Gamma-ray flashes in thunderstorms may lack a link to lightning

Mysterious gamma-ray flash may be missing a link to lightning

Written by Lee Billings

It is said that lightning never strikes the same place twice and that a watched pot never boils.

However, neither claim is true. Especially if your “pot” is a giant tropical lightning storm full of lightning strikes, and you’re observing it from high up in the stratosphere. Two recent studies nature It turns out that some storms are actually boiling, emitting powerful bursts of gamma rays instead of water vapor. And some of these emissions occur in previously unrecognized mysterious patterns: fleeting flickers that appear to cause ordinary lightning discharges.

“How lightning occurs in thunderstorms is a big mystery,” says Joseph Dwyer, a physicist at the University of New Hampshire who served as a reviewer for both studies. “Decades of balloon and aircraft measurements have failed to discover the presence of electric fields in storms large enough to produce sparks, yet thunderstorms still occur on Earth every day. More than 8 million flashes. We’re clearly missing something important. These new observations could be that “something.” ”

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Scientists have long known that thunderstorms can produce gamma rays, extremely high-energy light that is often associated with astrophysical phenomena such as exploding stars and matter-eating black holes. I did. For surface storms, the physics behind such emissions are relatively well understood. Swirling, wind-blown water droplets and ice crystals accumulate electrical charge in a storm, with positively charged particles rising to the top of the cloud and negatively charged particles sinking to the bottom of the cloud. The result is a vast electric field of about 100 million volts. This is powerful enough to accelerate the electrons in the storm to nearly the speed of light, slamming the charged particles into air molecules, ejecting more electrons, and setting off a series of collisions. The energy that ultimately produces gamma rays.

Researchers observed two forms of gamma-ray emissions in thunderstorms. One is a relatively long-lived “glow” lasting several hundred seconds, and the other is a powerful burst on the microsecond scale, known as a terrestrial gamma-ray flash (TGF), bright enough to be visible to humans. Earth observation satellite.

But scientists also knew that this picture was incomplete, built on piecemeal measurements from airborne and ground-based instruments. “Great uncertainties remain in the electrical properties of storms, from the details of how charges are separated by particles in clouds to the physics of lightning generation and channel development.” says Vanna Chmielewski, an atmospheric scientist at the National Oceanic and Atmospheric Institute. The government’s National Institute for Severe Storms was not involved in the new study. “Many of these processes are difficult to accurately capture in laboratory settings or models, given the number of factors, the known variability even within a single storm, and the limited observational data sets available for validation. It is difficult, if not impossible.”

To get a clearer picture, in 2023 a team led by atmospheric physicists Nikolai Ostgaard and Martino Marisardi from the University of Bergen in Norway will monitor the gamma-ray emissions of large storms from up close and above. 10 aircraft tracked the thunder cloud. A modified NASA-owned U-2 reconnaissance plane flew over the Caribbean Sea and Central America. The program, called ALOFT (Airborne Lightning Observatory for Fly’s Eye Geostationary Lightning Mapper Simulator and Terrestrial Gamma-Ray Flashes), constitutes the most comprehensive and intensive airborne surveillance of thunderstorm gamma-ray emissions ever conducted.

“ALOFT was designed to definitively answer the question, ‘Are these gamma-ray flashes or glows common or rare?'” said co-author Steve Comer. “And it turned out to be a huge success. The gamma ray production process is much more important than we thought.”

This flight revealed the expected brilliance and TGF, but so much more. Both phenomena turned out to be much more abundant than predicted, and most of the TGF was too dim to be noticed by monitoring satellites. Also, the glow was not emanating steadily from isolated areas in the storm, as had been expected, but rather bubbling in an hours-long surge of radiation over an area about 100 kilometers wide. And among the hundreds of recorded events, researchers also caught a glimpse of something new. So-called flicker gamma-ray flashes (FGFs) are pulsed emission spikes that last milliseconds and appear to spring from the light. Most interestingly, Ostgaard said, “all transient gamma-ray events were followed by intense lightning.”

The researchers suggest that the dynamism and mass of these high-energy phenomena suggests that an avalanche of electrons at their source not only drives lightning to occur, but also acts to limit the thunderstorm’s large-scale electric field. It states that “Thunderstorms produce so much ionizing radiation that they partially discharge in some areas, causing electric fields to surge in other areas of the storm,” Dwyer says. “These field enhancements could be large enough to cause lightning strikes.”

Overall, ALOFT’s results paint a new, more nuanced picture that requires follow-up research, Marisardi says. “The acceleration of charged particles to relativistic energies within thunderclouds, of which gamma rays are a proxy, is an essential and very common process, at least in convective tropical clouds,” he says. “Our observations suggest the interesting hypothesis that glowing thunderclouds may be a prerequisite for the development of lightning. This glow evolves locally in unstable phenomena such as FGFs and TGFs. , which could lead to the occurrence of lightning. We hope to test this hypothesis in another flight campaign.”

He said future flights could investigate storm emissions at higher latitudes, as well as two other major tropical lightning hotspots in central Africa and Southeast Asia.