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What Comes Out of the Top of a Thunderstorm

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What Comes Out of the Top of a Thunderstorm

Gamma Rays from Severe Weather

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.

Right: Nighttime cloud-to-ground lightning. Credit: C. Clark, NOAA

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.

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"Like so many things in science, we came across these (TGFs) totally by accident," commented Dr. Jerry Fishman, principal investigator on the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. "We designed BATSE to collect gamma-rays from the deepest recesses of space, which it does very well. What we didn't know it would do was also collect gamma rays released from severe weather systems on Earth."

Left: The Compton Gamma Ray Observatory, designed to detect gamma ray sources in deep space, has also noticed gamma rays coming from the Earth. Credit: NASA

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.

"The first TGF actually was our second 'trigger' just a few days after launch," commented Dr. Robert Mallozzi of the BATSE science team. "The first event we observed was a real gamma-ray burst in outer space. The second event had a location coming from the Earth. We thought something was really wrong."

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"I ran back to all of my notebooks to make sure the detectors were connected correctly on the satellite," said Dr. John Horack of the BATSE science team. "At first, we weren't totally convinced that these events weren't an instrument malfunction. Then we took a look at the simultaneous weather images for each of these events."

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.

Left: INSAT, India's weather satellite, shows thunderclouds around Sri Lanka. The oval represents BATSE's detection area at the time of the photograph. BATSE scientists narrowed the detection of a TGF to the gridded region near the center of the oval. Links to 640x630-pixel, 109K JPG. Credit: Indian Space Research Organization, April 26, 1991.

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.

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BATSE TGF Observations

NASA Information on Sprites and Jets

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.

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."

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.

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

The upcoming International Conference on Atmospheric Electricity in Guntersville, Ala. (June 7 - 11) 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.

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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Leslie Mullen
Curator: Bryan Walls
NASA Official: John M. Horack