Jul 30, 1996

Catching Earth's Gamma-rays in the Act



Revision: March 3, 2017

Scientists using the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (above, shown during its 1991 deployment from the Space Shuttle) are readying themselves for an exciting summer of 1996. Along with observers of lightning strikes and severe weather, BATSE scientists are looking to catch some of the Earth's most powerful and elusive emissions of energy: gamma-ray flashes from thunderstorms.

Terrestrial gamma-ray flashes (or TGF's) are a newly discovered part of the Earth's spectacular show displayed during intense thunderstorms. BATSE, an astrophysics experiment designed to probe the cosmos for powerful gamma-ray bursts, accidentally discovered the TGF's shortly after launch of the satellite in April 1991. Because BATSE contains detectors which observe the entire sky, some detectors are inevitably pointed at the Earth during some part of the spacecraft orbit, allowing the TGF's to be detected.

TGF's are short blasts of gamma-ray energy, lasting a few milliseconds - about as long as the sound from a snap of the fingers. The energy of the radiation detected is significantly higher than that of even large solar flares or cosmic gamma-ray bursts. And the TGF's only seem to occur in the vicinity of large-scale thunderstorms.

thunderstorm from shuttle
"At first, we weren't totally convinced that these events weren't an instrument malfunction," said John Horack of the BATSE Science Team. "Then we took a look at the simultaneous weather images." What these images revealed was that each time a TGF was detected, a massive thunderstorm was seen in the vicinity of the Earth's surface from which the TGF was determined to originate.

About the same time as the discovery of TGF's, "sprites" and "jets," or huge emissions of upward moving lightning were also discovered around massive thunderstorms. Some were observed by television cameras onboard the space shuttle. The photograph at right shows such a massive thunderstorm taken by STS-55 astronauts aboard Columbia in May of 1993. Scientists now believe that these jet phenomena and TGFs are probably related in some unknown way. The problem is that no TGF has ever been directly observed in conjunction with an upward moving lightning stroke or jet.

This is an effort to "catch a TGF in the act," detecting both the gamma-ray emission and the sprite or lightning strike simultaneously. Working in conjunction with BATSE are optical sensors such as the Space Sciences Laboratory Optical Transient Detector and other lightning detectors using radio frequencies to detect large strikes. "Hopefully through this intensive, multi-instrument campaign, we'll be able to detect both the TGF and a lightning strike simultaneously, and verify our theory that these two are somehow connected," said Horack. "Then we can get on to the business of explaining why these things occur in nature."