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, 15 January 2005.)