The serendipitous flybys of two satellites near a tropical thunderstorm have given researchers an unprecedented look at terrestrial gamma-ray flashes—a mysterious, high-energy phenomenon that scientists first observed in 1991. First thought to be generated at high altitudes, researchers have recently pinned down the origin of the fleeting lightning-linked bursts—one of the most energetic surges of natural electromagnetic radiation on Earth—to altitudes below 20 kilometers, in the layer of the atmosphere where most weather happens. Now, analyses of data gathered in 2006 by two satellites—one carrying a down-gazing camera and the other a gamma ray detector—as well as a ground-based lightning detector in North Carolina, reveal that these flashes start out, as does most lightning, as a small channel of charged particles within the storm cloud (golden zigzag line, left; lightning-generated radio waves are depicted as concentric rings). If the electric field within the storm is abnormally strong, large cascades of negatively charged electrons can be accelerated to nearly the speed of light. When those electrons slam into molecules in the air, they produce gamma rays (pink, center) and lengthen the channel (gold, center) through which the visible flash of lightning (yellow, right) travels. Although terrestrial gamma-ray flashes typically last between 300 and 400 microseconds, the burst observed by the satellite sensors in 2006 lasted a mere 70 microseconds, the researchers report online and in a forthcoming issue of Geophysical Research Letters. Besides shedding new light on how terrestrial gamma-ray bursts unfold, the new analysis suggests that the phenomena are about twice as common as previously suspected, the researchers say.
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