Bolt From The Black

Scientists are toying with the idea that lightning gets its spark from outer space

July 22, 2005|By Michael Stroh | Michael Stroh,Sun Staff

Gaze at the sky on a stormy summer night and consider this: The lightning bolts streaking from the clouds may actually have gotten their start in outer space.

That's one theory, anyway.

It turns out that more than 250 years after Benjamin Franklin lofted his kite, scientists are still struggling to understand the basic physics behind this dazzling -- and occasionally deadly -- electrical display. "We know more about how a star explodes across the universe than we do about how lightning works," says Joseph Dwyer, who studies the phenomenon at the Florida Institute of Technology.

One of the most fundamental mysteries is how a cloud spawns lightning in the first place. Scientists know that thunderclouds contain electric fields, most likely generated by colliding particles of ice. The problem is that instrument-studded balloons and aircraft dispatched over the decades have never found a field powerful enough to spark lightning. If storm clouds are too wimpy to touch off the bolts, what does? A growing number of researchers are wondering whether a provocative, 13-year-old Russian theory -- once written off as impossibly weird -- might clear up the conundrum.

The theory holds that highly energized particles bombarding the Earth from space provide the initial spark for lightning to form. These particles, known as cosmic rays, are essentially atomic debris from solar flares and ancient exploding stars scattered across the Milky Way.

"You wouldn't even imagine such a connection," says Thomas Marshall, a physicist at the University of Mississippi in Oxford who studies electrical fields in storm clouds. "I think we're making progress."

It's not to say that scientists haven't learned anything about lightning since Franklin first proved it was an electrical phenomenon.

They know, for example, that a bolt can cook the surrounding air to 50,000 degrees Fahrenheit, about five times hotter than the surface of the sun. This rapid heating is what produces the sound we hear as thunder. A lightning bolt, scientists say, is also potent enough to dislodge a 5-ton stone and has enough energy to keep a 100-watt bulb glowing for three months.

They also know it can kill. An average of 62 people die from lightning-related injuries in the United States each year, according to new research from scientists at the Centers for Disease Control and Prevention. In the study, which analyzed lightning injuries from 1995 to 2000, Florida and Texas recorded the highest number of deaths. In Maryland, lightning killed fewer than 10 people during the period, researchers found.

Over the last century, high-speed cameras and other tools have allowed scientists to dissect the basic anatomy of a lightning strike. It begins with a "stepped leader," a negatively charged channel that emerges from the base of a thunderstorm and zigzags toward the positively charged Earth. The completed circuit triggers a surging upward "return stroke." Visible as a blistering flash, this is the first part of the process that most people see.

Some lightning strokes stop there. But often bolts known as "dart leaders" continue to ply the electrical channel forged by the original stepped leader. This back and forth registers on the human eye as a series of flickers that can last as long as a second.

The majority of lightning strokes travel from one part of a cumulonimbus thundercloud to another, producing a bright flash known as sheet lightning. Only about one-third of bolts jump from cloud to ground.

This helps explain why progress in studying the phenomenon has been so slow, says Martin Uman, a lightning researcher at the University of Florida who has written several textbooks on the subject. While lightning flashes somewhere on the planet an estimated 100 times each second, "you don't know when or where," he says.

So, like many physicists who want to study the phenomenon up close, Uman attempts to coax it from the sky.

He and other scientists spend the sticky Florida summers launching 3-foot rockets from a wooden tower erected on the sprawling grounds of a military base near Jacksonville -- a part of the country known as "lightning alley" because it generates so many strikes each year.

As the missiles scream into the sky, they unspool a gossamer-thin, Kevlar-coated copper wire. The wire serves as an electrical conduit. If conditions are right and researchers lucky, the wire will suddenly explode to life as it lures a bolt from the clouds. "If you're up close it sounds like a high-powered rifle," says Uman.

Researchers huddling inside a metal trailer a few dozen yards away videotape and measure the stroke. The trailer is fully grounded, but these so-called "triggered lightning" experiments still have the power to startle. "On the video you can often hear people scream," says Uman, who co-directs the International Center for Lightning Research and Testing, as the outdoor lightning lab is known.

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