Thursday, December 6, 2012

Earth Gamma Ray Blasters

NASA's Fermi Gamma-ray Space Telescope has been catching the brief outbursts of high-energy light mysteriously produced above thunderstorms. The outbursts, known as terrestrial gamma-ray flashes (TGFs), last only a few thousandths of a second, but their gamma rays rank among the highest-energy light that naturally occurs on Earth. The enhanced GBM discovery rate helped scientists show most TGFs also generate a strong burst of radio waves, a finding that will change how scientists study this poorly understood phenomenon.

Lightning emits a broad range of very low frequency (VLF) radio waves, often heard as pop-and-crackle static when listening to AM radio. The World Wide Lightning Location Network (WWLLN), researchers routinely detect these radio signals and use them to pinpoint the location of lightning discharges anywhere on the globe to within about 20 km.

Scientists have known for a long time TGFs were linked to strong VLF bursts, but they interpreted these signals as originating from lightning strokes somehow associated with the gamma-ray emission.

The researchers identified much weaker radio bursts that occur up to several thousandths of a second before or after a TGF. They interpret these signals as intracloud lightning strokes related to, but not created by, the gamma-ray flash.

Scientists suspect TGFs arise from the strong electric fields near the tops of thunderstorms. Under certain conditions, the field becomes strong enough that it drives a high-speed upward avalanche of electrons, which give off gamma rays when they are deflected by air molecules.

Because the WWLLN radio positions are far more precise than those based on Fermi's orbit, scientists will develop a much clearer picture of where TGFs occur and perhaps which types of thunderstorms tend to produce them.

The GBM scientists predict the new operating mode and analysis techniques will allow them to catch about 850 TGFs each year. While this is a great improvement, it remains a small fraction of the roughly 1,100 TGFs that fire up each day somewhere on Earth, according to the team's latest estimates.