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Space telescope catches antimatter from terrestrial thunderstorms
Normally astronomers look deep into space, but in the latest finding from the NASA Fermi
Gamma-ray Space Telescope presented on Monday, Jan. 10, during a news briefing at the American
Astronomical Society meeting, they detected an antimatter signal from Earth. Created in energetic
processes above thunderstorms, when such an antimatter beam strikes the spacecraft, it actually
becomes a source of the gamma-ray light it was designed to observe. Scientists at the Max Planck
Institute for Extraterrestrial Physics (MPE) were responsible for the development of the detectors
and the power supplies of the Fermi Gamma-ray Burst Monitor (GBM), which led to this discovery,
and contributed to the calibration and data analysis for this particular result.
While Fermi flew over Egypt, the GBM intercepted a particle beam from a terrestrial gamma-ray flash
(TGF) that occurred in a thunderstorm below its horizon.
Credit: NASA/Goddard Space Flight Center
When an antimatter particle strikes Fermi, it will collide with a particle of normal matter. Both
particles are immediately annihilated and transformed into gamma rays of characteristic energy. The
GBM detected gamma rays with energies of 511,000 electron volts, a signal indicating that an electron
has met its antimatter counterpart -- a positron. For comparison, visible light has energies between
2 and 3 electron volts.
The scientists think that the antimatter particles were formed in a terrestrial gamma-ray flash (TGF),
a brief burst of gamma rays produced inside thunderstorms that is somehow associated with lightning.
"These gamma photons have typical energies of 20-40 million electron volts, and these are usually
detected as TGF," says Andreas v. Kienlin, scientist at the MPE who led the development of the
Theorists estimate that about 500 TGFs occur worldwide each day, but most go undetected. Even though
Fermi's GBM is designed to observe high-energy events in the distant universe, the GBM team has
identified 130 TGFs since Fermi's launch. During one TGF, which occurred on Dec. 14, 2009, Fermi was
located over Egypt. But the active storm was in Zambia, some 2,800 miles (4,500 km) to the south.
Because the distant storm was below Fermi's horizon, any gamma that rays it produced could not have
been detected by the GBM. High-speed electrons and positrons produced in the TGF, however, could
travel along the Earth's magnetic field to strike the spacecraft and produce a pulse of gamma rays
that was picked up by Fermi's GBM. "This signal is the first direct evidence that thunderstorms make
antimatter particle beams," said Michael Briggs, a member of the GBM team at the University of Alabama.
The detection of positrons thus proves that copious high-energy particles are indeed being ejected
from the atmosphere. In fact, scientists now think that all TGFs emit electron/positron beams.
"However, we still don't know how these TGFs are produced, and we also do not quite understand how
the classical lightning forms," says Jochen Greiner from the MPE, the German principal investigator
of the GBM. Even though turbulence in thunderclouds can produce large voltages, they are not strong
enough to ionize the air and to lead to sparks. TGFs could provide the trigger.
A paper on the findings will appear in a forthcoming issue of Geophysical Research Letters.
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