In September 2021, an unprecedented thunderstorm blew across Utah’s West Desert. Lightning from this storm produced at least six gamma ray flashes that beamed downward to Earth’s surface and activated detectors at the University of Utah-led Telescope Array. The storm was noteworthy on its own—the array usually clocks one or two of the lightning-triggered gamma rays per year—but recent upgrades led to a new observation by the Telescope Array scientists and their lightning collaborators.

For the first time ever, they captured video footage of lightning-triggered downward terrestrial gamma-ray flashes (TGFs). A special camera running at 40,000 frames per second gave an unprecedented look at how gamma rays burst downwards to the Earth’s surface from cloud-to-ground lightning strikes. They found that not only were multiple gamma rays produced at later lightning stages than previously thought, but the rays were also associated with a pulse of optical light that had never been recorded.

“This is an important step in lightning research that could lead us to the physics producing these downward gamma rays,” said lead author Dr. Rasha Abbasi, now an assistant professor of physics at Loyola University Chicago. Abbasi began the research on TGFs as a postdoctoral scholar at the University of Utah.

The study published on July 20, 2023, in Geophysical Review Letters.

Telescope Array Collaborative leads the field

Gamma rays are no joke. The highest-energy light waves on the electromagnetic spectrum can knock electrons off atoms and cause serious harm to human cells. Until BATSE satellite detected the first terrestrial gamma ray flash in 1994, scientists thought only galactic events like exploding stars could produce gamma rays. Over time, physicists determined that TGFs were produced in the initial milliseconds of upward intracloud lightning, which beamed the rays into space. Since discovering upward TGFs, researchers have sought to understand the rarer phenomenon of downward TGFs, when cloud-to-ground lightning produces gamma rays that beam to the Earth’s surface.

Ground-based observations are ideal for studying downward TGFs because they’re just a few kilometers from the lightning storms themselves.

“The ability of the Telescope Array Surface Detector to detect downward TGFs is a great example of serendipity in science,” said John Belz, professor of physics and astronomy at the U and co-author of the study. “The TASD was designed to do astroparticle physics, by studying the particle showers produced by energetic atomic nuclei from deep space. Purely by happenchance, the astroparticle showers share many properties—including energy, duration, and size—with the gamma ray showers known as downward TGFs. So in a sense, we are able to operate two groundbreaking science facilities for the price of one.”

Telescope Array collaborators from the University of Utah, Loyola University Chicago, the Langmuir Laboratory for Atmospheric Research at New Mexico Tech and the National Institute for Space Research-Brazil (INPE), have installed a suite of lighting instrumentation to the existing Telescope Array, a ground-based grid of surface detectors primarily designed to observe ultra-high energy cosmic rays.

The array covers an area the size of New York City and is the largest of its kind in the northern hemisphere. The groups have added a lightning mapping array, a broadband very high-speed interferometer, a fast antenna that measures the change in the electric field over the whole lightning flash, and most recently, multiple high-speed video cameras. 

“In one second, your eye sees a lightning flash—that’s it. These instruments measure the change in the electric field over the whole lightning flash that happens in the blink of an eye,” said Abbasi. “We’re interested in the science of the gamma ray initiation. What stage of the lightning produces the gamma ray? Why do gamma rays happen in correlation with some lightning flashes, but they don’t in others?” 

The efforts have paid off. The study analyzed one of these TGF events. They identified that the cloud-to-ground flash with the observed TGF was formed by lightning’s fast downward leader, followed by an intense return stroke. The TGF occurred while the downward leader was already branching well below the cloud base, even halfway in its propagation to the ground.

“This new finding is a consequence of collaborations between astroparticle physicists and lightning scientists,“ said Marcelo Saba, co-author at the INPE. “It is exciting to merge efforts together.”

What’s next?

Telescope Array’s massive footprint is large enough to detect particle showers as they rain down across a wide area of Earth’s surface. Its 500-plus surface detector stations cover 700 km² (~270 miles²) outside of Delta, Utah, in the southwestern part of the state. It’s in the process of expanding to four times its original size. The researchers expect that the expansion, in addition to recently installed lightning sensors, will unlock new discoveries.

“For example, do downward-TGFs and upward-TGFs represent different manifestations of the same phenomenon and share a common origin? Is it possible to achieve simultaneous detection of upward and downward TGFs? Under what meteorological conditions do thunderstorms produce TGFs? What are the underlying physical mechanisms connecting multiple lightning stages to TGF initiation?” asked Abbasi. “We hope to detect more events to address these questions still left unanswered.”

The operation and analyses of this study have been supported by the U.S. National Science Foundation. The Telescope Array experiment is supported by the Japan Society for the Promotion of Science, the University of Tokyo; the U.S. National Science Foundation; the National Research Foundation; the Russian Academy of Sciences; Belgian Science. The foundations of Dr. Ezekiel R. and Edna Wattis Dumke, Willard L. Eccles, and George S. and Dolores Doré Eccles all helped with generous donations. The State of Utah supported the project through its Economic Development Board, and the University of Utah through the Office of the Vice President for Research. Analyses of this work have been conducted by collaborators from the Telescope Array, including lightning scientists from the Langmuir Laboratory for Atmospheric Research at New Mexico Tech, and the National Institute for Space Research-Brazil.

• Lisa Potter Research communications specialist, University of Utah Communications
  949-533-7899 lisa.potter@utah.edu