Scientists have known about dark lightning since at least the early 1990s, when satellites detected a powerful gamma-ray flash in our atmosphere. At first, we thought the event must have been something like a gamma-ray burst. However, NASA’s Fermi space telescope detected a terrestrial gamma-ray flash (TGF). This allowed us to trace the phenomenon’s source to a terrestrial thunderstorm.
Since then, we’ve learned that perhaps one in 1,000 lightning bolts produces a TGF. That means dark lightning happens 1,000 times a day or more. Thankfully, they don’t pose much risk to people on the ground. However, a new study compared the frequency of dark lightning bolts with a map of the most common airline routes, concluding that dark lightning does pose some risk to plane passengers.
Atmospheric scientist Melody Pallu, the first author of the report, discussed the results at the latest American Geophysical Union conference. Dark lightning might strike a plane maybe once every 1 to 4 years, said Pallu, but that’s probably “an upper limit of the real probability.” However, Pallu said, the calculations didn’t address the fact that pilots usually avoid thunderstorms. So, that estimate of once every few years could be 10 times the actual rate.
Planes are carefully designed to withstand lightning strikes of terrifying power. But they can’t protect against everything, and gamma rays are no joke. Dark lightning happens at cruising altitude for some planes. If a plane should pass near a bolt of dark lightning, said Pallu, its passengers could receive 0.3 sieverts of radiation in an instant. That’s more than a year’s worth of radiation exposure. It doesn’t reach the level of exposure that can cause radiation sickness, but it’s enough to elevate a person’s lifetime risk of cancer.
Airline pilots usually try to avoid thunderstorms. They’ll divert from their standard course to do so. Even so, from time to time, lightning strikes a plane — usually the nose, tail, or the end of a wing. When this happens, the plane’s body acts as a Faraday cage, diverting the lightning bolt’s electromagnetic pulse (EMP) to protect the people inside. Since most lightning is cloud-to-cloud, the zorch simply passes over and through the plane’s metal body, and then exits the opposite end of the plane. Dark lightning, however, is different.
How Does Dark Lightning Happen?
Even now, says Pallu, we’re not sure precisely how thunderstorms generate these gigantic, invisible bolts of dark lightning. But according to NASA, the leading hypothesis is that regular lightning can act as an invisible particle accelerator, producing an electric field that throws off a jet of electrons moving at near-light-speed. Those electrons crash into the atmosphere, and the energy they throw off is so much greater than normal, it’s no longer in the visible spectrum.
Most lightning actually happens within the cloud-tops. Deep inside a thundercloud, positive and negative charges build up when ice crystals rub against one another. But because of the updraft, these charged particles can become separated by height. Lightning happens when an electrical charge concentrated in one place finds a path to somewhere that has an opposite charge. As soon as it does, the system finds a way to equalize that electrical “pressure.” The same process is at work in high-voltage electrical arcing, spark plugs, and even neurons firing.
The light from lightning comes from bremsstrahlung (“braking radiation“). This happens when fast-moving electrons with a lot of kinetic energy are deflected and slowed down by the atmosphere. The electron releases that difference in energy as a photon. For normal lightning, our eyes see that as a glow in the visible spectrum.
Dark lightning happens when braking radiation from lightning in the upper atmosphere soaks so much energy that the electron releases a photon in the gamma energy band. Extremely severe storms with lots of lightning can create lots of gamma rays this way. When these gamma rays strike oxygen atoms, they create an electron and a positron, and the process begins anew — spiced up by antimatter. As NASA explains in the video above, it’s an invisible, “self-generated, self-sustained particle accelerator.”
In the end, Pallu added, we need more research to understand dark lightning and its risk to human health. Scientifically, dark lightning is a fascinating reminder that some of the most energetic interactions on the planet occur in parts of the electromagnetic spectrum we can’t even see.
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