December 6, 2012
NASA Gives Fermi Space Telescope Better Vision
[Watch Video: Fermi Finds Radio Bursts From Terrestrial Gamma-Ray Flashes]
Lee Rannals for redOrbit.com — Your Universe Online
NASA's Fermi Gamma-ray Space Telescope is new and improved, and ready to start sending back some images of high-energy light mysteriously produced above thunderstorms.
The space agency updated the telescope with improved data analysis techniques and a new operating mode to the Gamma-ray Burst Monitor (GBM) aboard Fermi. This will enable the telescope to start analyzing terrestrial gamma-ray flashes (TGFs).
TGFs only last for a few thousandths of a second, but their gamma rays are among the highest-energy light that naturally occurs on Earth. The enhanced GBM discovery rate helped scientists show that most TGFs generate a strong burst of radio waves. These findings will change how scientists study TGFs.
"In mid-2010, we began testing a mode where the GBM directly downloads full-resolution gamma-ray data even when there is no on-board trigger, and this allowed us to locate many faint TGFs we had been missing," lead researcher Valerie Connaughton, a member of the GBM team at the University of Alabama in Huntsville, said in a statement.
The team uploaded new flight software to operate the GBM in this mode continuously, rather than in selected parts of Fermi's orbit. They were able to discover the new technique with GBM data that was gathered from August 2008 to August 2011. The scientists then compared the gamma-ray data to radio emissions over the same period.
Scientists have long known that TGFs were linked to strong very low frequency (VLF) radio wave bursts, but they interpreted these signals as originating from lightning strokes.
"Instead, we've found when a strong radio burst occurs almost simultaneously with a TGF, the radio emission is coming from the TGF itself," co-author Michael Briggs, a member of the GBM team, said in the statement.
The team identified weaker radio bursts that occur up to several thousandths of a second before or after a TGF. They interpreted these signals as intracloud lightning strokes related to gamma-ray flash.
They suspect TGFs arise from the strong electric fields near the tops of thunderstorms. Certain conditions see that the field becomes strong enough to drive a high-speed upward avalanche of electrons.
"What's new here is that the same electron avalanche likely responsible for the gamma-ray emission also produces the VLF radio bursts, and this gives us a new window into understanding this phenomenon," Joseph Dwyer, a physics professor at the Florida Institute of Technology in Melbourne, Fla., and a member of the study team, said in the statement.
GBM scientists believe the new operating mode and analysis techniques will allow them to catch about 850 TGFs each year. There are about 1,100 TGFs that fire up each day, according to the team's estimates.