The air between the charges normally acts as an insulating layer, which means that no sparks can fly — no lightning — unless something causes that insulation to break down. Scientists have known from lab experiments that super-strong electric fields can temporarily convert the air’s electrically neutral molecules into a conductive pathway.
The trouble is, lightning researchers — despite decades of measurements from balloons, aircraft and rockets — have been unable to locate in thunderclouds electric fields sufficiently strong to trigger this insulator-to-conductor transformation.
To learn what might trigger the transformation, they began measuring radiation of the lightning that thunderstorms routinely emit and discovered something unexpected: the gamma rays and X-rays of dark lightning.
Nuclear explosions and collapsing stars — those are the sorts of extreme events that had been known to spew out gamma rays, not mere thunderstorms.
How is it that some storms produce these unusually strong rays? Dwyer speculates that super-fast electrons — perhaps revved up after being struck by cosmic rays that hit Earth’s atmosphere from deep space — may be the key. The theory is that these energetic electrons collide with atoms inside thunderclouds to create X-rays and gamma rays. These collisions lead to chain reactions that could be the mysterious basis for dark lightning.
Astronomers with access to gamma-ray detectors on satellites will be pivotal to discovering what causes dark lightning.
According to gamma-ray researcher J. Eric Grove of the Naval Research Laboratory in Washington, the gamma-ray flashes that Dwyer’s model describes match closely the best recent satellite measurements of thunderstorm emissions of these high-energy rays. But he also notes that recent data from an Italian satellite implies that thunderstorms might be producing gamma-ray flashes far more energetic than Dwyer’s theory can account for, adding mystery even as it helps confirm dark lightning’s existence.
Grove hopes additional data from a sensor aboard the Fermi Gamma-ray Space Telescope, which he has worked on for years, will provide more information. “We need more gamma-ray and electric-field experiments in and around thunderstorms to really understand this,” Grove says.
Until then, a full understanding of the natural phenomenon that Ben Franklin first analyzed will have to wait.
Amato is a freelance science writer and organizer of the monthly DC Science Cafe.