Lightning sprite phenomenon explained

DukeUniversity inforefrontofresearch onnaturalenergyflow


DURHAM, N.C. - Magnetic field measurements by a German researcher and analyses by a Duke University engineer explain how dual electrical discharges associated with high-altitude lightning can sometimes be separated by unusually long intervals of one-tenth of a second or more.

The studies show that strong cloud-to-ground electrical currents can persist between the first and the follow-up discharges, maintaining enough energy flow for the second burst to trigger a sprite -the faint, colorful and exceedingly brief flashes that are now known to erupt high in Earth's atmosphere in a region just below the ionosphere, which begins at an altitude of about 50 miles.

While previously sensed by airline pilots, lightning sprites have been scientifically documented only in the past decade.

Previous research

Previous research, some of it by Steven Cummer, a Duke assistant professor of electrical and computer engineering, has linked sprite development to the extra-powerful thunderstorms observed in the Midwest.

Earlier work also ties each sprite to an unusually strong cloud-to-ground lightning bolt, followed by a second discharge at heights of 25 to 50 miles.

Cummer, who uses an antenna in a remote forest location near Duke to study such Midwestern storms, documented that the energy in the second discharge, rather than the initial bolt, corresponds with the waxing and waning of a sprite.

In a new report, published in the journal Geophysical Research Letters, Cummer and Martin Fuellekrug of Frankfurt University's Institute for Meteorology and Geophysics answer a question that had continued to perplex sprite investigators.

About 75 percent of the time, sprites evolve quickly in a "very causal" way, Cummer said. That explainable chain of events begins when an exceptional lightning burst builds up a high-altitude electric field sufficient to create the second spark that turns into a sprite.

"But with that remaining 25 percent there can be a really long delay from any lightning stroke to when the sprite starts, from longer than 10 to as long as 200 milliseconds [thousandths of a second], which is longer than most lightning processes happen," he said.

In such cases, Cummer's antenna studies, which analyze low-frequency electromagnetic returns characteristic of lightning strikes, would show the initial bolt and the second discharge as peaks on a graph separated by an unaccountable gap in activity. "It was a mystery what was connecting the two," he said.

"It had been known, independent of sprites, that some lightning discharges have what are called `long continuing currents' that can last 100 milliseconds or longer," he said. "But most measurements of that current are much smaller than the lightning peak current, just not enough current to make anything like a sprite happen.

"There were suspicions that something like these long continuing currents were acting in some sprite-producing lightning. But those would need to be at least 10 times, and probably more like 50 times, bigger than ordinary continuing currents. And nobody had observed continuing currents long enough and strong enough to make these delayed sprites," Cummer said.

Cummer found signs of the missing currents by teaming up with Fuellekrug, who works with magnetic field sensors exceptionally sensitive to ultra-low electromagnetic frequencies. Stationing those sensors in the summer of 1998 at Santa Cruz, Calif.; Soccoro, N.M.; and Saskatoon, Saskatchewan, Fuellekrug focused on three cases in Michigan, Minnesota and Oklahoma where high-altitude sprites followed lightning strikes below by more than 40 milliseconds.

The lightning and sprite events were linked by their timing and locations, the first being logged by the National Lightning Detection Network, while the sprites were video-imaged by University of Alaska researchers.

Applying mathematical modeling analysis to Fuellekrug's much more sensitive measurements, Cummer found continuing cloud-to-ground currents in one event that varied from about 4,000 to 7,000 amperes over a period of about 150 milliseconds. "That number is extremely big," he said. "Most measurements of continuing currents like this in less spectacular lightning are on the order of 100 to 200 amps."

Continuing current

His analysis showed 10,000 amperes of continuing current flowed between lightning and sprite discharge during another event. "In an ordinary lightning discharge, you may have a peak current of 10,000 to 20,000 amps," Cummer noted, but for a much shorter time. "These continuing currents are approaching the peak currents in ordinary lightning, but we're talking about durations that are more than 100 times longer."

The difference between a large and small interval in this study may still seem like an indistinguishable instant. "You can probably have a sense of 100 milliseconds, which is 0.1 seconds, by looking at a stopwatch. But that tenth of a second is still pretty short," he said.

"The question is: Does this always happen in the case of long-delayed sprites? We've only sampled three. Beyond the sprite implications, these measurements raise the ceiling of just how big continuing lightning currents can get.

"There's no question that these kinds of discharges are the ones that can start forest fires. If you pump that much current into a tree for that long, you give it so much time to heat up you can't help but start a fire," he said.

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