Cosmic rays reveal thunderstorm secrets

Ever since Benjamin Franklin flew his kite in a thunderstorm and extracted a spark from the other end we’ve known that lightning is a discharge of electricity. Yet just how that electricity bolts from cloud to ground has remained a mystery.

But astronomers studying cosmic rays at Radboud University in the Netherlands may have stumbled on the answer. Just as X-rays are used to probe the structure of matter in labs, the cosmic rays provided an image of the electric field inside roiling thunderclouds. The rays may also provide the track for the lightning strike. The findings were published in Physics Review Letters.

“It’s like having a particle accelerator in the sky, except the particles are much higher in energy than anything we can produce here on Earth,” says Pim Schellert, lead author of the study.

Franklin’s 1752 experiment was a dangerous one. While Franklin survived with nothing more than a shock, the following year Georg Richmann died in Saint Petersburg while attempting to replicate it. But Franklin’s risk was worth it as his discovery transformed our study of electricity.

The “electric fire” that could be extracted from rubbing a silk scarf with a glass rod was the same as that generated in thunderclouds. But instead of being a parlour game, used to charge wine glasses so drinkers received a shock, post-Franklin the study became a serious discipline.

Today we know lightning is all a matter of generating electric fields. Yet researchers still cannot explain a lightning bolt. There simply isn’t enough of a measurable charge difference between cloud and ground to explain how the spark overcomes the air resistance and jumps across.  

“We thought, ‘this is physics, right? There should be a reason for this’.” 

“Something’s going on that we don’t understand,” says Joseph Dwyer, a physicist specialising in lightning at the University of New Hampshire.

One problem is getting good information about the electric fields in clouds. Weather balloons wandering drunkenly through a storm can’t capture the dynamic changes in the electric field, which can snap into a different shape and intensity in seconds. The balloon itself can disturb the field and even set the lightning off.

Cosmic rays on the other hand can give us an overview of what’s happening in the cloud. These so-called “rays” are actually positively charged nuclei, remnants of atoms ejected from exploding supernovae at extraordinary speeds – sometimes at energies 40 million times higher than particles whizzing around the Large Hadron Collider.

When one of these high-energy cosmic rays smashes into our atmosphere it leaves a trail of destruction in its wake. Disintegrating nitrogen and oxygen atoms crash into other air molecules, generating a chain reaction that culminates in a shower of charged particles. Swept up in the Earth’s magnetic field, they drift like falling leaves in a breeze, giving off radio waves as they go.

And it is those radio waves that Schellert, a PhD student, and his supervisor, Heino Falcke, were studying to learn more about high-energy cosmic rays. Using the LOFAR (Low-Frequency Array) radio telescope near Exloo in the Netherlands, they were able to track the trajectory of the cosmic ray, as well as measure its energy – important clues for identifying the particle that produced the ray and where in the cosmos it came from. Everything worked fine in fair weather. But when a thunderstorm was nearby their data went haywire. In particular, the polarity of the radio waves – the direction they corkscrew through the air – switched during a thunderstorm, playing havoc with their cosmic ray model.

“We thought, ‘this is physics, right? There should be a reason for this’,” said Falcke.

They realised the storm cloud’s electric field must be responsible and tried to model what that field looked like. The computer simulation which best fit the observations painted a two-layered picture of a thundercloud, with a positively charged top layer and negatively charged one underneath – a structure that balloons had indeed suggested.

So have cosmic rays solved the mystery of lightning bolts? Not entirely. The size of the measured electric field is still not big enough to overcome air resistance and account for the massive discharge between cloud and ground.

However cosmic rays might themselves provide the lightning trigger. The particle showers they leave in their wake may be the slipstream that the lightning bolt follows.

“It’s a viable idea,” says Dwyer, “but I think the jury’s still out on that one.”

An alternative is that raindrops and ice crystals, whose irregular or spiky shapes mimic the effects of lightning rods, may create a trail for the lightning bolt.

“A lot of people assume that Benjamin Franklin solved the lightning problem,” Dwyer says. “But there’s a lot left to explain – and we’re still working on it.”

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