Surprising amount and variety of gamma radiation produced by large tropical storms, observations reveal

In the 1990s, NASA satellites built to detect high-energy particles from supernovas and other celestial objects discovered a surprise: bursts of high-energy gamma radiation originating right here on Earth.

Although it didn’t take long for researchers to figure out that these supercharged radioactive particles were coming from thunderstorms, how often this phenomenon occurred remains a mystery. Satellites were not built to detect gamma radiation coming from Earth, and to do so they had to be in the right place at the right time.

After years of making do with platforms that weren’t ideal for the task, a group of scientists got the opportunity to fly a modernized NASA-owned U2 spy plane above the storms to take a look. of eye. In two new articles published in Naturethe team discovered that gamma radiation produced by thunderstorms is much more common than previously thought and that the dynamics creating this radiation hold a treasure trove of mysteries yet to be solved.

“There’s a lot more going on in thunderstorms than we ever imagined,” said Steve Cummer, the William H. Younger Distinguished Professor of Engineering at Duke University, co-author of both papers. “It turns out that virtually all large storms generate gamma rays all day long in many different forms.”

The general physics behind how thunderstorms create flashes of high-energy gamma radiation is no mystery. As thunderstorms develop, swirling air currents bring water droplets, hail and ice into a mix that creates an electrical charge, much like rubbing a balloon on your shirt.

Positively charged particles end up at the top of the storm while negatively charged particles fall to the bottom, creating a huge electric field that can be as powerful as 100 million AA batteries stacked end to end.

When other charged particles, such as electrons, are in such a strong field, they accelerate. If they accelerate to high enough speeds and hit an air molecule, they knock out more high-energy electrons. The process cascades until the collisions have enough energy to create nuclear reactions, producing extremely powerful and extremely fast flashes of gamma rays, antimatter, and other forms of radiation.

But that’s not the end of the gamma radiation story from storms. Planes flying near thunderstorms have also seen a faint glow of gamma radiation from the clouds. These storms appear to have enough energy to produce a faint quiver of gamma radiation, but something prevents it from creating an explosive blast like a popping kernel of corn.

“A few aerial campaigns have attempted to determine whether or not these phenomena are common, but the results have been mixed, and several campaigns over the United States have detected no gamma radiation,” Cummer said. “This project was designed to answer these questions once and for all.”

The research group secured the use of a NASA ER-2 high-altitude airborne scientific aircraft. A modernized U2 spy plane, a holdover from the Cold War, it flies more than twice as high as commercial planes and about three miles above most thunderstorms. It’s also extremely fast, giving the team the ability to choose the exact storms they believe are most likely to produce results.

“The ER-2 aircraft would be the ultimate observation platform for gamma rays from storm clouds,” said Nikolai Østgaard, professor of space physics at the University of Bergen in Norway and principal investigator of the project. “By flying at 20 km (12.4 miles), we can fly directly above the cloud tops, as close as possible to the gamma ray source.”

Because the ER-2 was the perfect solution and the team was going to fly over the right storms, the researchers figured that if these phenomena were rare, they would see almost none at all. But if they were common, they would see a lot of things.

And they saw a lot.

Over the course of a month, the ER-2 made 10 flights over severe storms in the tropics south of Florida, and nine of them observed gamma radiation this summer, also more dynamic than expected.

“The dynamics of gamma-ray storm clouds clearly contradicts the old quasi-stationary picture of glows and instead resembles that of a huge boiling pot of gamma-ray radiation, both in pattern and behavior,” said Martino Marisaldi, professor of physics and technology at the University. from Bergen.

Given the size of a typical storm in the tropics, which is much larger than storms at other latitudes, this suggests that more than half of all storms in the tropics are radioactive. The researchers posit that this low output of gamma radiation acts like boiling steam in a pot of water and limits the amount of energy that can be stored inside.

The researchers were also excited to see many examples of intense, short-lived gamma-ray bursts from the same storms. Some of them looked precisely like those originally detected by NASA satellites. These almost always occurred in conjunction with an active lightning discharge.

This suggests that the large electric field created by lightning likely overcharges the already high-energy electrons, allowing them to create high-energy nuclear reactions.

But there were also at least two other types of short bursts of gamma radiation that had never been observed before. One type is incredibly short, less than a thousandth of a second, while the other is a sequence of about 10 individual bursts that repeat over a duration of about a tenth of a second.

“These two new forms of gamma radiation are what I find most interesting,” Cummer said. “They do not appear to be associated with the development of lightning. They emerge spontaneously in one way or another. The data suggests that they may in fact be linked to the processes that trigger lightning, which still remains a mystery to scientists. scientists.”

If there’s anyone worried about being turned into the Hulk by all that gamma radiation, Cummer added, they shouldn’t be. The amount of radiation produced would only be dangerous if a person or object was quite close to the original source.

“Radiation would be the least of your problems if you were there. Planes avoid flying into active storm cores because of the turbulence and extreme winds,” Cummer said. “Even knowing what we know now, I don’t worry about flying any more than I did before.”