“Dark lightning,” coined by Florida Tech Professor Joe Dwyer, could blast airline passengers with gamma rays who fly near the top of a thunderstorm, with doses equal to about 10 chest x-rays. At the middle of a storm, an altitude of about 16,000 feet, “the radiation could be comparable to 400 chest x-rays or roughly equal to a full-body CT scan,” he says.
Although planes that fly into thunderstorms at dangerous levels are pretty rare, and the bursts of radiation occur only over extremely brief periods, such scenarios have caught the attention of Dwyer and his team.
“We know that commercial airplanes are typically struck by lightning once or twice a year,” says Dwyer. “What we don’t know is how often planes happen to be in just the right place to receive a high radiation dose. We believe it is very rare, but we’ll continue to look for answers to this question.”
Dwyer and co-investigating Florida Tech professors Ningyu Liu and Hamid Rassoul have made advances over the past five years. In spring 2013, they presented their latest models at a press conference of the European Geosciences Union in Vienna,. Their findings resulted in dozens of follow-up stories in the international media.
To understand exactly how thunderstorms manage to produce high-energy radiation, the scientists showed how, instead of creating normal lightning, thunderstorms can sometimes produce an exotic kind of electrical breakdown that involves high-energy electrons and their anti-matter equivalent called positrons. The interplay between the electrons and positrons causes an explosive growth in the number of these high-energy particles. They emit the observed terrestrial gamma ray flashes while rapidly discharging the thundercloud, sometimes faster than normal lightning. Even though copious gamma-rays are emitted by this process, very little visible light is produced, creating a kind of electrical breakdown within the storms Dwyer calls “dark lightning.”
With funding in part by the Defense Advanced Research Projects Agency’s NIMBUS program, the team’s recent modeling work of dark lightning calculates the radiation doses received by individuals inside aircraft that happen to be in exactly the wrong place at the wrong time. The model also explains many of the observed properties of terrestrial gamma ray flashes. Their next step is to make direct measurements using aircraft and balloons inside the thunderstorms, a potentially risky endeavor.
Not to worry, Dwyer concludes: “The radiation from dark lightning is not something that people need to be frightened about and it’s no reason to avoid flying. I have no problem getting on a plane with my family.”