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| Fermi Space Telescope [Image Credit: NASA] |
Washington: Thanks to improved data analysis techniques and a new
operating mode, the Gamma-ray Burst Monitor (GBM) aboard NASA's Fermi Gamma-ray
Space Telescope is now 10 times better at catching the brief outbursts of
high-energy light mysteriously produced above thunderstorms.
The outbursts, known
as terrestrial gamma-ray flashes (TGFs), last only a few thousandths of a
second, but their gamma rays rank among the highest-energy light that naturally
occurs on Earth. The enhanced GBM discovery rate helped scientists show most
TGFs also generate a strong burst of radio waves, a finding that will change
how scientists study this poorly understood phenomenon.
Before being
upgraded, the GBM could capture only TGFs that were bright enough to trigger
the instrument's on-board system, which meant many weaker events were missed.
"In mid-2010, we
began testing a mode where the GBM directly downloads full-resolution gamma-ray
data even when there is no on-board trigger, and this allowed us to locate many
faint TGFs we had been missing," said lead researcher Valerie Connaughton,
a member of the GBM team at the University of Alabama in Huntsville (UAH). She
presented the findings Wednesday in an invited talk at the American Geophysical
Union meeting in San Francisco. A paper detailing the results is accepted for
publication in the Journal of Geophysical Research: Space Physics.
The results were so
spectacular that on Nov. 26 the team uploaded new flight software to operate
the GBM in this mode continuously, rather than in selected parts of Fermi's
orbit.
Connaughton's team
gathered GBM data for 601 TGFs from August 2008 to August 2011, with most of
the events, 409 in all, discovered through the new techniques. The scientists
then compared the gamma-ray data to radio emissions over the same period.
Lightning emits a
broad range of very low frequency (VLF) radio waves, often heard as
pop-and-crackle static when listening to AM radio. The World Wide Lightning
Location Network (WWLLN), a research collaboration operated by the University
of Washington in Seattle, routinely detects these radio signals and uses them
to pinpoint the location of lightning discharges anywhere on the globe to
within about 12 miles (20 km).
Scientists have known
for a long time TGFs were linked to strong VLF bursts, but they interpreted
these signals as originating from lightning strokes somehow associated with the
gamma-ray emission.
"Instead, we've
found when a strong radio burst occurs almost simultaneously with a TGF, the radio
emission is coming from the TGF itself," said co-author Michael Briggs, a
member of the GBM team.
The researchers
identified much weaker radio bursts that occur up to several thousandths of a
second before or after a TGF. They interpret these signals as intracloud
lightning strokes related to, but not created by, the gamma-ray flash.
Scientists suspect
TGFs arise from the strong electric fields near the tops of thunderstorms.
Under certain conditions, the field becomes strong enough that it drives a
high-speed upward avalanche of electrons, which give off gamma rays when they
are deflected by air molecules.
"What's new here
is that the same electron avalanche likely responsible for the gamma-ray
emission also produces the VLF radio bursts, and this gives us a new window
into understanding this phenomenon," said Joseph Dwyer, a physics
professor at the Florida Institute of Technology in Melbourne, Fla., and a
member of the study team.
Because the WWLLN
radio positions are far more precise than those based on Fermi's orbit,
scientists will develop a much clearer picture of where TGFs occur and perhaps
which types of thunderstorms tend to produce them.
The GBM scientists
predict the new operating mode and analysis techniques will allow them to catch
about 850 TGFs each year. While this is a great improvement, it remains a small
fraction of the roughly 1,100 TGFs that fire up each day somewhere on Earth,
according to the team's latest estimates.
Likewise, TGFs
detectable by the GBM represent just a small fraction of intracloud lightning,
with about 2,000 cloud-to-cloud lightning strokes for every TGF.
The Fermi Gamma-ray
Space Telescope is an astrophysics and particle physics partnership and is
managed by NASA's Goddard Space Flight Center in Greenbelt, Md. Fermi was
developed in collaboration with the U.S. Department of Energy, with important
contributions from academic institutions and partners in France, Germany,
Italy, Japan, Sweden and the United States.