Each day, thunderstorms around the world produce about a thousand quick
bursts of gamma rays, some of the highest-energy light naturally found
on Earth. By merging records of events seen by NASA's Fermi Gamma-ray
Space Telescope with data from ground-based radar and lightning
detectors, scientists have completed the most detailed analysis to date
of the types of thunderstorms involved.
"Remarkably, we have found that any thunderstorm can produce gamma
rays, even those that appear to be so weak a meteorologist wouldn't look
twice at them," said Themis Chronis, who led the research at the
University of Alabama in Huntsville (UAH).
The outbursts, called terrestrial gamma-ray flashes (TGFs), were
discovered in 1992 by NASA's Compton Gamma-Ray Observatory, which
operated until 2000. TGFs occur unpredictably and fleetingly, with
durations less than a thousandth of a second, and remain poorly
understood.
In late 2012, Fermi scientists employed new techniques that
effectively upgraded the satellite's Gamma-ray Burst Monitor (GBM),
making it 10 times more sensitive to TGFs and allowing it to record weak
events that were overlooked before.
"As a result of our enhanced discovery rate, we were able to show
that most TGFs also generate strong bursts of radio waves like those
produced by lightning," said Michael Briggs, assistant director of the
Center for Space Plasma and Aeronomic Research at UAH and a member of
the GBM team.
Previously, TGF positions could be roughly estimated based on Fermi's
location at the time of the event. The GBM can detect flashes within
about 500 miles (800 kilometers), but this is too imprecise to
definitively associate a TGF with a specific storm.
Ground-based lightning networks use radio data to pin down strike
locations. The discovery of similar signals from TGFs meant that
scientists could use the networks to determine which storms produce
gamma-ray flashes, opening the door to a deeper understanding of the
meteorology powering these extreme events.
Chronis, Briggs and their colleagues sifted through 2,279 TGFs
detected by Fermi's GBM to derive a sample of nearly 900 events
accurately located by the Total Lightning Network operated by Earth
Networks in Germantown, Maryland, and the World Wide Lightning Location
Network, a research collaboration run by the University of Washington in
Seattle. These systems can pinpoint the location of lightning
discharges -- and the corresponding signals from TGFs -- to within 6
miles (10 km) anywhere on the globe.
From this group, the team identified 24 TGFs that occurred within
areas covered by Next Generation Weather Radar (NEXRAD) sites in
Florida, Louisiana, Texas, Puerto Rico and Guam. For eight of these
storms, the researchers obtained additional information about
atmospheric conditions through sensor data collected by the Department
of Atmospheric Science at the University of Wyoming in Laramie.
"All told, this study is our best look yet at TGF-producing storms,
and it shows convincingly that storm intensity is not the key," said
Chronis, who will present the findings Wed., Dec. 17, in an invited talk
at the American Geophysical Union meeting in San Francisco. A paper
describing the research has been submitted to the Bulletin of the
American Meteorological Society.
Scientists suspect that TGFs arise from strong electric fields near
the tops of thunderstorms. Updrafts and downdrafts within the storms
force rain, snow and ice to collide and acquire electrical charge.
Usually, positive charge accumulates in the upper part of the storm and
negative charge accumulates below. When the storm's electrical field
becomes so strong it breaks down the insulating properties of air, a
lightning discharge occurs.
Under the right conditions, the upper part of an intracloud lightning
bolt disrupts the storm's electric field in such a way that an
avalanche of electrons surges upward at high speed. When these
fast-moving electrons are deflected by air molecules, they emit gamma
rays and create a TGF.
About 75 percent of lightning stays within the storm, and about 2,000
of these intracloud discharges occur for each TGF Fermi detects.
The new study confirms previous findings indicating that TGFs tend to
occur near the highest parts of a thunderstorm, between about 7 and 9
miles (11 to 14 kilometers) high. "We suspect this isn't the full
story," explained Briggs. "Lightning often occurs at lower altitudes and
TGFs probably do too, but traveling the greater depth of air weakens
the gamma rays so much the GBM can't detect them."
Based on current Fermi statistics, scientists estimate that some
1,100 TGFs occur each day, but the number may be much higher if
low-altitude flashes are being missed.
While it is too early to draw conclusions, Chronis notes, there are a
few hints that gamma-ray flashes may prefer storm areas where updrafts
have weakened and the aging storm has become less organized. "Part of
our ongoing research is to track these storms with NEXRAD radar to
determine if we can relate TGFs to the thunderstorm life cycle," he
said.
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