Gamma-Ray Emission from Tropical Thunderstorms Takes More Forms Than Previously Thought

There’s more to thunderclouds than rain and lightning. Along with visible light emissions, thunderclouds can produce intense bursts of gamma rays that last for millionths of a second. The clouds can also glow steadily with gamma rays for seconds to minutes at a time. Using a battery of detectors aboard NASA’s ER-2 research aircraft, scientists have found a new kind of gamma-ray emission that’s shorter in duration than the steady glows and longer than the microsecond bursts; they’re calling it a flickering gamma-ray flash.

Previous research has reported two types of gamma-ray emissions by thunderclouds: high intensity bursts known as terrestrial gamma-ray flashes, and moderate intensity longer-duration gamma-ray glows.

However, characteristics of these emissions and how they are produced are not fully understood.

Using data collected by aircraft during ten flights in July 2023, researchers investigated gamma-ray emissions produced during ocean and coastal thunderstorms over the Caribbean and Central America.

“The ER-2 aircraft would be the ultimate observing platform for gamma-rays from thunderclouds,” said University of Bergen’s Professor Nikolai Østgaard.

“Flying at 20 km, (12.4 miles) we can fly directly over the cloud top, as close as possible to the gamma-ray source.”

“There is way more going on in thunderstorms than we ever imagined,” added Duke University’s Professor Steve Cummer.

“As it turns out, essentially all big thunderstorms generate gamma rays all day long in many different forms.”

“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.”

“This project was designed to address these questions once and for all.”

Professor Østgaard, Professor Cummer and their colleagues identified a different type of gamma-ray emission called flickering gamma-ray flashes, which consist of pulses with a longer duration than those of terrestrial gamma-ray flashes.

In total, 24 flickering gamma-ray flashes were observed while passing over thunderclouds emitting gamma-ray glows during five of the ten flights; 17 of these flickering gamma-ray flashes were followed by lightning activity.

The researchers suggest that flickering gamma-ray flashes may start as gamma-ray glows before increasing suddenly in intensity and becoming a sequence of pulses, and may also have a role in some lightning initiation.

As flickering gamma-ray flashes share similar features to gamma-ray glows and terrestrial gamma-ray flashes, they propose that flickering gamma-ray flashes could provide evidence of links between the two phenomena.

In a separate study, University of Bergen’s Dr. Martino Marisaldi and colleagues investigated the properties of gamma-ray glows detected by the aircraft.

These included a thundercloud system occupying an area of more than 9,000 km² that was observed glowing for at least three hours.

They found that emissions were common and not uniform over a glowing region.

More than 500 individual gamma-ray glows were observed over the entire study area during nine of the ten flights, with each glow each lasting between one and 10 seconds.

These findings contradict those of previous studies, which reported that gamma-ray glows last up to hundreds of seconds and are emitted uniformly over areas of up to 20 km.

Together, the findings improve our understanding of gamma-ray emissions from thunderclouds, suggesting a causal link between the glows and the flashes, and a possible role for these emissions in the subsequent initiation of lightning.

“Those two new forms of gamma radiation are what I find most interesting,” Professor 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.”

The results appear in two papers in the journal Nature.

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N. Østgaard et al. 2024. Flickering gamma-ray flashes, the missing link between gamma glows and TGFs. Nature 634, 53-56; doi: 10.1038/s41586-024-07893-0

M. Marisaldi et al. 2024. Highly dynamic gamma-ray emissions are common in tropical thunderclouds. Nature 634, 57-60; doi: 10.1038/s41586-024-07936-6