In the 1990s, NASA scientists got an unexpected surprise. While scanning the cosmos for high-energy gamma radiation bursts from supernovas and black holes, they stumbled upon a curious finding. Apparently, bursts of gamma radiation were coming from thunderstorms on Earth. Yet, the full extent of this phenomenon remained a mystery — until now.
In two new papers published in Nature, researchers have unveiled that gamma radiation in thunderstorms is far more prevalent than previously thought. Using a retrofitted U2 spy plane to get a better look at these storms, scientists discovered that thunderstorms constantly generate gamma radiation in various forms.
“There is way more going on in thunderstorms than we ever imagined,” said Steve Cummer, an engineering professor at Duke University and a coauthor on both papers. “As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms.”
Gamma Rays in the Sky
Gamma rays, the highest energy form of light, are typically associated with violent cosmic events. But thunderstorms produce them too, albeit in a different form.
For the past three decades, scientists have known about these short but intense flashes of gamma radiation, known as terrestrial gamma-ray flashes (TGFs). They also understood that thunderstorms produce a lower-level, faint glow of gamma rays. The mechanism driving these bursts has long been attributed to the relativistic runaway electron avalanche (RREA). This phenomenon occurs when a single high-energy electron is accelerated by a thunderstorm’s electric field, colliding with air molecules to create more electrons and gamma rays in a cascading effect.
During a storm, updrafts carry water droplets, hail, and ice into clouds, creating an electric charge. Much like rubbing a balloon on a sweater, this mixture of particles builds up electrical energy. The result is a massive electric field capable of accelerating particles — such as electrons — to incredible speeds. When these high-energy electrons collide with air molecules, they unleash gamma rays and, in some cases, even antimatter.
However, until recently, gamma rays coming from thunderstorms seemed sporadic, given the limitations of previous satellite technology. NASA’s satellites — the kind meant to monitor cosmic events, such as cosmic gamma-ray bursts from exploding stars — needed to be in the right place at the right time to detect gamma radiation from Earth. And aircraft missions trying to fly close to thunderstorms often failed because the danger of getting too close to an active storm limited their scope.
Storm Chasing
The research team was led by Nikolai Østgaard, professor of space physics at the University of Bergen in Norway. The team used a NASA ER-2 High-Altitude Airborne Science Aircraft — a modified U2 spy plane. Flying at 12.4 miles above the Earth, which is three miles higher than most thunderstorms, the plane gave the team an ideal vantage point. Over a month, the ER-2 flew over storms in the tropics south of Florida. It recorded gamma radiation during 9 out of 10 flights. What seemed like a rare event incredibly turned out to occur in most thunderstorms.
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“The ER-2 is the perfect platform for these measurements. It can fly at extremely high altitudes (twice as high as regular aircraft) and so can be just a few miles above the thunderstorm in a much safer environment but still very close to the source region to catch any weaker TGFs. And it can fly back and forth across a single form for a long time so that we can finally get a good picture of how much gamma-ray production can happen in an active storm,” Cummer told ZME Science.
There were important logistical and technical challenges. For example, one of the main difficulties was choosing which storms to target. Because it took hours for NASA’s ER-2 aircraft to reach many of these storms, there was always the risk that the storm would dissipate before they could collect data. The research campaign only had about 60 flight hours available, so the team had to be incredibly selective to avoid wasting valuable time on storms that wouldn’t yield results.
“The planners did an amazing job, given how successful the measurements were,” Cummer said.
Østgaard explains in an email for ZME Science how they managed to overcome this challenge using real-time data from the ER-2. The plane was equipped with a system that sent low-resolution data back to the team on the ground, allowing them to monitor when the aircraft entered a gamma-ray-emitting cloud. “We would know when the ER-2 entered a gamma-ray glowing cloud and instruct the pilot to return to the cloud as long as it was glowing,” Østgaard said.
New Insights from Above the Clouds
Incredibly, the team hit the jackpot on their very first flight. The ER-2 encountered a glowing cloud that continued emitting gamma rays for at least 30 minutes, sparking excitement in the control room. “Lots of ‘whoaas’ in the room then,” Østgaard recalled.
Martino Marisaldi, a professor of physics at the University of Bergen, described the gamma activity as resembling a “boiling pot,” constantly churning with energy.
The team believes that thunderstorms may release gamma rays like steam escaping from a kettle, allowing storms to release energy without culminating in a massive burst of lightning. “It is way, way more important than we thought,” Cummer added.
However, gamma rays produced by thunderstorms on Earth are fundamentally different from those generated by cosmic events like black holes or exploding stars in terms of their scale and intensity. Cosmic gamma rays are born from cataclysmic processes, releasing energy on an astronomical scale. These events create some of the most powerful explosions in the universe. Their gamma rays emit across vast distances that can be detected billions of light-years away.
By contrast, the gamma rays produced by thunderstorms are more localized and short-lived, driven by electric fields within the storm. Yet, Cummer points out the striking similarity between the two:
“Something as ordinary as a thunderstorm is one of the only other natural processes in the known universe that generates bursts of particles with such high energy. Obviously on a smaller spatial scale, but that is still amazing to me — exploding stars and thunderstorms make the same kinds of high-energy particles,” he told ZME Science in an email.
The Mystery Deepens
While the discovery of frequent gamma radiation is fascinating, the team’s work also revealed something even more intriguing. They detected two entirely new types of gamma-ray bursts. These bursts, shorter and more intense than the ones captured by satellites, occurred in unexpected ways, including before traditional lightning strikes.
Traditional terrestrial gamma-ray flashes (TGFs) were already known to occur alongside lightning, but the recent observations uncovered gamma-ray bursts happening in completely different scenarios. These bursts were detected without any clear connection to lightning and even appeared before any associated lightning strikes.
These newly observed TGFs are too dim to be picked up by satellites, which is why they hadn’t been noticed before. The ER-2 aircraft, flying close to thunderstorms, was able to detect more than 100 of these dimmer TGFs, far more than expected.
“At least many and maybe all of these non-lightning-associated TGFs seem to be followed very closely in time (less than a second) by a traditional lightning discharge. This is hinting that the TGF process may play a role in initiating a lightning discharge, which is something that has never been fully understood,” Cummer said.
Should We Worry?
The findings might sound alarming, but there’s no need to worry about a sudden spike in radiation exposure. The amount of gamma radiation generated in thunderstorms isn’t enough to harm anyone flying near a storm. “The radiation would be the least of your problems if you found yourself there,” Cummer said. As he pointed out, pilots already avoid flying through the turbulent cores of thunderstorms.
However, the risks for equipment like satellites or military aircraft that operate at high altitudes cannot be ruled out.
“The presence of nonnegligible gamma radiation in and above thunderstorms for long durations is definitely something that needs to be considered for any equipment that needs to operate in that environment. That said, there aren’t many things that I know of that operate at those altitudes. But it is certainly a risk that needs to be factored into the design if anything that would,” Cummer added.
As scientists continue to dig into the data, the findings point to one clear conclusion: thunderstorms are far more complex than we ever imagined. In fact, the new research may raise more questions than it answers. The discovery of previously unknown types of gamma radiation hints at deeper processes within storms.
“It has always been amazing to me that thunderstorms can generate high energy gamma-rays, electrons, even antimatter in the form of positrons, and even nuclear processes in the form of knocking neutrons out of oxygen and nitrogen atoms. But these seemed to happen at rates that made the processes very interesting but maybe not important in the big picture. But now we have seen that this high energy process is almost omnipresent in big thunderstorms and plays a major role in the evolution of thunderstorm electric fields and discharging thunderstorms. Then the cherry on top is the hint that the process may play a role in initiating lightning and actually be the thing that, at least sometimes, creates the spectacular lightning flash that we (or at least I) know and love,” Cummer concluded.
The findings appeared in Nature.