New red sprites model:
Like setting off '10,000 sky rockets at the same time'
BY DAVID F. SALISBURY
Red sprites the
dazzlingly bright but subliminally brief form of
lightning that occurs high in the atmosphere above large
thunderstorms may not be the amorphous blobs of light
that scientists had first thought.
Researchers from
Stanford's Very Low Frequency Research Group, who have
been studying this recently discovered phenomenon for
several years, now propose that sprites may consist of
thousands of fiery streamers, each only a few meters
wide.
"It's as if you set
off ten thousand sky rockets at the same time," said
Timothy F. Bell, a senior research associate in the
group.
The new model was reported
in a paper presented on Dec. 8 at the American
Geophysical Union meeting in San Francisco by Bell and
Stanford postdoctoral research affiliate Victor Pasko and
electrical engineering Professor Umran S. Inan.
When a large bolt of
lightning flashes from the top of a thunderhead to the
ground in a matter of milliseconds, it leaves behind
large amounts of uncompensated electrical charge in the
atmosphere. This creates an intense electrostatic field
in the region above the thunderstorm. If the lightning
discharge is large enough, the electrostatic field causes
the air to ionize at thousands of points where the field
is strongest.
Normally air, which is
made up primarily of electrically neutral molecules, has
a relatively high electrical resistance. But an electric
field that is strong enough will accelerate ambient
electrons to energies sufficient to knock additional
electrons off the air molecules in collisions, causing
them to become electrically charged. Such ionized air
molecules conduct electricity much more readily than
normal air molecules.
In the region between 30
and 50 miles in altitude above a thunderstorm, the
Stanford researchers predict that small-scale spark
channels will form at the breakdown points. These
channels, which give off a blue glow, are propelled
upward (although a few may streak downward) with
velocities as fast as one-tenth of the speed of light and
leave behind glowing red streamers of ionized gas. The
researchers estimate that the average thickness of these
streamers is about 30 feet at an altitude of 40 miles. At
lower altitudes they become thinner and at higher
altitudes they grow wider. The streamers glow for a few
milliseconds before burning out.
Current observations of
sprites, which can be more than 25 miles wide and 25
miles in height, have been made from a considerable
distance with high-speed cameras capable of capturing the
images of these extremely brief flashes. The best
resolution of these images has been several hundred feet,
much larger than the size of the streamers that the
Stanford model predicts. Inan's group hopes to acquire a
special telescope that can capture details in sprites as
small as a few feet across. That will allow the
scientists to determine if the structure that they have
predicted does in fact exist.
The model does explain
several recent observations. Other research groups have
detected extremely fast upward motions in sprites and
measured intense bursts of blue light measured in the
first 1 to 2 milliseconds of sprite formation. The model
also explains the fact that the Stanford researchers have
measured radio waves in the extremely low frequency (ELF)
band from sprites that are as intense as those generated
by the lightning flashes that create them. ELF signals
range from about 3 hertz to 3 kilohertz and are
associated primarily with geophysical processes like
lightning, aurorae and the radiation belts surrounding
Earth.
"These signals prove
that sprites are a substantial phenomenon. They are not
just made up of reflected light like a rainbow. There are
ionization channels there. If you ventured into a sprite,
you would get one hell of a shock," said Umran. SR
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