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Network uses radio waves to log lightning
Posted January 28, 2007
Scientists creating a network to triangulate and pinpoint lightning strikes around the world are using a very narrow band of radio waves to detect the phenomena over long distances.
The small network of very low frequency (VLF) receivers includes stations in the Antarctic, including at Palmer Station. VLF generally refers to radio frequencies in the range of 3 to 30 kilohertz.
Stanford University graduate student Ryan Said is building the lightning detection and geo-location network as part of his Ph.D. thesis for the VLF Research Group, one of several groups in the Space, Telecommunications and Radioscience (STAR) Laboratory, a research team within the Department of Electrical Engineering at the university.
Umran Inan is the director of the lab and the principal investigator for the lightning triangulation program.
Every lightning strike generates a strong electromagnetic pulse, according to Said. He takes the recorded radio pulses created by an individual lightning strike from three or more geographically separate receiver stations to triangulate its location, he explained.
“So, with our receiver at Palmer, another receiver in Alaska, and a third receiver in Indiana, I can detect and triangulate most lightning activity in the United States,” he said during e-mail and phone interviews.
“At Palmer, with the incredibly quiet noise environment, we can detect most cloud-to-ground lightning flashes as far as Canada.”
Pinpointing lightning strikes is not new. The U.S. National Lightning Detection Network, operated by a commercial business called Vaisala, uses more than 100 ground-based sensors to monitor lightning continuously across the continental United States. The system is used for everything from air traffic control to help with forecasting severe weather.
But such a ground-based, high radio frequency network cannot cover the distance across oceans, Said pointed out. “That’s where the VLF content shines because it can travel these great distances and allow us to geo-locate regions of the globe that aren’t easily accessible by close-range receivers.”
He is still tweaking the accuracy and efficiency of the system and will publish the final results by June 2008.
Lightning is an electrical discharge between positive and negative regions of a thunderstorm. As the ice particles within a cloud grow and interact, they collide, fracture and break apart.
The smaller particles tend to acquire a positive charge, while the larger particles acquire a negative charge. These particles then separate. The upper portion of the cloud acquires a net positive charge, and the lower portion of the cloud becomes negatively charged. This separation produces enormous electrical potential both within the cloud and between the cloud and ground. Eventually the electrical resistance in the air breaks down and a flash begins.
The network also offers further opportunities for atmospheric research, according to Said.
“From a scientific research standpoint, lightning strikes are a source of several interesting physical phenomena, and having a database of lighting strike locations and times will aid in the investigation of these phenomena,” he noted.
One such phenomenon is called an LEP event — lightning-induced electron precipitation. A small portion of the energy created by a lightning strike as it travels along the Earth’s ionosphere “leaks” into the region of space that’s closest to the planet, the magnetosphere, which is dominated by the Earth’s magnetic field.
In the magnetosphere, high-energy particles from solar winds bounce back and forth between the northern and southern hemispheres like ping pong balls, trapped by the magnetic field.
However, some of the leaked energy from the lightning strike that travels along the ionosphere (which exists along the inner edge of the magnetosphere) will interact with these trapped particles, essentially driving them deep enough into the atmosphere to cause the LEP event. In some limited cases, the lightning geo-location system can detect the electron precipitation.
More receivers are planned for the network.
Another VLF Research Group graduate student, Andrew Gibby, will travel to the Antarctic Peninsula later this year to install a parallel antenna and data receiver at Vernadsky Station, in coordination with the Ukrainian National Antarctic Program.
The network already includes stations in Antarctica, Alaska, California and Indiana — enough to provide coverage across North and South America as well as the eastern Pacific Ocean.
International partners include Israel, though the establishment of a global geo-location network is out of the scope of his project at this time, according to Said.
“With a few stations we can geo-locate in a huge region at a relatively low cost,” he said. “Global coverage is a long-term goal.”
NSF-funded research in this story: Umran Inan, Stanford University.