January 6, 1999
|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.
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