in exactly the right place at exactly the right time. And the plane’s electronics will probably not be affected by the radiation, so the flight itself would not be in danger. Then again, the typical burst of gamma rays is roughly equivalent to a year of background radiation on earth, and the X-rays are hundreds of times more energetic than the typical medical X-ray—about the equivalent of getting an entire CT body scan in just a few seconds.
Three scientists presented the findings today at the American Geophysical Union fall meeting here, noting that they and their fellow researchers are all “surprised” at how common the bursts are, according to Joseph Dwyer, a physicist who has just moved from the Florida Institute of Technology to the University of New Hampshire.
Although scientists have known for about 20 years that lightning can emit so-called terrestrial gamma-ray flashes, they thought the events were rare. But better sensors in space now show that the bursts happen in all kinds of thunderstorms, “even those that appear to be so weak that a meteorologist wouldn’t look twice at them,” noted researcher Themis Chronis, from the University of Alabama in Huntsville.
The improved sensors, onboard spacecraft such as the NASA Fermi Gamma-ray Space Telescope, and on ground-based radar and lightning detectors, can now pick up the bursts much more readily (see the video below). The clinching data, however, came from a brazen set of airplane flights. For five days, pilots flew an Airbus airliner fitted with sensors right through big thunderstorms, which confirmed many details about the energy in the bursts. The pilots flew back and forth between France and Italy, putting the plane inside thunderstorms, in one case for five hours straight, managing to keep the plane airborne through severe turbulence. X-rays and gamma rays were detected in numerous lightning bolts that flashed close to the plane.
As best as scientists can tell, gamma bursts come from the same mechanism that causes lightning to flash. Positive and negative charges accumulate in the thunderclouds, and when the difference in charge becomes strong the entire electrical field breaks down the insulating properties of air, sparking a bolt of lighting. When lightning flashes within the storm clouds themselves, an avalanche of fast moving electrons is liberated. They move upward out of the top of the thunderclouds, and when they are deflected there by air molecules, they emit gamma rays. “The storms act like a natural particle accelerator,” Chronis said. He noted that gamma rays are usually not detected in lower portions of thunderclouds, probably because so much air and moisture is present that it attenuates any emissions.
Scientists are less sure about the mechanism that causes the X-ray emissions. Indeed, “many mysteries remain” about how the bursts form, and even about how thunderstorms work, Dwyer said. For example, it appears that most of the X-ray transmission comes from the “leader phase” of the lightning—the spark that moves through the clouds. But researchers cannot explain why a given burst is weak or strong.
Dwyer and his fellow scientists are eager to learn more, and they will get their chance soon. A Geostationary Lightning Mapper sensor will go up on the GOES-R satellite that will be launched in late 2015.
Dwyer stressed that airline passengers, and even pilots, should not be concerned about being exposed to radiation from the bursts. The cross-section of air that is affected is only about the size of a football field, and the burst lasts for only thousands of a second. Exposure is possible, but as in life, timing is everything. Other risks associated with flying are still much more pressing. However, research pilots who fly planes into thunderstorms might have much more to worry about; given how intense the bursts are, even a few exposures could add up to a lot.
Video courtesy of NASA Goddard Space Flight Center