Most tropical lightning storms are radioactive
New observations from a retrofitted U2 spy plane reveal a surprising amount and variety of gamma radiation is produced in large tropical thunderstorms
Duke University
DURHAM, N.C. – In the 1990s, NASA satellites built to spot high-energy particles coming from supernovas and other celestial-sized objects discovered a surprise — high energy gamma radiation bursts coming from right here on Earth.
While it didn’t take long for researchers to figure out that these radioactive supercharged particles were coming from thunderstorms, how commonly the phenomenon happened remained a mystery. Satellites weren’t built to find gamma radiation coming from Earth, and they had to be in just the right place at just the right time to do so.
After years of making do with platforms not ideal for the task, a group of scientists secured an opportunity to fly a retrofitted U2 spy plane owned by NASA over storms to take a proper look. In two new papers published October 3 in Nature, the team discovered that gamma radiation produced in thunderstorms is far more common than anyone thought and that the dynamics creating the radiation hold a treasure trove of mysteries yet to be solved.
“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, who was a coauthor on both papers. “As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms.”
The general physics behind how thunderstorms create high-energy flashes of gamma radiation is not a mystery. As thunderstorms develop, swirling drafts drive water droplets, hail and ice into a mixture that creates an electric charge much like rubbing a balloon on your shirt. Positively charged particles end up at the top of the storm while negatively charged particles drop to the bottom, creating an enormous electric field that can be as strong as 100 million AA batteries stacked end-to-end.
When other charged particles — such as electrons — find themselves in such a strong field, they accelerate. If they accelerate to high enough speeds and happen to strike an air molecule, they knock off more high-energy electrons. The process cascades until the collisions have enough energy to create nuclear reactions, producing extremely strong and extremely fast flashes of gamma rays, antimatter and other forms of radiation.
But that’s not the end of the thunderstorm gamma radiation story. Aircraft flying close to thunderstorms have also seen a faint glow of gamma radiation coming from clouds. These storms seem to have enough energy to produce a low-level simmering of gamma radiation, but something prevents it from creating an explosive burst like a popping corn kernel.
“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.”
The research group secured the use of a NASA ER-2 High-Altitude Airborne Science Aircraft. A retrofitted U2 spy plane left over from the Cold War, it flies over twice as high as commercial aircraft and about three miles above most thunderstorms. It’s also extremely fast, giving the team the opportunity to pick the exact thunderstorms they thought were most likely to produce results.
“The ER-2 aircraft would be the ultimate observing platform for gamma-rays from thunderclouds,” said Nikolai Østgaard, professor of space physics at the University of Bergen in Norway and lead investigator of the project. “Flying at 20 km [12.4 miles], we can fly directly over the cloud top, 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, then they’d barely see any at all. But if they were common, then they’d see a lot.
And they saw a lot.
Over the course of a month, the ER-2 flew 10 flights over large storms in the tropics south of Florida, and 9 of them yielded observations of this simmer of gamma radiation, which was also more dynamic than 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, professor of physics and technology at the University of Bergen.
Given the size of a typical thunderstorm in the tropics, which get much larger than storms at other latitudes, this suggests that more than half of all thunderstorms in the tropics are radioactive. The researchers postulate that this low-level production of gamma radiation acts like steam boiling off a pot of water and limits how much energy can be built up inside.
The researchers were equally excited to see numerous examples of short duration and intense gamma radiation bursts coming from the same thunderstorms. Some of these were precisely like those that were originally detected by the NASA satellites. These almost always occurred in conjunction with an active lightning discharge. This suggests that the large electric field created by lightning is likely supercharging the already high-energy electrons, enabling them to create high-energy nuclear reactions.
But there were also at least two other types of short gamma radiation bursts that had never been seen 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 the course of about a tenth of a second.
“Those two new forms of gamma radiation are what I find most interesting,” Cummer said. “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.”
If there is anybody out there worried about getting turned into the Hulk by all of this gamma radiation, Cummer added, they shouldn’t be. The amount of radiation being produced would only be dangerous if a person or object were quite close to the origination 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 was supported 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 contract 325582.
CITATIONS: Østgaard N., Mezentsev A., Marisaldi M., Grove J. E., Quick M., et al., Flickering Gamma-Ray Flashes, the Missing Link between Gamma Glows and TGFs. Nature Letter, DOI: 10.1038/s41586-024-07893-0 (2024)
Marisaldi, M., Østgaard, N., Lang, T., Sarria D., Mezentsev, A., et al. Highly dynamic gamma-ray emissions are common in tropical thunderclouds, Nature Letter, DOI: 10.1038/s41586-024-07936-6 (2024)
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Journal
Nature
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Flickering Gamma-Ray Flashes, the Missing Link between Gamma Glows and TGFs
Article Publication Date
3-Oct-2024
Missing link found in gamma emission phenomena from thunderclouds
Groundbreaking research reveals new insights into gamma-ray emissions during thunderstorms
image:
«Artistic view of ALOFT observations» by University of Bergen / Mount Visual, licensed under CC BY 4.0
An illustration of NASAs research plane ER-2 flying over thunderstorms.
view moreCredit: «Artistic view of ALOFT observations» by University of Bergen / Mount Visual, licensed under CC BY 4.0
Groundbreaking Discoveries in Gamma-Ray Emissions from Thunderstorms
In the recent edition of Nature, groundbreaking results about the gamma-ray emissions produced during thunderstorms are presented. Overall, these findings reveal that gamma-ray emission from thunderclouds is much more complex, diverse, and dynamic than previously thought. Understanding these phenomena is crucial to uncovering the secrets of lightning.
Flickering Gamma-Ray Flashes: A New Discovery
Entitled “Flickering Gamma-Ray Flashes, the Missing Link between Gamma Glows and TGFs,” the paper by Østgaard et al. [2024] reports unique observations of a new phenomenon called Flickering Gamma-Ray Flashes (FGFs).
During thunderclouds, two different hard radiation phenomena have so far been known to originate: Terrestrial Gamma-ray Flashes (TGFs) and gamma-ray glows. This third phenomenon, observed and named FGFs by Østgaard et al. [2024] resembles the other two, while at the same time revealing certain characteristics separating FGFs from the others. Most noteworthy may be that FGFs are pulses of gamma-rays not associated with any detectable optical or radio signals.
The Missing Link Found?
«We think that FGFs could be the missing link between TGFs and gamma-ray glows, whose absence has been puzzling the atmospheric electricity community for two decades», says lead author and Professor Nikolai Østgaard at the University of Bergen.
Dynamic Gamma-Ray Emissions in Tropical Thunderclouds
In another study presented in this edition of Nature, Marisaldi et al. [2024] focus on the phenomenon referenced above and known as gamma-ray glows. The paper, entitled «Highly dynamic gamma-ray emissions are common in tropical thunderclouds», shows that contrary to what has been believed until now, tropical thunderclouds over ocean and coastal regions commonly emit gamma rays for hours over areas up to a few thousand square kilometers.
A New Understanding of Gamma-Ray Glows
«The dynamics of gamma-glowing thunderclouds starkly contradicts the former quasi-stationary picture of glows, and resembles that of a huge gamma-glowing boiling pot both in pattern and behavior», says Professor Martino Marisaldi at the University of Bergen.
The ALOFT Campaign
The groundbreaking results presented by Østgaard et al. [2024] and Marisaldi et al. [2024] are based on observations from the ALOFT (Airborne Lightning Observatory for FEGS and TGFs) campaign.
ALOFT, a collaboration between NASA and the University of Bergen, involved flying the NASA ER-2 aircraft from the MacDill Air Force Base, Florida, over tropical thunderstorms around the Gulf of Mexico, Central America, and the Caribbean in the summer of 2023. The payload included lightning detectors, gamma-ray scintillators, and a mix of passive and/ or active microwave sensors. A total of 10 flights were conducted above thunderclouds around Mexico, El Salvador, Nicaragua, and Florida.
The ALOFT campaign was financed by the Research Council of Norway and led by Professor Nikolai Østgaard and Professor Martino Marisaldi from the University of Bergen, Timothy Lang (Marshall Space Flight Center, NASA), and Franzeska Becker (ER-2 Mission Management).
The instrumentation on Board ER-2 during the ALOFT campaign included:
- Gamma-ray detectors UIB-BGO (PI: Nikolai Østgaard, UiB) and iStorm (PI: Eric Grove, NRL)
- An optical instrument named FEGS (PI: Mason Quick, Marshall Space Flight Center, NASA)
- Two electric field sensors known as EFCM (PI: Hugh Christian, University of Alabama, Huntsville) and LIP (PI: Christopher Schultz, Marshall Space Flight Center, NASA)
- Four cloud characterization instruments: AMPR (PI: Timothy Lang, Marshall Flight Center, NASA), CRS and EXRAD (PI: Gerry Heymsfield, Goddard Space Flight Center, NASA), and CoSSIR (PI: Rachael Kroodsma, Goddard Space Flight Center, NASA)
The flight campaign was coordinated with ground-based radio observations managed by a large network of collaborators.
Journal
Nature
Article Title
«Flickering Gamma-Ray Flashes, the Missing Link Between Gamma Glows and TGFs» by Østgaard et al. & «Highly dynamic gamma-ray emissions are common in tropical thunderclouds» by Marisaldi et al.
Article Publication Date
2-Oct-2024
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