Team makes breakthrough in understanding rare lightning-triggered gamma-rays

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. Credit:
Telescope Array collaboration

In the western Utah desert, the Telescope Array sprawls
across an area the size of New York City, waiting for cosmic
rays. The facility detects the high-energy particles that
collide with Earth’s atmosphere constantly; the cosmic rays
trigger the 500-plus sensors once every few minutes.


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 had unexpectedly recorded an extremely rare
phenomenon—, 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.

A bolt of insight
The bright flash of light is only one stage of lightning;
there’s a substructure that happens too fast for the eye to
see. ‘Step leaders’ proceed toward the ground in stages.
Negative electric charge builds at the leader tip until it is
sufficient to cause the air to break down and form a new
conducting path. The study found that terrestrial gamma rays
are produced within the first 1-2 milliseconds of the initial
breakdown stage, which is the least understood part of
lightning. Credit: National Oceanography and Atmospheric
Administration

“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 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 detectors and expanding the effort, I
think we can become a significant player in this area of
research,” said Belz.

Explore further:

NASA’s Firestation on way to the International Space
Station

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

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