What Comes Out Of The Top Of A Thunderstorm -- Gamma Rays From Severe Weather

May 26, 1999

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.

Full size image available through contact

After eight years of observing, the BATSE team has recorded about 70 TGFs. According to Mallozzi, our view of TGFs is somewhat limited. The Compton Observatory is the only satellite spacecraft that has detected TGFs and its orbit follows a restricted path over the Earth's surface, within 28 degrees from the Equator. This means a large portion of the Earth is invisible to BATSE. BATSE would never see TGFs coming off the American Plains, for instance - an area that often experiences tornadoes and thunderstorms.

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. Credit: European Space Agency.

Full size image available through contact

Also, the time and energy levels it takes to trigger BATSE may significantly reduce the number of TGFs detected. BATSE's detectors continually monitor gamma rays coming from deep space in 64 millisecond intervals. There are always some gamma rays coming from space, so BATSE only triggers into "gamma-ray burst mode" if the number of gamma rays in that 64 millisecond interval exceeds a prescribed background level. Because TGFs only occur for 1 or 2 milliseconds, their energy averaged out over 64 milliseconds often may not be great enough to send BATSE into trigger mode. BATSE may detect only the very brightest TGFs, just a small sample of the total number possible.

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.

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.

Red Sprite photographed by a team at
the University of Alaska, Fairbanks.

Full size image available through

"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."

Television cameras onboard the Space Shuttle and high-altitude aircraft have observed some sprites and jets, and many scientists now believe that these phenomena and TGFs are probably somehow related. The problem is that no TGF ever has been directly observed in conjunction with an upward moving sprite or jet.

The upcoming International Conference on Atmospheric Electricity in Guntersville, Ala. will offer many explanations for sprites and jets, but the reason for TGFs remain a mystery. Discovered only recently, the powerful but elusive TGFs prove there is much more to a thunderstorm than what meets the eye.
For more information, please contact:
Author: Leslie Mullen
Curator: Linda Porter
NASA Official: Gregory S. Wilson

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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