January 7, 1998
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