While pouring over data in 2013, Telescope Array physicists discovered a strange particle signature; the photon equivalent of a light drizzle punctuated by a fire hose. The array had unexpectedly recorded an extremely rare phenomenon—gamma rays, the highest-energy light waves on the electromagnetic spectrum, produced by lightning strikes that beam 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 observed the so-called downward terrestrial gamma ray flashes (TGFs) in more detail than ever before.

The Telescope Array detected 10 bursts of downward TGFs between 2014 and 2016, more events than have been observed in rest of the world combined. The Telescope Array Lightning Project is the first to detect downward TGFs at the beginning of cloud-to-ground lightning, and to show where they originated inside thunderstorms. The Telescope Array is by far the only facility capable of documenting the full TGF "footprint" on the ground, and show 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, 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 study published online on May 17 in The Journal of Geophysical Research: Atmospheres.

An accidentally perfect laboratory

The work builds on a study published by the group last year that established a strong correlation between similar bursts of energetic particle showers detected between 2008 and 2013, and lightning activity recorded by the National Lightning Detection Network. The physicists were stunned.

"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," said John Belz, professor of physics at the U and principal investigator of the National Science Foundation-funded Telescope Array Lightning Project. "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."

The researchers brought in lightning experts from the Langmuir Laboratory for Atmospheric Research at New Mexico Tech to help study the lightning in more detail. They installed a nine-station Lightning Mapping Array developed by the group, which produces 3-D images of radio-frequency radiation that lightning emits inside a storm. In 2014, they installed an additional instrument in the center of the array, called a "slow antenna", that records changes in the storm's electric charge 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", said Paul Krehbiel, long-time lightning researcher at the New Mexico Institute of Mining and Technology and co-author of the study. "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."

An extremely rare phenomenon

Until a FERMI satellite recorded the first TGF in 1994, physicists thought only violent celestial events, such as exploding stars, could produce gamma rays. Gradually, scientists determined that the rays were produced in the initial milliseconds of upward intracloud lightning, which beamed the rays into space. Since discovering these upward TGFs, physicists have wondered whether cloud-to-ground lightning could produce similar TGFs that beam downward to the Earth's surface.

Previously, only six downward TGFs have ever been recorded, two of which came from artificially-induced lightning experiments. The remaining four studies with natural lightning report TGFs originating much later, after the lightning had already struck the ground. The array's observations are the first to show that downward TGFs occur in the initial breakdown stage of lightning, similar 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," said Abbasi.

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

What's next

The researchers have many questions left unanswered. For example, not all lightning strikes create the flashes. Is that because only one particular type of lightning initiation produces them? Are the scientists only seeing a subset of TGFs that happen to be large enough, or point in the right direction, to be detected?

The team hopes to bring additional sensors to the Telescope Array to enhance the lightning measurements. In particular, installing a radio-static detecting "fast antenna" would enable the physicists to see the substructure in the electric field changes at the beginning 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.

More information:
R. U. Abbasi et al, Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders, Journal of Geophysical Research: Atmospheres (2018). DOI: 10.1029/2017JD027931

The researchers combined new Hubble observations with archival Hubble images for 50 star-forming spiral and dwarf galaxies in the local universe, offering a large and extensive resource for understanding the complexities of star formation and galaxy evolution. The project, called the Legacy ExtraGalactic UV Survey (LEGUS), has amassed star catalogs for each of the LEGUS galaxies and cluster catalogs for 30 of the galaxies, as well as images of the galaxies themselves. The data provide detailed information on young, massive stars and star clusters, and how their environment affects their development.

"There has never before been a star cluster and a stellar catalog that included observations in ultraviolet light," explained survey leader Daniela Calzetti of the University of Massachusetts, Amherst. "Ultraviolet light is a major tracer of the youngest and hottest star populations, which astronomers need to derive the ages of stars and get a complete stellar history. The synergy of the two catalogs combined offers an unprecedented potential for understanding star formation."

How stars form is still a vexing question in astronomy. "Much of the light we get from the universe comes from stars, and yet we still don't understand many aspects of how stars form," said team member Elena Sabbi of the Space Telescope Science Institute in Baltimore, Maryland. "This is even key to our existence—we know life wouldn't be here if we didn't have a star around."

The research team carefully selected the LEGUS targets from among 500 galaxies, compiled in ground-based surveys, located between 11 million and 58 million light-years from Earth. Team members chose the galaxies based on their mass, star-formation rate, and abundances of elements that are heavier than hydrogen and helium. The catalog of ultraviolet objects collected by NASA's Galaxy Evolution Explorer (GALEX) spacecraft also helped lay the path for the Hubble study.

The team used Hubble's Wide Field Camera 3 and the Advanced Camera for Surveys over a one-year period to snap visible- and ultraviolet-light images of the galaxies and their most massive young stars and star clusters. The researchers also added archival visible-light images to provide a complete picture.

The star cluster catalogs contain about 8,000 young clusters whose ages range from 1 million to roughly 500 million years old. These stellar groupings are as much as 10 times more massive than the largest clusters seen in our Milky Way galaxy.

The star catalogs comprise about 39 million stars that are at least five times more massive than our Sun. Stars in the visible-light images are between 1 million and several billion years old; the youngest stars, those between 1 million and 100 million years old, shine prominently in ultraviolet light.

The Hubble data provide all of the information to analyze these galaxies, the researchers explained. "We also are offering computer models to help astronomers interpret the data in the star and cluster catalogs," Sabbi said. "Researchers, for example, can investigate how star formation occurred in one specific galaxy or a set of galaxies. They can correlate the properties of the galaxies with their star formation. They can derive the star-formation history of the galaxies. The ultraviolet-light images may also help astronomers identify the progenitor stars of supernovas found in the data."

One of the key questions the survey may help astronomers answer is the connection between star formation and the major structures, such as spiral arms, that make up a galaxy.

"When we look at a spiral galaxy, we usually don't just see a random distribution of stars," Calzetti said. "It's a very orderly structure, whether it's spiral arms or rings, and that's particularly true with the youngest stellar populations. On the other hand, there are multiple competing theories to connect the individual stars in individual star clusters to these ordered structures.

"By seeing galaxies in very fine detail—the star clusters—while also showing the connection to the larger structures, we are trying to identify the physical parameters underlying this ordering of stellar populations within galaxies. Getting the final link between gas and star formation is key for understanding galaxy evolution."

Team member Linda Smith of the European Space Agency (ESA) and the Space Telescope Science Institute, added: "We're looking at the effects of the environment, particularly with star clusters, and how their survival is linked to the environment around them."

The LEGUS survey will also help astronomers interpret views of galaxies in the distant universe, where the ultraviolet glow from young stars is stretched to infrared wavelengths due to the expansion of space. "The data in the star and cluster catalogs of these nearby galaxies will help pave the way for what we see with NASA's upcoming infrared observatory, the James Webb Space Telescope, developed in partnership with ESA and the Canadian Space Agency (CSA)," Sabbi said.

Webb observations would be complementary to the LEGUS views. The space observatory will penetrate dusty stellar cocoons to reveal the infrared glow of infant stars, which cannot be seen in visible- and ultraviolet-light images. "Webb will be able to see how star formation propagates over a galaxy," Sabbi continued. "If you have information on the gas properties, you can really connect the points and see where, when, and how star formation happens."

More information:
legus.stsci.edu/