Scientists have discovered something mind-bending about lightning – sometimes its flashes are invisible, just sudden pulses of unexpectedly powerful radiation. Termed “dark lightning”, it is now clear to a growing number of lightning researchers and astronomers that along with bright, visible thunderbolts, thunderstorms sometimes unleash sprays of X-rays and intense bursts of gamma rays (called terrestrial gamma-ray flashes, or TGF), a form of radiation normally associated with such cosmic spectacles as collapsing stars. The radiation in these invisible blasts can carry a million times as much energy as the radiation in visible lightning, but that energy dissipates quickly in all directions rather than remaining in a stiletto-like lightning bolt. A person could be struck by dark lightning and they would never even know it.
Dark lightning appears to compete with normal lightning as a way for thunderstorms to vent the electrical energy that gets pent up inside their tumultuous interiors. Scientists estimate there is about one dark lightning burst for every thousand visible lightning flashes.
Now for the bad news. Unlike with regular lightning, though, people struck by dark lightning, most likely while flying in an airplane which fly near the tops of storms where dark lightning originates, would not get hurt. But according to scientists’ calculations, they might receive in an instant the maximum safe lifetime dose of ionizing radiation — the kind that wreaks the most havoc on the human body. It is estimated that one dark lightning burst is equivalent to 10 chest x-rays. Not good.
“It may be possible that hundreds of people, without knowing it, may be simultaneously receiving a sizable dose of radiation from dark lightning.”
How is it that some storms produce these unusually strong rays? Joseph Dwyer, a lightning researcher at the Florida Institute of Technology, 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. Science Daily explains:
“Instead of creating normal lightning, thunderstorms can sometimes produce an exotic kind of electrical breakdown that involves high-energy electrons and their anti-matter equivalent called positrons. The interplay between the electrons and positrons causes an explosive growth in the number of these high-energy particles, emitting the observed terrestrial gamma ray flashes while rapidly discharging the thundercloud, sometimes even faster than normal lightning. Even though copious gamma-rays are emitted by this process, very little visible light is produced, creating a kind of electrical breakdown within the storms called “dark lightning.”
Scientists have seen hints of dark lightning from gamma ray measurements from satellites since at least the 1990. 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.