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NSF PR 99-20 - April 2, 1999
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First Estimates Developed Of Lightning-Associated
"Sprites"
Radio signals help scientists
estimate how many occur in thunderstorms
For the first time, scientists have developed a reliable
estimate of the number of "sprites" spawned by a single
thunderstorm. Sprites, the luminous red glows that
are the high altitude companions of some lightning
strikes, are the focus of a new study by researchers
Steven Reising of the University of Massachusetts,
and Umran Inan and Timothy Bell of Stanford University
in California. The team's findings appear in the April
1 issue of Geophysical Research Letters (GRL), published
by the American Geophysical Union. The work was funded
by the National Science Foundation (NSF), the U.S.
Air Force, and NASA.
Sprites accompany roughly one in every 200 lightning
strikes. They tower up to 55 miles above a thundercloud,
occurring simultaneously with a lightning strike,
and can be seen with the naked eye, sometimes from
as far away as 400 miles. Sprites are electrical phenomena
that appear above thunderclouds, reaching the lower
ionosphere. These striated, glowing ribbons appear
at several-minute intervals. They are found above
all the major landmasses of the earth, according to
Reising.
"Sprites are spectacular luminous evidence of electrodynamic
coupling between the neutral atmosphere in which weather
processes occur and the higher altitude (60-90 km)
ionized regions of the earth's atmosphere known as
the mesosphere and the lower ionosphere," explains
Sunanda Basu, director of NSF's aeronomy program,
which funded the research. "The importance of the
new finding is that the radio signals produced by
lightning discharges that lead to sprites are distinctly
different from those due to other lightning discharges."
Researchers focused on a thunderstorm which occurred
on August 1, 1996, in western Kansas, above which
a total of 98 sprites were recorded in a 90-minute
period. The team recorded the radio signals emitted
by each lightning strike. For each visible sprite,
they examined the corresponding radio wave measurements
using custom-designed radio antennas and receivers.
Researchers found that the lightning strikes that
produce sprites also tend to carry a distinctive radio
signature. The radio signals the team "read" were
emitted by the lightning itself, rather than by its
companion sprite. The information gleaned in the study
may have a bearing on climate monitoring and atmospheric
chemistry.
"This marks the first time that independent measurements
not requiring video have been used to estimate the
number of sprites produced by a single thunderstorm,"
said Reising. A typical lightning strike occurs in
one-tenth of a millisecond. But those associated with
sprites emit a much longer-lived electrical current.
"These electrical currents last for at least several
milliseconds," explained Reising. "In a relative sense,
that's a long period of time, and radio measurements
can easily tell the difference. We can't rely on video
alone to count all the sprites, because many times,
sprites are visually blocked by the clouds." Also,
it would be nearly impossible, and extremely costly,
to videomonitor every thunderstorm in the hemisphere,
or around the world, he added.
Sprites do not interfere with spacecraft launches,
aircraft or telecommunications satellites. However,
chemical changes could be produced in the atmosphere
by sprites. But in order to address that issue, scientists
first need a reliable estimate of how many sprites
actually occur. "Using four relatively low-cost receivers,
you can count the number of lightning strikes and
sprites in the Western hemisphere, 24 hours a day,
and at very low cost," said Raising. "A storm in Brazil
could be monitored by stations in California and Antarctica.
You can do this from 12,000 kilometers away - a quarter
of the way around the world."
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