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X-RAY THUNDERBOLT. Scientists have long suspected that lightning might generate x rays. However, until recently the observation of such x-rays has remained elusive, largely owing to the unpredictable nature of lightning. In the last few years a series of experiments by Joseph Dwyer and his colleagues at the Florida Institute of Technology and the University of Florida has shown that lightning indeed emits large bursts of x rays with energies up to about 250 keV (about twice that of a chest x ray). These x rays are mostly produced not by the bright return strokes, but by the leaders that precede the stroke, as they propagate from the cloud to the ground.

Now, Dwyer and his colleagues have discovered that these bursts of x rays are produced at the precise moment that the lightning steps forward along its jagged path. For unknown reasons, lightning does not travel to the ground in a continuous manner, but instead traverses the distance in a series of discrete steps. It is this stepping process that gives lightning its jagged, sometimes forked, appearance, and Dwyer has now shown that this same stepping process also makes x rays. The x rays are likely produced by strong electric fields that accelerate electrons to close to the speed of light. These so-called runaway electrons collide with air producing bremsstrahlung ("braking radiation" in German) x-rays. Dwyer says that higher energy gamma rays are also emitted sometimes, but that these seem to come from the thunderstorm cloud itself and not from the lightning stroke. (Dwyer et al., Geophysical Review Letters, 16 January 2005.)

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