Lightning Strikes and Rays Follow

Shocking discovery. Fission reactions create gamma rays showers that last for minutes to hours after a flash of lightning.

MONTREAL, CANADA--Electrical storms can light up a midsummer night, but the dazzling forks are just part of the show. Invisible gamma rays shoot out of the sky minutes to hours after lightning strikes, researchers have learned. The team suspects that nuclear reactions fizzing in the atmosphere may be the source, and lightning could be the trigger.

Lightning packs a billion-volt punch, rending the sky with massive electric fields. The fields accelerate electrons, which streak upward and smack into atoms with such energy that the excited atoms give off gamma rays. These gamma ray bursts, which follow just microseconds after lightning strikes, have been known for a while. However, at a meeting here of the American Physical Society on 22 March, Mark Greenfield of the International Christian University in Tokyo described how he and his colleagues used sodium iodide detectors on the roof of their building to track delayed gamma rays.

The rays follow lightning by several minutes to an hour. This delay, along with the 50-minute half-life of the gamma ray shower (the time it takes for half the gamma rays to fly off), suggests that they come from nuclear fission reactions in the atmosphere. How the reactions get going isn't known, but Greenfield and colleagues suspect that protons may accelerate from the lightning's electric field in the same way that electrons do and slam into common elements in Earth's atmosphere, setting off fission reactions. For example, they suggest, high-energy protons crashing into argon-40 could trigger a chain reaction in which argon-40 decays into chlorine-39, which then decays into excited argon-39, which immediately gives off a gamma ray. Chlorine-39 has a 56 minute half-life, which fits nicely with the observed half-life of the gamma ray shower.

Still, questions remain. For one thing, it's not clear whether lightning has sufficient energy--on the order of mega-electron volts, or about a million times the energy of an electron in a flashlight bulb--to crank up the protons. Nor is it certain which atoms are giving off the gamma rays. Even so, other researchers say the idea is worth pursuing. "[Greenfield] seems to be on to something very intriguing," says Joseph Dwyer, a physicist at Florida Institute of Technology in Melbourne. The nuclear fission reactions may help corroborate new theories of how lightning is triggered, he says.

Related site
NASA article on lightning and gamma ray bursts

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