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May
26, 1999: Springtime in North America often brings severe
weather such as tornadoes, thunderstorms, high winds, and damaging
hail. But just as powerful and fascinating as what comes out
of the bottom of storm clouds are the flashes of gamma-rays that
have been observed coming out of the top. Terrestrial gamma-ray flashes (or TGFs) are short blasts of gamma-ray energy associated with thunderstorms. They only last a few milliseconds - about as long as the sound from a snap of the fingers - and can only be detected by satellites orbiting the Earth. NASA scientists inadvertently discovered TGFs while they were monitoring bursts of gamma-ray energy coming from the depths of space. |
The Compton Gamma-Ray Observatory, launched in April 1991,
was designed to observe the universe in gamma rays much as the
Hubble Space Telescope observes in visible light. Because BATSE
observes in all directions, portions of the Earth are inevitably
in its field of view. BATSE accidentally discovered the TGFs
shortly after launch of the satellite. |
What these weather images revealed was that each time a TGF
was detected, a massive thunderstorm was in the same vicinity.
Because TGFs seem to only occur in the vicinity of large-scale
thunderstorms, scientists believe the two phenomena are somehow
related. |
Right: A BATSE graph (with a line of thunderclouds in the background) showing a spike of gamma rays detected from a TGF. TGFs only last for one or two miliseconds. Click on picture to see a 600 x 475 pixel JPG. But BATSE never was designed to monitor gamma rays originating from Earth. In order to study TGFs in greater detail, we would need a gamma-ray detector designed to view the Earth with a 1 to 2 millisecond trigger. Satellite detectors like BATSE only would see powerful TGFs generated high in the atmosphere. Just as cosmic gamma rays are scattered or absorbed by the Earth's atmosphere before they reach the ground, if thunderstorms generate TGFs close to the ground, those gamma rays would likewise be scattered by the inner atmosphere before they reached outer space. Left: METEOSAT image of thunderclouds off the east coast of Africa and north of Madagascar. BATSE's detection area at the time of the photograph is indicated by the oval. A TGF originated somewhere within the grided area. Links to 640x636-pixel, 66K JPG. Credit: European Space Agency, Feb. 24, 1993. It is tempting to associate the quick and powerful TGFs with lightning bolts, but scientists say that lightning alone is not energetic enough to generate them. Sometimes "red sprites" and "blue jets" - huge colorful emissions associated with upward-moving lightning - are also seen coming from the tops of massive thunderstorms. "Pilots had reported seeing flashes of red and blue lights
for years, but no one ever believed them," said Mallozzi.
"It wasn't until recently that we've been able to get pictures
of these phenomena." |
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