Researchers recently flew a NASA retrofitted U2 spy plane over storms, and in two papers published on October 3 in 'Nature', they revealed that gamma radiation in thunderstorms is far more frequent than previously thought. The findings also suggest that there are still many unknowns in the dynamics that generate this radiation.
"There is way more going on in thunderstorms than we ever imagined," said Steve Cummer, the William H. Younger Distinguished Professor of Engineering at Duke University and coauthor of both papers. "As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms."
The basic physics behind thunderstorms producing gamma radiation isn't new. As thunderstorms form, updrafts drive water, hail, and ice, creating an electrical charge. Positively charged particles rise to the top of the storm, while negative charges settle at the bottom, forming an immense electric field. When particles like electrons accelerate in this field, they can knock electrons off air molecules, eventually producing gamma rays, antimatter, and other radiation.
Aircraft flying near thunderstorms have also detected low-level gamma radiation glowing faintly from the clouds. However, there was uncertainty about how widespread this phenomenon is. Some aircraft campaigns found no gamma radiation, but the current project aimed to clarify this once and for all.
Researchers utilized NASA's ER-2 High-Altitude Airborne Science Aircraft, a Cold War-era U2 spy plane, to fly above storms and detect gamma rays. Flying over 12 miles high, the aircraft allowed the team to study gamma rays from as close as possible to the source. The team expected to see very little gamma radiation if the phenomenon were rare, but their observations confirmed its commonality.
During 10 flights over tropical storms, 9 yielded gamma radiation. "The dynamics of gamma-glowing thunderclouds starkly contradicts the former quasi-stationary picture of glows, and rather resembles that of a huge gamma-glowing boiling pot," said Martino Marisaldi, professor of physics at the University of Bergen.
The data suggest that over half of all tropical thunderstorms produce gamma radiation. The researchers theorize that this low-level radiation limits the energy buildup inside storms, similar to steam releasing from a boiling pot.
The study also uncovered two new forms of short gamma radiation bursts, neither of which had been seen before. These bursts, which last less than a thousandth of a second, might even help scientists understand the mystery of how lightning forms.
"Those two new forms of gamma radiation are what I find most interesting," said Cummer. "There are hints that they may be linked to the processes that initiate lightning flashes, which are still a mystery."
Cummer reassured that despite these findings, people need not worry about exposure to gamma radiation from thunderstorms. The radiation would only be dangerous if someone were extremely close to the source. "Even knowing what we now know, I don't worry about flying any more than I used to," he said.
This research was supported by the European Research Council and the Research Council of Norway.
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