Lightning Strikes High in the Clouds

Somewhere in the Midwestern United States, another powerful thunderstorm is brewing. As the sun-warmed afternoon air rapidly rises upward, a giant anvil shaped cloud begins to grow, reaching as high as 40,000 feet into the sky.

Inside this behemoth, a cauldron of swirling wind, rain and ice produce the electrical field necessary to spark a white-hot bolt of lightning. Instantly, an intense path of electricity carves its way from the cloud to the ground, producing a deafening crack of thunder as the air around the bolt flashes to 40,000 degrees.

Meanwhile 1,000 miles away, Steven Cummer, Duke assistant professor in electrical engineering, is busy recording the radio waves from this lightning with an antenna and data recorder housed deep in Duke Forest. He knows that there is a lot more to lightning than, literally, meets the eye.

Recently, physicists have discovered that lightning not only emerges from the bottom of clouds but high above them. These above-cloud lightning phenomena, known as sprites, were first observed by Air Force pilots flying in the darkness over powerful storms. Sprites occur high in the Earth's atmosphere, up to five to 10 times the altitude of cloud tops.

Observers may only see these sprites out of the corner of their eye, flashing a faint red or blue for only a fraction of a second. However, scientists using low light, high-speed cameras have captured these elusive phenomena on film. What these pictures reveal are oddly shaped bursts of light, often shooting more than 50 miles high, that sometimes resemble angels, carrots or curtains floating in mid-air.

Cummer is working with colleagues at Stanford University to develop a model of what happens electrically when a sprite goes off.

"Since their discovery, a big question was whether sprites were just glowing light or if they actually contained their own electrical current, like ordinary lightning," he says.

The answer has emerged from a set of complicated and competing theories on sprites' origins. Scientists have discovered that sprites appear 1 to 150 milliseconds after a powerful lightning discharge. The strong thunderstorms that rage over the Midwest during the summer are likely candidates for producing sprites. However, not all lightning is capable of producing a sprite, Cummer said.

"Ordinary lightning occurs when the friction of ice and water particles bumping into each other build up negative charge at the bottom of a cloud, creating an unstable electric field. The air between the cloud and ground then breaks down, creating a path of electric current," he says. Lightning that most often produces sprites comes from a center of positive charge. This type of lightning happens less often than negative discharges at a ratio of about 9 negatives to 1 positive. He says these discharges tend to be much more powerful and carry more charge.

Sprites are like lightning near the Earth, but because they occur high in the Earth's atmosphere, where there are fewer air molecules, they tend to be longer and wider than near ground lightning. They are colored red or blue because of the different chemistry of the upper atmosphere.

Cummer's research involves measuring the electromagnetic radiation given off by powerful, sprite-producing lightning bolts.

"We found the same type of pulse in a sprite, that turns on when the sprite flashes in the sky and turns off when the sprite disappears."

Although sprites tend to occur with positive lightning discharges, their actual origin is still a mystery.

"Sometimes a sprite will occur long after a discharge leading us to believe something is also happening inside the cloud to produce them," Cummer said. "By measuring the radio waves produced by the lightning, I hope to be able to say something about what is happening inside the cloud."

Shortly after they were discovered, NASA was concerned about whether or not sprites could pose problems for a space shuttle coming down through the atmosphere over a storm.

Cummer's research has helped to identify the nature of electricity in a sprite. Currently, Cummer is building the antenna and data recorder that will be placed in Duke Forest to measure lightning radiation from remote locations.

"With this device, I'll be able to take part in lightning research campaigns in the Midwest while remaining at Duke," he says.

A similar lightning recording device will go beyond the Earth's atmosphere all the way to Mars in 2005 to listen for potential dust storm lightning and similar sprite phenomena. Meanwhile, Cummer hopes his research will further illuminate the mechanisms behind sprites and the implications for lightning between thunderclouds and outer space.

Written by Kelly Malcolm.