In the 1990s, NASA satellites designed to detect high-energy particles from celestial events like supernovas made an unexpected discovery - bursts of high-energy gamma radiation coming from thunderstorms on Earth.
Researchers quickly connected these gamma-ray bursts to thunderstorms, but questions lingered about whether this was a common phenomenon. Satellites weren’t optimized to track gamma radiation from Earth, and they had to be in the right place at the right time to capture the events.
Now, thanks to a new approach, a group of scientists has been able to get a clearer picture. By retrofitting a NASA U2 spy plane to fly over thunderstorms, the team made some surprising discoveries.
Thunderstorms produce gamma radiation
In two papers published in the journal Nature , the experts have revealed that thunderstorms produce gamma radiation far more frequently than previously thought.
Additionally, the complex dynamics driving these radioactive particles have opened up a trove of new mysteries.
“There is way more going on in thunderstorms than we ever imagined,” said co-author Steve Cummer, a professor of engineering at Duke University . “As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms.”
Physics of thunderstorm radiation
The basic physics of how thunderstorms create high-energy gamma radiation flashes isn’t entirely new. When thunderstorms build, rising air currents and swirling particles like water droplets, hail, and ice create electric charges.
These charges separate, with positive charges rising to the top of the storm and negative ones sinking to the bottom, forming an electric field strong enough to equal 100 million AA batteries stacked together.
If charged particles like electrons find themselves in this electric field, they accelerate. Once they hit air molecules at high speeds, they knock off more electrons, creating a chain reaction that eventually leads to nuclear reactions. This results in flashes of gamma rays, antimatter , and other forms of radiation.
Radioactive glow in thunderstorms
But there’s more to the story. Aircraft flying near thunderstorms have detected a faint gamma radiation glow from the clouds - energy that’s strong enough to create gamma rays but lacks the explosive burst seen in more intense gamma events. It’s like a simmering pot, producing gamma radiation without the full blast.
“A few aircraft campaigns tried to figure out if these phenomena were common or not, but there were mixed results, and several campaigns over the United States didn’t find any gamma radiation at all,” Cummer said. “This project was designed to address these questions once and for all.”
To solve this mystery, the research team used a NASA ER-2 High-Altitude Airborne Science Aircraft, a retrofitted U2 spy plane. This plane, a relic of the Cold War, flies twice as high as commercial jets and about three miles above most thunderstorms. Its speed allowed the researchers to pinpoint the most promising storms for study.
“The ER-2 aircraft would be the ultimate observing platform for gamma-rays from thunderclouds,” said Nikolai Østgaard, a professor of space physics at the University of Bergen in Norway and the project’s lead investigator. “Flying at 20 km [12.4 miles], we can fly directly over the cloud top, as close as possible to the gamma-ray source.”
The team wasn’t sure what to expect. If gamma radiation was rare, they might not see much. But if it was common, they expected to find plenty. And they were not disappointed.
Radioactive thunderstorms are common
Over a month of flights, the ER-2 flew over 10 major tropical storms south of Florida, detecting gamma radiation in 9 of those storms. The radiation came in more dynamic and varied forms than previously expected.
“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 both in pattern and behavior,” said Martino Marisaldi, a professor of physics and technology at the University of Bergen.
Tropical storms are much larger than storms at other latitudes, and the team now believes that more than half of all tropical thunderstorms are radioactive, with low-level gamma radiation simmering away like steam from a boiling pot.
This simmering effect may act as a pressure valve, releasing energy from the storm before it can reach a full gamma-ray explosion.
Lightning may trigger the radiation
But the team also observed several intense, short bursts of gamma radiation. Some matched the kind of bursts first discovered by NASA satellites, occurring alongside lightning discharges.
This suggests that lightning supercharges the electrons in the storm, triggering nuclear reactions that release gamma radiation.
Yet, intriguingly, two other types of gamma bursts had never been seen before. One type lasts less than a thousandth of a second, while the other consists of a series of around 10 bursts, occurring over a tenth of a second.
“Those two new forms of gamma radiation are what I find most interesting,” said Cummer. “They don’t seem to be associated with developing lightning flashes.”
“They emerge spontaneously somehow. There are hints in the data that they may actually be linked to the processes that initiate lightning flashes, which are still a mystery to scientists.”
Are these thunderstorms dangerous?
For anyone worried about the potential dangers of gamma radiation during thunderstorms, Cummer offered some reassurance. The amount of radiation produced isn’t enough to pose a danger to people on the ground or in planes, unless they were extremely close to the source.
“The radiation would be the least of your problems if you found yourself there. Airplanes avoid flying in active thunderstorm cores due to the extreme turbulence and winds,” Cummer said. “Even knowing what we now know, I don’t worry about flying any more than I used to.”
This research has revealed just how much more there is to learn about thunderstorms and the radioactive gamma radiation they produce. With the new data in hand, scientists are eager to continue exploring the mysteries of these high-energy storms.
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