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IN THE time it takes to blink, there are 10 flashes of fork lightning in the Earth’s atmosphere. Each bolt travels at around a third of the speed of light, reaches temperatures as hot as the sun’s corona, and carries about 10,000 times as much current as a household circuit. No wonder lightning triggers more than a third of North America’s power cuts each year, and kills hundreds of people worldwide.

The sheer electrical power of lightning has had researchers baffled. In 1752, Benjamin Franklin flew a kite into a storm cloud and showed that it became electrically charged. But how the comparatively modest build-up of charge in a cloud has the strength to set off such an impressive light show has been something of a mystery. “Nobody understands what’s going on here. You have a lot of people guessing, but we are really clueless. After a couple of hundred years, it’s actually quite embarrassing,” says Joe Dwyer, who studies lightning at the Florida Institute of Technology in Melbourne.

Now the mystery is about to be solved. X-ray emissions detected from lightning bolts have provided the first confirmation of a surprising and once controversial theory: that lightning comes from outer space.

Lightning is the release of pent-up charge stored in an electric field. In a thundercloud, electric fields are generated by collisions between ice particles (see “Icy origins”). The bulk of the charge is negative, inducing a positive charge in the ground thousands of metres below. Eventually the air between becomes ionised and conducts charge either from cloud to cloud or cloud to ground as a lightning bolt.

But there is a flaw in this explanation. Air only ionises spontaneously in electric fields of around 2500 kilovolts per metre. Centuries of often dangerous measurements with kites, balloons and aircraft have produced many measurements of fields in thunderclouds. (In 1753, a Russian scientist was killed as he tried to reproduce Franklin’s kite-flying experiment.) But no one has ever found an electric field in a storm cloud that is anywhere near strong enough to ionise air molecules. The fields found are typically between 100 and 400 kilovolts per metre, less than a tenth of what is needed.

Some argue that lightning researchers have simply missed small regions of high field strength. The massive storms of the Great Plains in the US can be 10 kilometres high and span some 100,000 square kilometres. Even relatively small thunderstorms cover 2500 square kilometres. “If you have ever seen a research plane inside a cloud, it’s just like a needle in a haystack,” says atmospheric physicist Clive Saunders of the University of Manchester, UK. So it is possible that the strong fields are in there somewhere, far from the measuring devices. But an increasingly popular explanation is that the strong fields are not seen because they are simply not there. “The more measurements we make that don’t see high fields, the less likely it is that we’re missing them,” says lightning modeller John Helsdon at South Dakota School of Mines and Technology in Rapid City.

A few years ago, lightning researchers began to look for some other way the electrical breakdown of air might be triggered, and they have identified a chief suspect: cosmic rays. These are highly energetic particles that zip through space at close to the speed of light. Thousands of them bombard every square metre of Earth’s atmosphere each second, many having travelled intergalactic distances. In 1992, Alex Gurevich of the P. N. Lebedev Physical Institute in Moscow had proposed a way that cosmic rays might seed lightning. When a cosmic ray strikes Earth’s atmosphere, it could hit an air molecule, ionising it and producing an extremely energetic electron. In the electric fields near a storm cloud, such an electron could be accelerated to near the speed of light, then hit and ionise other air molecules, producing more and more electrons in a chain reaction. The ensuing avalanche of electrons would ionise the air, allowing charge to flow. Gurevich called the idea “runaway breakdown”.

“Thousands of cosmic rays bombard every square metre of Earth’s atmosphere each second, many having travelled intergalactic distances”

The theory was originally considered fairly maverick, but in the absence of other explanations, it is now becoming mainstream. The main advantage of runaway breakdown is that it requires a far smaller electric field to get started – around 300 kilovolts per metre, similar to that routinely measured in storm clouds. And electrons moving near the speed of light emit energetic radiation such as X-rays and gamma rays, providing a way to test the idea. In 2001, Charlie Moore and his colleagues at the New Mexico Institute of Mining and Technology in Socorro found the first direct evidence for runaway breakdown when they recorded X-rays shooting from the “leader” of a nearby lightning bolt. The leader is the nearly invisible path of current that moves towards the ground in halting steps, each between 50 and 100 metres long. Each step is separated by a 50 microsecond pause. Usually, the leader is negative, but as it approaches the ground, positive charges collect and are drawn upwards. When these two paths meet, the circuit between ground and sky is complete and lightning is unleashed.

There had been anecdotal reports of lightning emitting X-rays for decades, but Moore’s reports got researchers very excited. “Moore’s observations got a lot of people thinking that maybe the energetic radiation was real. But it needed to be confirmed or denied. We still couldn’t be sure it wasn’t a mistake,” says Martin Uman, co-director of the International Center for Lightning Research and Testing in Camp Blanding, Florida.

Dwyer, Uman and colleagues set out to test the theory a different way. They triggered lightning themselves by sending rockets up into storm clouds, which, says Uman, has some advantages over waiting for a natural strike. “You can wait a whole summer and only get a couple of lightning strikes close enough to see,” he says. In 2002, following Moore’s reports, they began watching their man-made strikes with an X-ray detector. To the team’s amazement, of the 37 strikes captured in the summer of 2002, 31 emitted X-rays. It seemed that the rocket was setting off runaway breakdown, just as, perhaps, cosmic rays do in natural strikes.

Since the original observations, Uman, Dwyer and their collaborators have refined their measurements of the X-rays’ energies. Last summer they recorded microsecond bursts of X-rays with energies of around 150,000 electronvolts. This energy coincides with the energy of electrons in a runaway breakdown cascade over a distance of about 50 metres – the typical distance for a step of the zig-zag of a lightning bolt. Dwyer was particularly pleased. “The stepping process determines where lightning will go and how it branches. But stepping has been very mysterious and no one knows exactly how or why it does it.” Runaway breakdown explains this: the field builds up over a short distance in the air until breakdown is triggered, then the charge moves to the next location and the build-up begins again.

However, just as evidence has begun to pile up for runaway breakdown, some kinks have arisen in the theory that cosmic rays are setting off the cascades. In February, Thomas Marshall of the University of Mississippi in Oxford and colleagues reported a series of balloon measurements taken inside a mountain thunderstorm in New Mexico. They compared the timing and position of a lightning flash with the position of a remote balloon carrying electric field meters that they had sent into a storm cloud. Two lightning flashes happened just moments after the local electric field passed the runaway breakdown threshold of 281 kilovolts per metre.

Cosmic kick-off

But there was one puzzling observation. For one of the three strikes that the team studied, even though the local electric field grew to 345 kilovolts per metre it sat there for a full 40 seconds before lightning shot to the ground. Why the delay? “It may just be waiting for a bigger cosmic ray to kick it off,” says Marshall. This points to a major unknown in the runaway breakdown theory: if runaway breakdown is triggered by cosmic rays, just how energetic must a cosmic ray be to do the trick?

Gurevich suggested in 2003 that you may need a particle with an energy of at least 10¹⁶ electronvolts. But a cosmic ray of this energy hits a square kilometre of Earth only once every 50 seconds: not often enough to account for the world’s lightning activity. And when Dwyer recently went back and took a closer look at the energies of X-rays recorded from lightning leaders between 2002 and 2004, he found they pointed to runaway electrons with energies around 20 times smaller than those Gurevich predicted for a cosmic ray cascade. “It’s very perplexing,” says Marshall.

However, Dwyer’s observations only apply to X-rays measured from lightning bolt leaders. Gurevich’s cosmic ray predictions might still apply to the initiation of runaway breakdown up in the clouds. In summer 2003, Dwyer recorded a burst even more energetic than an X-ray burst, a gamma-ray burst, coming from the cloud above a lightning bolt. Such a high-energy event fits Gurevich’s predictions much better.

“Researchers are convinced that lightning is caused by some kind of runaway breakdown and this could be triggered by cosmic rays”

Gamma-ray bursts in clouds have been well studied because satellites can see them from above. A team working with NASA’s RHESSI satellite, for example, have reported seeing around 50 gamma-ray flares from the atmosphere every day, many with the 10¹⁶-electronvolt energies needed to match Gurevich’s predictions. Originally, lightning theorists had only been interested in gamma-ray flashes because they believed they might be connected with another lightning mystery: sprites. These faint flashes of light dance above the clouds at an altitude of between 40 and 90 kilometres (storm clouds are 10 to 16 kilometres above the ground).

Sprite researcher Steve Cummer at Duke University has now compared the RHESSI results to radio emissions collected below the clouds at his research station in Durham, North Carolina. Very low frequency radio can be used to detect whether there has been a lightning bolt within about 4000 kilometres of a detector. Cummer says the strike positions and timing overlap with the gamma-ray signals RHESSI saw from above. What was more, of 26 gamma-ray flashes that Cummer studied up close, none was connected with a sprite. They all seemed to come from the height of thunderclouds rather than much higher sprite territory.

“Certainly it’s looking like terrestrial gamma flashes are associated with processes that may have something to do with lightning initiation,” says Cummer. But there are still some hiccups with the theory: there are not enough gamma-ray bursts to account for all the lightning activity on Earth, for example.

Nevertheless, researchers are convinced that lightning is caused by some kind of runaway breakdown, and the circumstantial evidence is that this could be triggered by a cosmic ray high up in the clouds. But so far, they cannot be sure that runaway breakdown is always sparked off by cosmic rays.

For Paul Krehbiel at the New Mexico Institute of Mining and Technology, researchers will have found the real “smoking gun” when they show that both lightning bolts and the cascades of particles triggered by incoming cosmic rays originate at the same position. To see this, researchers need field stations capable of detecting the showers of charged particles set off by cosmic rays as well as lightning. Gurevich and his colleagues have recently set up just such a station in Kazakhstan, and Dwyer and Uman are combining the power of 32 telescopes scattered over a square kilometre to capture the X-rays and the cascades close together. “Maybe we’ll find bursts of gamma rays coming out of every thunderstorm,” Uman says.

Dwyer would be very surprised to see the theory disproved now. “We know that runaway breakdown is occurring, and we know that lightning is occurring,” he says. “Do these two things have anything to do with each other? That’s just a question of being at the right place at the right time.”