Merging data on high-energy bursts seen on Earth by NASA's Fermi Gamma-ray Space Telescope with data from ground-based radar and lightning detectors, scientists have completed the most detailed analysis to date of the types of thunderstorms producing terrestrial gamma-ray flashes, or TGFs.
TGFs occur unpredictably and fleetingly, with durations less than a thousandth of a second, and remain poorly understood. Yet the gamma rays they produce rank among the highest-energy light naturally produced on Earth.
Earlier Fermi studies helped uncover lightning-like radio signals emitted by TGFs. This made it possible to use ground-based lightning location networks to pin down storms producing the flashes, opening the door to a deeper understanding of the meteorology powering these extreme events.
Scientists gathered a sample of nearly 900 Fermi TGFs accurately located by ground networks, which can pinpoint the location of lightning discharges -- and the corresponding signals from TGFs -- to within 6 miles (10 km) anywhere on the globe. From this group, they identified 24 TGFs that occurred within areas covered by Next Generation Weather Radar (NEXRAD) sites.
The researchers found that even weak and marginally electrified storms are capable of producing TGFs.
The new study also confirms previous findings indicating that TGFs tend to occur near the highest parts of a thunderstorm, between about 7 and 9 miles (11 to 14 kilometers) high. However, TGFs associated with lightning at lower altitudes would be so weakened by traveling a longer path through the atmosphere that Fermi couldn't detect them. If true, the estimated number of 1,100 TGFs occurring each day may be much larger than previously thought.
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