It is capable of producing a million times as much energy and radiation as the lightning you can see.
Everyone has heard of and witnessed lightning — a powerful and natural electrical discharge between clouds or between a cloud and the ground — typically seen as a bright flash. But have you ever heard of dark lightning?
For years, scientists have been trying to figure out how thunderstorms can produce bursts of intense terrestrial gamma-ray flashes (TGFs) since they were accidentally detected in the early 1990’s while trying to measure gamma rays from distant supernovae and black holes. For a long time, they thought the bursts were coming from very high up in the atmosphere.
“It was logical to think that if we can see them from space, they must come from the top of the atmosphere," explains physicist and lightning expert Joseph Dwyer of the Florida Institute of Technology. “We now know that they come from deep within the atmosphere, from garden-variety thunderstorms,” he explained.
It turns out TGFs are produced in all types of thunderstorms, no matter how large or small, and they occur about once for every one thousand flashes of conventional lightning. This phenomenon is termed dark lightning because it is silent and invisible.
In a storm cloud, lightning occurs between two oppositely charged layers (positive at the top, negative in the center). However, as the storm charges up, the strong electric field transforms the cloud into a giant floating particle accelerator. What this means is that electrons are accelerated to almost the speed of light which then crash into air molecules, producing even more fast moving electrons — in a sort of avalanche effect.
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At relativistic speeds (close to the speed of light), the electrons emit Bremsstrahlung gamma rays, where some disintegrate into an electron and a positron. These newly formed electrons join the rest of the fast moving electrons, whereas the positrons plunge towards the negative center of the cloud — which forms even more electrons. “You create this feedback loop: the positrons make the electrons, the electrons make the positrons,” explained Dwyer. “So you get sort of an avalanche of avalanches.”
TGFs are extremely bright pulses of gamma rays, producing 1017 electrons in tenths of a millisecond, that emanate from Earth’s atmosphere. Although they last only briefly, they are capable of not only temporarily blinding satellite-based equipment located hundreds of kilometers away, but can also deliver a radiation dose equal to a full-body CT scan to nearby airplane passengers.
Dwyer and his team used a model to calculate the radiation dose a person aboard a plane in a storm would receive. Near the top of the storm, the dose is equal to 10 chest X-rays — the same dose we receive from natural background sources each year. However, near the center of the storm, the dose is roughly ten times larger, comparable to a full-body CT scan — the largest dose ever received during medical procedures.
“On rare occasions...it may be possible that hundreds of people, without knowing it, may be simultaneously receiving a sizable dose of radiation from dark lightning,” said Dwyer. “You'd have to be inside the thunderstorm [to be at risk], and not only inside but in the worst part of the storm at precisely the wrong time.” What this means is that we do not have to be overly concerned about dark lightning, since pilots are trained to avoid thunderstorms.
There are, however, competing theories on what produces TGFs, so this is definitely not the last we are going to hear about this mysterious phenomenon.
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