Flash of insight

Natural History, Sept, 2005 by Rebecca E. Kessler

Physicists once thought gamma rays--the most energetic form of electromagnetic radiation--originated primarily from distant celestial sources. But new technology has been registering gamma rays in Earth's own atmosphere, at a rate of at least fifty bursts a day. Now Steven A. Cummer, an electrical engineer at Duke University in Durham, North Carolina, and his colleagues have analyzed twenty-six of those so-called terrestrial gamma-ray flashes (TG Fs) and discovered another big gamma-ray surprise.

A process called runaway breakdown seems to set off TGFs: A cosmic ray, or high-speed atomic nucleus, strikes an ordinary air molecule within a strong electric field. The collision energizes and dislodges one of the molecule's electrons, which the electric field then accelerates to nearly the speed of light. The highly energetic electron strikes other air molecules, energizing yet more electrons. When a beam of such energetic electrons collides with an atom, gamma rays burst forth.

It used to be thought that TGFs were generated far above thunderstorms, immediately after a very strong bolt of lightning. What Cummer and his colleagues found, however, is that TGFs precede lightning bolts by a split second, and are associated with lightning several hundred times weaker than anyone expected--or even with no lightning at all. To the investigators, the link with moderate lightning implies that TGFs develop at surprisingly low altitudes: near the tops of thunderclouds, rather than some twenty miles above them, as had been predicted. And so runaway breakdown, they say, seems to be connected to the cause of both lightning and TGFs. Apparently not only gamma rays, but also lightning itself, are still a bit of a mystery. (Geophysical Research Letters 32: L08811, 2005)