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Particle Accelerators in Earth's Atmosphere

Our atmosphere is very active electrically, evidenced by lightning forming during thunderstorms. Though we can't see it, another short-lived and equally as interesting atmospheric phenomenon occurs alongside lightning in thunderstorms, and this one can create gamma rays, the most energetic type of electromagnetic radiation.

In 1994, scientists first discovered that gamma rays, typically created by cosmic events in space, were also being created in thunderstorms. The events causing the gamma rays were called terrestrial gamma ray flashes (TGFs) and unlike lightning, they last only a few milliseconds (if that).

Scientists have thought since then that the Earth-based gamma rays are created in conjunction with a burst of cosmic rays entering our atmosphere. The incoming rays strip electrons off of molecules. When the electrons are combined with a lightning strike, an electric field is created, and an "avalanche" of stray particles form into a narrow beam projected out towards space.

Particle accelerators on earth do roughly the same thing, using electromagnetic fields to propel charged particles at high speeds. Though the process is the same, particle accelerators created by man produce beams with energies of many billions of electron volts while TGFs result in radiation amounting to only a few million electron volts.

Models for the creation of TGFs expect the atmospheric avalanche to result in about 10 MeV (mega, or million, electron volts) of radiation production in the form of gamma rays. New research by a group of Italian scientists, however, has recorded observations of TGFs producing ten times the amount of radiation at 100 MeV, suggesting that scientists' understanding of how TGFs are created may be incorrect.

Using the Italian Space Agency's AGILE satellite, the Italians detected multiple TGF events resulting in nearly 100 MeV of radiation using separate instruments. Their findings challenge current understandings of TGFs and suggest that more may be going on during the event, including the creation of neutrons which is already known to happen during lightning strikes. The Italian scientists hope that their new observations will reinvigorate interest in studying this elusive type of atmospheric phenomenon.

TGFs are typically found in tropical (equatorial) regions where thunderstorms and lightning occur more frequently. They're found between the altitudes of 33,000 ft. (where airliners cruise) and 65,000 ft. - well below the U.S. defined 'edge of space' at 262,000 ft. Estimates say anywhere from 100 to 1,000 TGFs occur every day. (Compare that to about 8.5 million lightning strikes on the average day.)

Though gamma ray radiation can alter human DNA, the radiation generated in TGF events is not significant enough to affect those on the ground. The gamma rays also mostly dissipate at the edge of Earth's atmosphere so astronauts are also safe from the newly-generated radiation, though satellites in low orbit could potentially be affected.


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