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By
Peter Rejcek
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.”
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