Researchers have known for many years that thunderstorms can serve as small particle accelerators, generating antimatter, gamma rays, and various nuclear effects. However, they were unsure of how frequently this occurs. Recent observations made by a modified U2 spy plane have revealed that nearly all large thunderstorms emit gamma rays in various dynamic, surprising, and previously unknown manners.
In the 1990s, NASA’s satellites, designed to detect high-energy particles from supernovas and astronomical phenomena, made an unexpected discovery: high-energy gamma radiation bursts emanating from our planet.
Though researchers quickly identified thunderstorms as the source of these potent particles, the frequency of this occurrence was still unclear. Satellites were not equipped to detect gamma radiation from Earth and needed to be positioned just right for precise observations.
After years of relying on suboptimal platforms, a team of scientists finally gained the opportunity to utilize a NASA-modified U2 spy plane to properly investigate thunderstorms. In two recent studies published on October 3 in Nature, they found that gamma radiation produced in thunderstorms is much more prevalent than previously thought, and the mechanisms behind this radiation contain numerous unsolved mysteries.
“There’s far more happening in thunderstorms than we ever imagined,” remarked Steve Cummer, the William H. Younger Distinguished Professor of Engineering at Duke University and coauthor of both studies. “It turns out that virtually all large thunderstorms continuously generate gamma rays in a multitude of ways.”
The fundamental physics of how thunderstorms produce high-energy bursts of gamma radiation is well understood. As thunderstorms develop, swirling air drafts propel water droplets, hail, and ice into a mix that generates an electric charge—similar to rubbing a balloon on your shirt. Positively charged particles rise to the storm’s peak, while negatively charged particles sink to the base, creating a powerful electric field that can measure up to 100 million stacked AA batteries.
When other charged particles, like electrons, are subjected to such a strong electric field, they accelerate. If they reach sufficient speeds and collide with an air molecule, they can dislodge more high-energy electrons. This reaction continues to cascade until it generates enough energy for nuclear reactions, leading to intense and rapid flashes of gamma rays, antimatter, and various radiation types.
However, the story of gamma radiation from thunderstorms doesn’t end there. Aircraft flying near these storms have also detected a faint gamma radiation glow emanating from the clouds. While these storms seem capable of producing a low-level radiation simmer, something prevents it from resulting in an explosive burst like a popcorn kernel.
“Previous aerial campaigns attempted to determine whether this phenomenon was common, but results were mixed, and many campaigns over the United States found no gamma radiation,” Cummer noted. “Our project was aimed at definitively answering these questions.”
The research team gained access to a NASA ER-2 High-Altitude Airborne Science Aircraft. A retrofitted U2 spy plane from the Cold War, it soars at altitudes twice that of commercial flights, roughly three miles above most thunderstorms. Its high speed allowed the team to carefully select the thunderstorms they expected to yield results.
“The ER-2 is the ultimate platform for observing gamma rays from thunderclouds,” explained Nikolai Østgaard, a space physics professor at the University of Bergen in Norway and the project lead. “Flying at 20 km [12.4 miles], we can go directly over the tops of clouds, as close to the gamma-ray source as possible.”
Given the ER-2’s capabilities, the researchers hypothesized that if these events were rare, they wouldn’t observe many occurrences. Conversely, if they were common, they would likely see a significant number.
And they indeed observed a significant amount.
During a month-long study, the ER-2 conducted 10 flights over large thunderstorms in the tropics south of Florida; 9 of these flights detected this low-level gamma radiation simmer, surprisingly more variable than anticipated.
“The dynamics of these gamma-emitting thunderclouds drastically differ from the formerly held view of static glows, now resembling the behavior of a large, boiling pot in both pattern and activity,” stated Martino Marisaldi, a physics and technology professor at the University of Bergen.
Considering the size of typical tropical thunderstorms, which can be much larger than storms at other latitudes, this suggests that over half of all thunderstorms in the tropics are producing radiation. The researchers propose that this low-level gamma radiation acts like steam escaping from a pot of water, regulating how much energy accumulates inside.
Additionally, the team was thrilled to detect numerous instances of short, intense gamma radiation bursts coming from the same thunderstorms. Some closely resembled the bursts detected by the NASA satellites. These bursts predominantly occurred alongside active lightning discharges, indicating the potent electric field generated by lightning likely augments the already high-energy electrons, causing them to initiate strong nuclear reactions.
Moreover, researchers noted at least two other previously unseen types of short gamma radiation bursts. One variety lasted less than a thousandth of a second, while the other consisted of about ten individual bursts repeating over about a tenth of a second.
“The two new forms of gamma radiation are particularly intriguing,” Cummer remarked. “They don’t appear to be linked to developing lightning flashes, emerging spontaneously instead. Preliminary data suggests they could actually relate to the processes that trigger lightning—a phenomenon scientists are still trying to understand.”
Addressing concerns about potential dangers from gamma radiation, Cummer reassured that the levels produced wouldn’t pose a threat unless one were extremely close to the source.
“If you were near the source, the radiation would be the least of your worries—airplanes avoid flying within active thunderstorm cores due to the severe turbulence and winds,” Cummer said. “Knowing what we know now, I don’t have any greater worries about flying than I did before.”
This research was funded by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 320839, and the Research Council of Norway under contracts 223252/F50 (CoE) and 325582.
