Taking a dual look at lightning

Contact: Kevin Roark, (505) 665-0582 (99-179)

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LOS ALAMOS, N.M., Dec. 15, 1999 -- The zigzagging fingers of blinding light and the booming thunderclap that follows are the familiar face of lightning, but the true nature of lightning, it turns out, is more than skin deep.

Scientists at the Department of Energy's Los Alamos National Laboratory, along with colleagues from Sandia National Laboratories, are using the FORTE satellite to continue looking deep into the mechanism of lightning, both optically and electrodynamically. They hope to one day provide a reliable statistical model for the bolts-from-the-blue and a host of other extreme weather conditions.

More than a dozen projects associated with the use of FORTE and other technologies to understand lightning were presented at a poster session today at the fall meeting of the American Geophysical Union. Included in the presentation are experiments that look at lightning's optical signals and radio frequency signals and a vast amount of related statistical data.

The primary mission of FORTE is to demonstrate advanced technologies for treaty verification and to take radio frequency and optical measurements of lightning, technologies that turn out to be ideal for detailed investigation of lightning. Inclined 70 degrees to the equator, FORTE's orbit takes the craft over some of the most lightning-prone places on Earth: South America, Africa, and Southeast Asia, for example. Because it covers the whole globe and has a very large data-storage capability, FORTE provides an ideal platform to gather statistics on the characteristics of the radio-frequency emissions associated with lightning. To date FORTE has received and recorded more than three million VHF signals associated with electrical storms.

"The thrust of all our research is to gather as much data as possible on lightning," said Diane Roussel-Dupre, FORTE flight operations manager and planner. "Our aim is to categorize the different kinds of lightning, collect a vast variety of statistics, and correlate Los Alamos' FORTE and electric field change data with detection data from other sensors so that we can eventually better understand the mechanism that is responsible for lightning."

Using the optical and radio frequency detectors on FORTE in concert with ground-based detection and characterization technologies, scientists are building the ability to pinpoint the exact location of lightning strikes and correlate the electrodynamics and atmospheric conditions that are generally present with lightning.

"The ultimate utility of FORTE is in its ability to observe lightning by both optical and radio frequency means simultaneously," said Abe Jacobson, FORTE project leader. "By combining joint data from the Los Alamos RF sensors and Sandia's fast photodiode and CCD imager sensors, we are able to tread new ground in the dual phenomonology of lightning seen from space.

"In other words," added Jacobson, "we're now not just one blind man looking at an elephant, but two partially-sighted men, looking at different ends of the elephant, acknowledging that the signals come from a related source, and communicating that effectively."

In 1993, sensors aboard another Los Alamos satellite, ALEXIS, discovered TIPPs, Trans-Ionospheric Pulse Pairs, extremely short pairs of radio pulses originating near storm centers. FORTE data has confirmed that the signal pairs correlate with lightning and that the second of the two signals is the result of signal reflection off the earth's surface. New work shows that along with ground based data TIPPs have proven to be an effective tool in determining a more specific location of a lightning strike, something not possible when looking at typical radio-frequency signals.

Expanding on technology and data already in hand from the National Lightning Detection Network, Los Alamos researchers have developed their own ground-based network, called the Sferic Array. The Sferic Array very accurately measures the electric field changes near thunderstorms and is used to locate Compact Intercloud Discharges thought to be an indicator of the types and location of thunderstorms likely to produce lightning. The array currently consists of 11 detection stations in New Mexico, Nebraska, Texas and Florida.

"If we can associate the location of certain kinds of lightning or quantities of lightning to other extreme weather conditions like heavy hail and rain or tornadoes then we're one step closer to a predictive weather model," said Roussel-Dupre. "The larger the data set the better. With enough data it may also be possible to look at the role of lightning in relation to global climate change."