Very Low Frequency group
confirms filamentary structure of atmospheric sprites
BY DAVID F. SALISBURY
Stanford researchers have
put red sprites under a telescope and found that they
consist of thousands of fiery streamers, each only a few
meters wide.
For several years members of Stanford's Very Low
Frequency Research Group (VLF) have been studying this
dazzlingly bright but subliminally brief form of
lightning that occurs high in the atmosphere above large
thunderstorms.
Last year the researchers
proposed that sprites may have such a filamentary
structure. To test this hypothesis they acquired a small
telescope capable of viewing the features of red sprites
in unprecedented detail and installed it in a prime,
sprite viewing location, the Langmuir Laboratory of the
New Mexico Institute of Technology and Mining in Socorro.
"The laboratory is
located on top of an 11,000-foot peak and provides clear
views of thunderstorms in Kansas, Colorado, Texas, and
northwest Mexico," said Umran S. Inan, professor of
electrical engineering and head of the VLF research
group.
Until now observations of
sprites, which can be more than 25 miles wide and 25
miles in height, have been made from a considerable
distance with low-light-level video cameras capable of
capturing the images of these extremely brief flashes. In
these images, sprites appear as large blobs of red light.
But these images can not distinguish features that are
less than 400 to 600 feet in size, so details the size of
the predicted streamers were not visible.
The main problem with
getting detailed pictures of sprites is their brevity.
With a lifetime of milliseconds, individual sprites do
not last long enough for a telescope operator to zero in
on them.
"Fortunately, our
knowledge of sprite behavior came to the rescue,"
said Inan. "We knew from past observations that,
when a storm kicks up sprites, they tend to appear
repeatedly at about the same place for 10, 20 even 30
minutes."
That allowed the telescope
operator, Elizabeth Gerken, to catch sprites in the act
by positioning the telescope at the location of a
previous sprite and catching its successors. The 16-inch
wide, 6-foot long telescope, which was purchased off the
shelf from Orion Telescopes & Binoculars, can
distinguish details down to 30 feet in size from a
distance of 300 miles.
"The system worked
almost from the beginning," said Gerken, a graduate
student in electrical engineering and a Stanford graduate
fellow. Working typically past midnight in the chilly
night air of the mountain-top observatory, she
successfully recorded the most detailed images of this
elusive atmospheric phenomenon ever taken.
The pictures show the
streamers that the researchers had predicted, but their
shape and complex arrangement came as a complete
surprise. "What we are seeing is not explained by
any theory. We are all somewhat at a loss," said
team member Christopher Barrington-Leigh, a graduate
student in applied physics.
In the images, most of the
streamers are vertical, but many are tilted at a variety
of angles. Many streamers are present in one frame and
absent in the next meaning that they have lifetimes of
less than 17 milliseconds but some individual branches
seem to persist through several frames. Most leave solid
streaks. Others turn on and off, creating what appear as
dashed lines or beaded forms.
The researchers are poring
over the images, looking for patterns. Gerken, who has
spent the most time examining them, says that many
streamers have a branching, tree-like structure. She has
observed that the streamers tend to branch upward at the
top of the sprite, while they tend to branch downward
near the bottom.
According to the Stanford
researchers, the streamers are caused by a build-up of
electrostatic charge in the atmosphere created by strokes
of lightning. 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, then 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 predicted that small scale spark
channels will form at the electrical breakdown points.
These channels, which give off a blue glow, should be
propelled upward (although a few may streak downward)
with velocities as fast as one-tenth of the speed of
light, leaving behind glowing red streamers of ionized
gas.
The new pictures confirm
this general scenario, but the researchers say that they
need to do a lot of additional work to explain the rich,
unexpected detail that they have discovered. For example,
they also predicted that the streamers would glow for a
few milliseconds before burning out, but the new pictures
indicate that some of them last much longer than
predicted. SR
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