Scientists in Japan reported seeing two radioactive weather phenomena at the same time, for the first time, according to a new paper. The observation establishes a link between the two, adding to our knowledge of the wild physics that takes place inside thunderstorms.
The researchers reported the “unequivocal simultaneous detection” of a minute-long “gamma-ray glow” followed by a powerful, millisecond-long “terrestrial gamma-ray flash,” or TGF. Though scientists have observed these two events before, they don’t quite understand the connection between them—the glows and flashes have never been observed together. That is, until now.
“The main result is that we, for the first time, confirmed the relation between gamma-ray glows and terrestrial gamma-ray flashes,” Yuuki Wada, the study’s first author and a PhD student at the University of Tokyo, told Gizmodo in an email.
The Gamma-ray Observation of Winter Thunderclouds (GROWTH) collaboration is a group of scientists with radiation detectors who monitor winter thunderstorms near the Sea of Japan. These storms are unique for their low-altitude clouds, making them good targets to study. The detectors are special crystals the size of thin bricks that flash when struck with high-energy particles; photo-multiplier tubes amplify the flash into a signal readable by a data acquisition system.
During a storm on the evening of 9 January 2018, one of the GROWTH detectors began to record an increase in radiation for 60 seconds—the sign of a gamma-ray glow. A nearby detector started measuring the glow as the signal waned in the first detector. The glow in the second detector suddenly spiked and then stopped. The first detector, too, measured a quick jump in gamma rays, which lasted around 0.2 seconds—the terrestrial gamma-ray flash. Lightning struck between the detectors, allowing the researchers to take data on both processes simultaneously, according to the paper published in Communications Physics. Having two detectors let the scientists know approximately where the glow came from, Wada said.
Scientists have been aware of high-energy physics occurring inside thunderstorms since the 1980s, when gamma-ray satellites started measured terrestrial gamma-ray flashes blasting into space, but didn’t really know why so few lightning bolts came with the flashes. Meanwhile, gamma-ray glows have been observed in the air, at sea level, and on mountain tops, and are generally associated with passing thunderstorms.
And before you ask, no, the radiation from the glow isn’t harmful and has only a fraction of the dose you’d get from a medical x-ray. As for the lightning, well, you don’t want to get struck by lightning.
Other researchers have previously theorised that a mechanism called relativistic runaway electron avalanches creates the environment for the glows and flashes. The storm’s strong electric fields accelerate electrons to high energies, producing more high-energy electrons. The gamma rays come from these high-energy electrons interacting with nuclei in the atmosphere.
But when and why the flash occurs has been up for debate, explained Joseph Dwyer, professor in physics and astronomy at the University of New Hampshire, who was not involved in the study. Perhaps it’s due to the electric fields in the lightning bolt, or maybe it’s something a little weirder called the “relativistic feedback model,” where the electric field is strong enough that it sets off a runaway chain reaction, producing the antimatter counterpart to electrons, as well as a lot more electrons, to undergo the gamma-ray producing interactions.
The GROWTH data seemed to match the what would be expected from the relativistic feedback model, Dwyer explained, and that’s really exciting. “Whatever caused this surge in the glow appears to be a rare process, since there have been a lot of glows observed in Japan without it,” David Smith, associate professor of physics at the University of California, Santa Cruz. told Gizmodo. “And we have been desperately needing a rare process in the cloud that we can associate with TGFs, since one of the biggest mysteries of TGFs is why such a small fraction of lightning produces them.”
Dwyer and Smith both were impressed by the observation. “The quality of the data is better than I could have hoped to see,” Smith said.
Now, all the researchers need are more detectors and more detections, Wada explained. The team wants to know how rare the phenomenon is.
I cannot stress this enough: Thunderstorms are amazing marvels of particle physics.
Featured image: Yuuki Wada