FERMI IMPROVES ITS VISION FOR THUNDERSTORM GAMMA-RAY FLASHES
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, according to a NASA report.
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.
The GBM Instrument Operations Center is located at the National Space
Science Technology Center in Huntsville, Ala. The GBM team includes a
collaboration of scientists from UAH, NASA's Marshall Space Flight
Center in Huntsville, the Max Planck Institute for Extraterrestrial
Physics in Germany and other institutions.
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