Spy Plane Uncovers Radioactive Gamma Rays in Tropical Thunderstorms

Thunderstorms in the tropics are literally radioactive, according to a team of researchers that recently used a retrofitted spy plane to survey the phenomena.

The team’s research was published in two papers in Nature this morning. One of the papers describes the frequent emission of long-duration gamma-ray glows across sweeping parts of the atmosphere during tropical thunderstorms. The other paper goes into detail about the newly identified phenomenon—dubbed flickering gamma-ray flashes, or FGFs—and ponders the relationship between the flashes and other forms of gamma radiation from thunderclouds.

“FGFs are typically much weaker than the well-known Terrestrial Gamma-ray Flashes (TGF) observed from space since the 90s,” said Martino Marisaldi, a physicist at the University of Bergen in Norway and lead author of one of the papers (and co-author of the other), in an email to Gizmodo. “This is why you need to get close to the thundercloud (possibly on top of it, as we did) to detect them.”

The team used a NASA-owned spy plane to fly above tropical thunderstorms and take a gander that the gamma radiation from the storms. The plane—an ER-2, a modified version of Lockheed Martin’s U-2 aircraft—detected gamma-ray glows lasting for hours and covering an area of nearly 3,500 square miles (9,065 square kilometers).

Gamma-ray glows and terrestrial flashes were previously known to come from thunderstorms, but the newly described FGFs are a sort of goldilocks radiation, lasting longer than the terrestrial bursts but much shorter than the glows.

“FGFs are remarkably different from both TGFs and glows but carry some features of both,” Marisaldi said. “This is why we figure them as the missing link between the two phenomena. FGFs are radio silent, but they are often followed, within milliseconds, by radio pulses called Narrow Bipolar Events (NBE) which are often associated to the initiation of lightning. Therefore, it is intriguing to hypothesize a causal connection between the two phenomena.”

Previous research that had taken gamma radiation data from thunderstorms from space established a relationship between TGFs based on the number of times those gamma flashes preceded lightning. But using the spy plane, the recent team found over 100 TGFs that were too weak to spot from the ground.

“This means that TGF lightning ratio is not 1-to-10000 as established from space observations but 1-to-100 or even higher,” said Nikolai Østgaard, also a physicist at the University of Bergen and lead author of one of the papers (and co-author of the other), in an email to Gizmodo. “All the transient (TGF: microseconds, FGF and glow burst: tens of milliseconds) were followed by intense lightning activity, and might play an important role in lightning initiation.”

Lightning is a spark of electricity in Earth’s atmosphere that either stays up there or hits the ground in brilliant tendrils of energy. Despite about 40,000 thunderstorms happening on Earth each day, lightning—which often is produced by thunderstorms—remains an unknown quantity.

“The glows, which typically lasted a few seconds, were highly dynamic and bubbled up as though the storm were a boiling cauldron — contradicting the view that glows are slowly varying and featureless,” said Joseph Dwyer, a physicist at the University of New Hampshire at Durham, in an accompanying News & Views article. “Flickering [gamma-ray] flashes resemble the multi-pulsed TGFs that have been observed from satellites in space, suggesting that there is one underlying mechanism for this wide range of energetic phenomena.”

Dwyer added that the team’s research “effectively rules out” other theorized mechanisms for lightning initiation, like cosmic rays and lightning leaders. Positron feedback, Dwyer added, is “the last mechanism standing.”

The new research lays the groundwork to further pin down the relationship between the gamma outbursts and thunderclouds, as well as lightning and its sources. Because many of the recently observed gamma flashes occurred without lightning present, it appears that the electrified storms themselves are capable of generating gamma rays.

The team’s findings aren’t entirely shocking—lightning aside, different types of gamma radiation were previously known from tropical storms. But the new research fills out an increasingly complex portrait for thunderclouds, one of the most jaw-dropping natural phenomena on Earth.