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Deep within their roiling clouds, thunderstorms hold an elusive
surprise. Under just the right conditions, they produce some of the highest
energy radiation naturally found on Earth: terrestrial gamma-ray
or TGFs for short.
Studies by NASA's Fermi Gamma-ray Space Telescope
have shown that TGFs fire up about 1,100 times a day.
Now, new research combines Fermi detections
with ground-based radar and lightning location data.
These studies show that TGFs come from more diverse types of storms than previously thought
In a thunderstorm, collisions among rain and snow cause different
parts of the clouds to develop positive and negative electrical charges.
When the strength of the electric field overcomes the insulating properties in
the thundercloud, a lighting flash occurs.
Most lightning occurs entirely within the cloud and is called an intracloud flash.
All lightning produces a strong and sudden change in the storm's electric field,
but the upward portion of an intracloud flash sometimes sends a surge
of electrons rushing toward the upper part of the storm. Reaching
speeds nearly as fast as light, these accelerated electrongs give off
gamma rays when their paths are deflected by air molecules. Using
global lightning location networks, scientists can determine a TGF
position more accurately than with Fermi data alone. Two dozen
localized TGFs occurred within areas covered by next-generation weather
radar systems. This gives scientists the opportunity to begin studying
the kinds of storms that produce TGFs. These slices of
radar data capture different types of storms encompassing a wide range of
updraft strengths. Even the weakest of them produced a TGF.
Another finding: TGFs seem to occur
in the same altitude range, between 7 and 9 miles high.
Lightning can form at much lower altitudes, so there's every reason
to think TGFs can too, but gamma-rays from TGFs occuring
deeper in the atmosphere are greatly weekend, they're too dim for Fermi to detect,
which probably means the satellite is undercounting them.
TGFs may be far more common than we think. With this
knowledge, scientists can design experiments to track storms and
study how TGFs relate to their strength and evolution. This will give us
an even better understanding of planet Earth's most powerful natural particle
accelerator. [Beeping]
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