Breakthrough Reached in Understanding Unusual Lightning-Triggered Gamma-Rays

The Telescope Array, spread across an area measuring the size of New York City, waits to receive cosmic rays in the western Utah desert. It detects the high-energy particles that crash into Earth’s atmosphere continuously; the cosmic rays activate the 500-plus sensors once every few minutes.

A Telescope Array Surface Detector and its neighbors, deployed in Utah's west desert. The 507 detectors are arranged on a grid covering 700 square kilometers, about the same as the land area of New York City. (Image credit: Telescope Array collaboration)

Telescope Array physicists, when exploring data in 2013, noticed an abnormal particle signature; the photon equivalent of a light drizzle punctuated by a fire hose. The array had all of a sudden recorded a very rare occurrence - gamma rays, the highest-energy light waves on the electromagnetic spectrum, created by lightning strikes that direct the radiation downward toward the Earth’s surface. Five years later, an international team led by the Cosmic Ray Group at the University of Utah has witnessed the so-called downward terrestrial gamma-ray flashes (TGFs) in more clarity than previously.

The Telescope Array sensed 10 bursts of downward TGFs between 2014 and 2016, more events than have been witnessed across the globe combined. The Telescope Array Lightning Project is the first to spot downward TGFs at the start of cloud-to-ground lightning, and to reveal where they originated within thunderstorms. The Telescope Array is virtually the only facility designed to record the complete TGF “footprint” on the ground, and reveal that the gamma rays cover an area 3 to 5 km in diameter.

“What’s really cool is that the Telescope Array was not designed to detect these,” said lead author Rasha Abbasi, a researcher at the High-Energy Astrophysics Institute and the Department of Physics & Astronomy at the U. “We are 100 times bigger than other experiments, and our detector response time is much faster. All of these factors give us the ability that we weren’t aware of - we can look at lightning in a way that nobody else can.”

The research has been published online on May 17 in The Journal of Geophysical Research: Atmospheres.

An Accidentally Perfect Laboratory

The study was developed based on a research published by the group in 2017, in which a strong correlation was established between similar bursts of energetic particle showers observed between 2008 and 2013, and lightning activity detected by the National Lightning Detection Network. The physicists were amazed.

It was BOOM BOOM BOOM BOOM. Like, four or five triggers of the detectors occurring within a millisecond. Much faster than could be expected by cosmic rays. We realized eventually that all of these strange events occurred when the weather was bad. So, we looked at the National Lightning Detection Network and, low and behold, there would be a lightning strike, and within a millisecond we would get a burst of triggers.

John Belz, Professor of Physics & Chief Investigator of the National Science Foundation-funded Telescope Array Lightning Project

They sought the help of the lightning experts from the Langmuir Laboratory for Atmospheric Research at New Mexico Techto for in-depth study of the lightning. The researchers installed a nine-station Lightning Mapping Array created by the team, which generates 3D images of radio-frequency radiation emitted by the lightning inside a storm. In 2014, an additional instrument, known as a “slow antenna,” was installed at the center of the array. It recorded variations in the electric charge of the storm caused by the lightning discharge.

Taken together, the Telescope Array detections and the lightning observations constitute a major advance in our understanding of TGFs.  Prior to this, TGFs were primarily detected by satellites, with little or no ground-based data to indicate how they are produced. In addition to providing much better areal coverage for detecting the gamma rays, the array measurements are much closer to the TGF source and show that the gamma rays are produced in short duration bursts, each lasting only ten to a few tens of microseconds.

Paul Krehbiel, Co-Author & Lightning Researcher at New Mexico Institute of Mining and Technology

An Extremely Rare Phenomenon

Previously, physicists were of the notion that only violent celestial events, for instance, exploding stars, could produce gamma rays, until the first TGF was recorded in 1994 by a FERMI satellite. Slowly, researchers ascertained that the rays were produced in the first few milliseconds of upward intracloud lightning, which emitted the rays into space. From the time these upward TGFs were discovered, physicists have wondered whether similar TGFs that are emitted downward to the Earth’s surface could be produced by cloud-to-ground lightning.

Formerly, only six downward TGFs have ever been noted, two of which came from artificially-induced lightning experiments. The other four studies with natural lightning state TGFs originating a lot later, after the lightning had already hit the ground. The array’s observations are the first to reveal that downward TGFs take place in the early breakdown stage of lightning, akin to the satellite observations.

The downward-going TGFs are coming from a similar source as the upward ones. We safely assume that we have similar physics going on. What we see on the ground can help explain what they see in the satellites, and we can combine those pictures in order to understand the mechanism of how it happens.

Rasha Abbasi, Lead Author

“The mechanism that produces the gamma rays has yet to be figured out,” added Krehbiel.

What’s Next

The team has several questions that have not been answered. For instance, not all lightning strikes form the flashes. Is that because just one specific type of lightning initiation creates them? Are the researchers only observing a subset of TGFs that happen to be adequately large, or point in the correct direction, to be detected?

The team plans to add more sensors to the Telescope Array to improve the lightning measurements. Specifically, fixing a radio-static detecting “fast antenna” would enable the physicists to observe the substructure in the electric field changes at the start of the flash.

By bringing other types of lightning detectors and expanding the effort, I think we can become a significant player in this area of research,” said Belz.

This research resulted from a partnership between 126 co-authors from 33 universities and research institutions from the United States, Russia, Korea, Japan, and Belgium.

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