Like a Bolt from Above

Lightning scientists begin to solve electric mysteries

SOCORRO, N.M. — Ten thousand feet high in the New Mexico mountains, Jake Trueblood is getting ready to fire rockets into a thunderstorm.

In this lightning flash that researchers generated over Camp Blanding, Fla., luminous stroke sequences are blown to the left of the vertical wire that triggered the flash. D. Hill/Univ. of Florida
At a mountaintop lightning laboratory in New Mexico, Jake Trueblood preps rockets to trigger lightning flashes. A. Witze
SPARK FROM ABOVE Lightning flashes most commonly discharge within clouds, but people on the ground tend to care most about the less common but more dangerous cloud-to-ground flashes. No matter where a flash reaches, the steps involved in initiating electrical discharge are the same. E. Feliciano
LIGHTNING IN 3-D These images represent three-dimensional slices of a cloud-to-ground lightning flash that occurred in New Mexico on September 7. Green dots show electrical breakdown propagating through negative charge in the storm. Gray dots show where positive charge resides, and gold represents unidentified sources of radiation. New Mexico Tech

He lines up eight rockets, straight as soldiers, then connects each to a wire bobbin once used to guide missiles for the French military. Trueblood arms the rockets and heads underground, then waits for hours in a windowless chamber on whose metal roof the rockets sit.

Trueblood, a graduate student at the New Mexico Institute of Mining and Technology in Socorro, is waiting for a good strong electric field in the atmosphere. Then he’ll push a button that will send a whoosh of compressed air to a single rocket, sending it careening more than a thousand feet high. The goal is for the rapidly moving wire to trick the air into discharging its electricity in a lightning flash that will slam to the ground just above Trueblood’s head.

He and other lightning hunters aren’t out on the mountaintop this August day for the thrill. They’re here, at New Mexico Tech’s Langmuir Laboratory for Atmospheric Research, in search of knowledge. “We’re here because we’re trying to understand the simplest storms we know of — and we can’t,” says Graydon Aulich, a lightning researcher at the lab.

Golfers and picnickers are acutely aware of lightning and its dangers, but scientists still don’t understand it. “It’s really amazing when you think this is something that everyone knows about,” says Joseph Dwyer, a physicist at the Florida Institute of Technology in Melbourne. “But try to draw the basic picture, where the electric charges are in the cloud and where are the currents, and you realize you don’t even know how to draw the picture to start with.”

Now, however, studies like Trueblood’s are helping to flesh out that picture. Shooting rockets into thunderstorms has allowed researchers to better understand how lightning follows an electrified channel through the air, hits the ground and then returns along the same path whence it came. Balloonborne and other experiments have revealed that X-rays and gamma rays often accompany lightning, a discovery that hints at high-speed electrons kicking the whole process off. And ways of mapping lightning in three dimensions have uncovered secrets of how lightning travels within a cloud, as well as how it can send a dangerous “bolt from the blue” to hit the ground kilo­meters away — or even zoom upward to the edge of space.

Such discoveries not only fill out the picture of lightning physics, but also are helping engineers design better systems to divert deadly bolts away from buildings and the people within.

Charging up

In 1752, Benjamin Franklin famously attached a metal key to a kite and flew it in a thunderstorm, observing a spark and deducing that electric charge existed in the atmosphere. Two and a half centuries later, scientists have a slightly better idea of how lightning forms: Hail and small ice particles rubbing against one another within clouds transfer electric charge, with positive charges generally gathering on small ice crystals that are carried higher by updrafts and negative charges gathering on heavier hail particles that drift lower in the cloud. This charge separation builds up an electric field, which at some point must be reconciled by discharging electricity between opposite charges, like a static spark on clothing on a dry winter day.

Electrons begin by carving a series of ionized channels, known as stepped leaders, through the air; for a typical cloud-to-ground flash this means negative charge starts propagating downward from the negatively charged region in the lower part of the cloud. Once the stepped leader reaches the ground, or another region of opposite charge, electric current zaps between the two points, creating the visible lightning flash with temperatures over 25,000° Celsius.

Worldwide, some 100 flashes occur every second. Not all of these reach the ground — in fact, the most common type of lightning discharges within a single cloud — but those that do can be deadly. Roughly 55 people are killed by lightning each year in the United States alone.

Yet studying lightning is like, well, trying to capture lightning in a bottle. The flash may happen often, but not often enough over the places where scientists sit and wait to study it. Kenneth Eack, a physicist at New Mexico Tech, says researching lightning is like trying to conduct an experiment knowing that the electricity you need will be turned on in your building for 20 minutes at some point during the summer — but you don’t know which 20 minutes on which day, so all you can do is wait.

To better track lightning’s unpredictable appearances, in the mid-1990s researchers at New Mexico Tech began developing a three-dimensional lightning mapping system. Global Positioning System satellites had just started to come into common use, and on a flight back from a geophysics conference Paul Krehbiel and William Rison realized that they could use GPS receivers to precisely time and locate lightning flashes.

Today the researchers’ mapping array consists of a collection of plastic tubs and other containers, each holding a solar-powered detector to measure radio-frequency radiation arriving from sparks within a lightning discharge. With multiple stations, the scientists can build a three-dimensional picture of how lightning appears and branches across the sky. Between 16 and 18 stations typically dot the mountaintop at Langmuir Lab. Many other research groups have set up similar arrays using the New Mexico Tech technology, including a new facility in Catalonia, Spain.

Because the system typically picks up 60-megahertz signals in very high frequency, or VHF, bands, it became a lot easier to detect lightning signals when overlapping television broadcasts in the United States moved to digital, Krehbiel says. Other tweaks have also improved the array’s sensitivity over time, allowing the scientists to see lightning in better detail than ever before. Already, the researchers have spotted many more “precursors,” or attempts to get intracloud lightning discharges started. Many flashes try multiple times to discharge before they make it, Rison says.

Data from the array have also helped explain why negatively charged cloud-to-ground flashes often have multiple strokes, whereas positively charged ones (which are less common) usually don’t. Positive leaders are observed to branch out a lot and move forward only tenuously, whereas negative ones branch less and move forward more robustly, Krehbiel reported in Rio de Janeiro in August at a conference on atmospheric electricity. “Every time we look at the data we see something new,” he says.

Next spring, the group plans to set up a new array in north-central Colorado as part of a larger study on how electrified storms affect atmospheric chemistry. As one side result, the team will see how often wind turbines along the Colorado-Wyoming border get hit by lightning. Turbine blades can spark electrical discharges, a major hazard, while spinning through the air at up to 100 miles per hour. “There’s no data on what the mechanism is, and if we knew maybe we could design a wind turbine to be less susceptible to lightning,” says Rison.

X-ray vision

Fifteen hundred miles east of Langmuir, other scientists use the lightning mapping array — and a lot more — at the country’s other premier research facility, the International Center for Lightning Research and Testing at Camp Blanding, Fla.

Scientists at Camp Blanding, a military base where neighbors don’t tend to complain about rockets bringing lightning down on them, are in the middle of an intensive four-year research project funded mainly by the Defense Advanced Research Projects Agency. Among other things, radar scans the area for storms; high-speed cameras, including a video camera that can shoot 4 million frames per second, record any lightning flashes; sensitive instruments capture information on electric and magnetic fields; the mapping array locates each bolt in three dimensions; and X-ray equipment detects high-energy radiation accompanying a flash. All this equipment gathers 100 measurements on each flash, 24/7, over an area of about a square kilometer. “It’s the most [lightning] instrumentation anyone has ever had in one place in the history of man,” says the facility’s codirector, Martin Uman.

The work paid off on July 7 of this year, when the instruments captured a natural lightning flash with four strokes. “Everything lit up like gangbusters,” says Uman, of the University of Florida in Gainesville. Although they haven’t gone through the data yet, the researchers expect to learn much from this scientific gift from above.

Along with studying natural lightning, the Florida team also triggers lightning the way Trueblood does in New Mexico. Triggered lightning is in some sense artificial; electricity coursing down the rocket wire does not behave the same way as ionized channels forming naturally. But the return stroke of triggered lightning, when the current connects and creates a visible flash, is pretty much the same as nature provides, Uman says. So shooting rockets into thunderstorms, as cowboy as it might sound, provides an easier way to get lightning where you want it.

Camp Blanding scientists are tackling three seemingly simple yet devilishly complex questions about lightning: how it originates in the cloud, how it travels through the air and how it connects to the ground (a key question in protecting people and buildings from strikes). Of these, lightning’s birth is the least understood, Uman says. “People always thought it was an electrical breakdown like happens in the laboratory: Put in a big enough electric field and the air starts to become conductive,” he says. But in clouds, lightning seems to occur where electric fields are much lower than those needed for a lab discharge. And for a long time no one could figure out how lightning got started in these smaller electric fields.

In the last few decades, however, researchers have begun to explore versions of a theory known as “runaway breakdown.” According to this theory, cosmic rays hitting the atmosphere deliver a steady supply of high-energy electrons. The cosmic rays also knock into other electrons, which speed up in the thundercloud’s electric field. Soon the electrons are avalanching out of control. “Suddenly your electric field is big enough to accelerate electrons up to the speed of light,” Dwyer says. Crucially, this means that electrons can spark a discharge when the electric field is an order of magnitude less than theorists had thought necessary.

Still, scientists don’t understand all the steps in the runaway breakdown idea. And it’s not clear whether the fast electrons initiate the lightning discharge or simply accompany it, Eack says.

X-rays in lightning could help solve the dilemma. If Superman watched lightning strike, his X-ray vision would see high-energy radiation accompanying the bolt all the way down, Dwyer says. The Langmuir team reported the first surprising hints of these X-rays back in 2001, and Dwyer later confirmed them by setting up sensitive detectors at the Florida lightning facility. At times, the X-rays were so intense that they blinded his equipment.

Last year, Dwyer took the first pictures of these X-rays during a triggered lightning flash at Camp Blanding. Made with a pinhole camera that measures voltage, the images show bright honeycomb-shaped pixels descending with the bolt from above.

Different strokes

Superman would have another advantage over today’s top lightning researchers: He could fly into, above and below a thunderstorm, watching lightning move in all directions. And he’d see phenomena that earthbound scientists have just begun to discover in recent years: lightning flashes that travel not just within a cloud or directly from the cloud to the ground, but that break out to the side or even zoom straight up to space.

The most spectacularly named of these are the “gigantic jets,” which are essentially ordinary lightning flashes that manage to punch straight up from a thundercloud and travel some 80 kilometers upward. Videos from two amateur scientists show how the jets manage this feat.

Normally, in-cloud lightning flashes develop in the lower, negatively charged part of the cloud; if they happen to reach higher within the cloud, the positive charges higher up cancel out the negative charge, stopping the flash. But in two cases videotaped recently in Florida and Oklahoma, the negatively charged leader zoomed upward and then went sideways, trying to exit the side of the cloud. Had it succeeded, it would have become a “bolt from the blue,” where a flash zaps the ground many kilometers away from the cloud where it originated. Bolts from the blue are some of the most dangerous types of lightning, as no one is expecting a strike from clouds far away.

But in these two cases the leaders didn’t break out of the clouds, says Steven Cummer, an electrical engineer at Duke University. They fizzled instead. In the process, though, they shorted out much of the positive charge that usually sits near the top of the cloud acting like a lid on a pot to keep negatively charged leaders from breaking through. So when a second leader was born within the same cloud and zoomed upward, it had very little positive charge above to trap it in. That leader broke through the top of the cloud and kept zipping up as a gigantic jet, Cummer and his colleagues, including Duke postdoctoral researcher Gaopeng Lu, reported online in June in Geophysical Research Letters.

Gigantic jets aren’t the only things that fly upward from storm clouds; so too do mysterious flashes of high-energy gamma rays. “Both of these events are produced by the most ordinary of all lightning,” Cummer says.

The only way to see the rays, known as terrestrial gamma-ray flashes or TGFs, is from the viewpoint of a gamma-ray satellite looking down on Earth. The first such TGFs were reported in 1994, and as yet no one is entirely sure how they form. But recent work by Cummer and other researchers suggests that TGFs are born in the first milliseconds of a lightning flash, when there is but a single lightning channel moving almost directly upward. “The high electric field that’s driving the TGF is definitely connected in some way to that upward leader,” Cummer says.

Even more surprising, TGFs contain antimatter, the doppelgänger of normal matter. Earlier this year, scientists from the University of Alabama in Huntsville reported that positrons, the antimatter counterpart of electrons, are common in thunderstorms (SN Online: 1/10/11).

Dwyer thinks these positrons may be the key to understanding TGFs. In a new theory he has presented at several scientific conferences, he argues that the runaway electrons thought to trigger lightning also produce gamma rays, which in turn collide with ordinary air particles to produce electrons and positrons. “The whole discharge becomes self-sustaining, where you get huge bursts of gamma rays because of all the positrons you’re making,” Dwyer says. “If this is correct, then one of the keys to understanding thunderstorm physics is positrons. Who would have thought that a few years ago?”

Few lightning researchers would have foreseen most of these discoveries a few years ago. But back at Langmuir, Trueblood and Aulich know they aren’t going to be contributing to new breakthroughs on this particular August day. Having watched thunderstorms come and go all around the Magdalena Mountains, too distant to try triggering a lightning flash in, the scientists are packing it in. They call the local Federal Aviation Administration office and say it’s OK to start routing airplanes over the lab again. “We’ve gone cold,” Aulich says as he hangs up the phone.

Volcanic lightning

David Jon/NordicPhotos/Getty Images

Lightning researchers chase thunderstorms, and volcanologists lie in wait for eruptions — and it took a seismologist in Alaska to bring the two worlds together.

In 1992, Steve McNutt of the University of Alaska, Fairbanks picked up some funny signals on the seismometers he was using to study the erupting Mount Spurr, near Anchorage. The signals turned out to be static from lightning generated by the eruption, possibly the first time volcanic lightning was recognized as such. Since then McNutt has been tracking lightning discharges in the huge billowy plumes from erupting volcanoes.

Electrical charges separate into positive and negative regions inside a volcanic plume much as they do within a thundercloud, thanks to ice particles forming and rubbing against one another, among other factors. Electricity then discharges between the charge separation, creating dramatic lightning bolts within the eruption plume.

So far McNutt has gathered data on lightning from 394 eruptions at 154 volcanoes. Volcanic lightning turns out to be “a hell of a lot more common than people had thought,” he says. “With modern tools in place, we can start to exploit it for what it tells us about the eruption process.”

For example, McNutt and his colleagues have found that when an eruption involves a lot of water coursing up from inside the Earth along with magma, more lightning occurs. “The combination of ash particles, like seeds, and a lot of water sets up a very efficient mechanism to produce a lot of electricity and lighting,” he says. In essence, the water turns an eruption cloud into a sort of dirty thunderstorm.

McNutt has also teamed up with Ronald Thomas and other scientists from the New Mexico Institute of Mining and Technology in Socorro, who developed a system for mapping lightning in 3-D. To catch lightning in the act, the scientists set up several mapping stations at a safe distance from an erupting volcano. So far, they’ve caught flashes at four: Mount Augustine in Alaska in 2006, Chaitén in Chile in 2008, Mount Redoubt in Alaska in 2009 and Eyjafjallajökull in Iceland in 2010 (lightning at Eyjafjallajökull shown). Among other things, the scientists have found that electric charge persists in the plume as the eruption continues, suggesting that interactions among ash and other particles continue to build up electric fields so that lightning can keep occurring.

One day, lightning might even be used to detect eruptions at remote volcanoes. Last December, the U.S. Geological Survey and the University of Washington in Seattle unveiled an alert system that ties into the World Wide Lightning Location Network, a global array of laboratories that monitor lightning activity. By searching for lightning activity near known volcanoes, the system can e-mail notices of eruptions to scientists even before the ash cloud becomes visible to satellites.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.