A MYSTERIOUS property of lightning has been confirmed. Just before a flash of lightning lights the sky, a huge blast of X-rays or other high-energy particles is released. The phenomenon means physicists may have to rethink how lightning is made.
Past studies of lightning have seen occasional hints of X-ray bursts, but the signals were swamped by the electromagnetic noise that accompanies lightning. So space physicist Joseph Dwyer at the Florida Institute of Technology and his colleagues set up a scintillation detector in a field in Florida, where in the summer thunderstorms happen daily. An aluminium box with centimetre-thick walls shielded the detector from noise. To ensure that lightning struck close to the detector, they created a giant lightning rod by firing a rocket trailing a copper wire up into the thunderclouds.
In 31 out of 37 strikes observed, the team measured a short burst of energetic radiation lasting 10 to 100 microseconds, just before the flash. The huge amount of energy 鈥� more than 10,000 electronvolts per particle 鈥� took them by surprise. 鈥淭his is totally unexpected,鈥� says Dwyer.
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The burst means something odd is happening in the air as negative charges from the clouds work their way down towards the ground. One way X-rays could be generated is if electrons were being ripped away from air molecules and accelerated close to the speed of light. Such 鈥渞unaway electrons鈥� are seen in fusion experiments. But researchers did not expect to see them in lightning, where the surrounding air should act to slow the electrons.
Another implication of the finding is that anyone standing close to a lightning strike will get a small dose of X-rays, of about the same energy as an X-ray at the dentist, says Dwyer. 鈥淎lthough you have other things to worry about if you鈥檙e that close to lightning,鈥� he adds.