Featured Research

from universities, journals, and other organizations

Long-Range Magnetic Studies Explain How Delayed "Sprites" Get Their Energy

Date:
February 12, 2001
Source:
Duke University
Summary:
Magnetic field measurements by a German researcher and analyses by a Duke University engineer explain how dual electrical discharges associated with the creation of ghostly, high-altitude "sprites" can sometimes be separated by unusually long intervals lasting as much or more than one-tenth of a second.

Magnetic field measurements by a German researcher and analyses by a Duke University engineer explain how dual electrical discharges associated with the creation of ghostly, high-altitude "sprites" can sometimes be separated by unusually long intervals lasting as much or more than one-tenth of a second.

Their studies show that previously undocumented 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 actually trigger the sprite.

Sprites are 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 keen-eyed observers, such as airline pilots, they only have been scientifically documented in the past decade.

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 places like the Midwestern United States.

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

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

In a new report, published in the Thursday issue of 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 in a minority of cases. About 75 percent of the time, sprites evolve quickly in a "very causal" way, Cummer said in an interview. That explainable chain of events begins when an exceptional lightning burst builds up a high-altitude electric field sufficient to spawn 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 noted.

"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 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 different 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."

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," Cummer 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."

Sprites that erupt so long after an initial megabolt of lightning "aren't necessarily any more energetic than the ones that come soon after," Cummer said.

"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."


Story Source:

The above story is based on materials provided by Duke University. Note: Materials may be edited for content and length.


Cite This Page:

Duke University. "Long-Range Magnetic Studies Explain How Delayed "Sprites" Get Their Energy." ScienceDaily. ScienceDaily, 12 February 2001. <www.sciencedaily.com/releases/2001/02/010205073718.htm>.
Duke University. (2001, February 12). Long-Range Magnetic Studies Explain How Delayed "Sprites" Get Their Energy. ScienceDaily. Retrieved July 23, 2014 from www.sciencedaily.com/releases/2001/02/010205073718.htm
Duke University. "Long-Range Magnetic Studies Explain How Delayed "Sprites" Get Their Energy." ScienceDaily. www.sciencedaily.com/releases/2001/02/010205073718.htm (accessed July 23, 2014).

Share This




More Earth & Climate News

Wednesday, July 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Observation Boat to Protect Cetaceans During Ship Transfer

Observation Boat to Protect Cetaceans During Ship Transfer

AFP (July 22, 2014) As part of the 14-ship convoy that will accompany the Costa Concordia from the port of Giglio to the port of Genoa, there will be a boat carrying experts to look out for dolphins and whales from crossing the path of the Concordia. Duration: 01:02 Video provided by AFP
Powered by NewsLook.com
New Orleans Plans to Recycle Cigarette Butts

New Orleans Plans to Recycle Cigarette Butts

AP (July 21, 2014) New Orleans is the first U.S. city to participate in a large-scale recycling effort for cigarette butts. The city is rolling out dozens of containers for smokers to use when they discard their butts. (July 21) Video provided by AP
Powered by NewsLook.com
Shark Sightings a Big Catch for Cape Tourism

Shark Sightings a Big Catch for Cape Tourism

AP (July 21, 2014) A rise in shark sightings along the shores of Chatham, Massachusetts is driving a surge of eager vacationers to the beach town looking to catch a glimpse of a great white. (July 21) Video provided by AP
Powered by NewsLook.com
Spectacular Lightning Storm Hits London

Spectacular Lightning Storm Hits London

AFP (July 19, 2014) A spectaCular lightning storm struck the UK overnight Friday. Images of lightning strikes over the Shard and Tower Bridge in central London. Duration: 00:23 Video provided by AFP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins