RED SPRITES remained mysterious until a
team out of the University of Alaska at Fairbanks obtained the first
color images from a circling jet aircraft.
|The ancients imagined thunderstorms and lightning to be
evidence of titanic war among the gods in the sky above. The truth, it
turns out, is even odder and more interesting than such myths. Peek above
the black anvil clouds of large storms, as droves of scientists have been
doing in recent years, and (with the right equipment) one can indeed find
wonders: not gods, but exotic stabs of illumination called sprites and
elves. Geophysicists are now scrambling to explain what causes these phenomena
and how they may affect the earth's atmosphere.
For more than a century, sky watchers have reported seeing strange flickers of lightninglike emissions shooting upward from cloudtops, but such sightings proved extremely hard to verify. Researchers finally caught these events on videotape in 1989, and space shuttle astronauts orbiting the earth got a clear, if distant, view from above of these extraordinarily brief flashes, playfully named sprites.
But it wasn't until 1994 that a team out of the University of Alaska at Fairbanks, circling around midwestern thunderstorms in a jet aircraft, obtained the first color images of sprites. These snapshots show that sprites are colorful (usually red), quite enormous and often surprisingly delicate in form. Some sprites stretch upward 95 kilometers (58 miles) from the cloudtops. They always seem to appear just as a lightning bolt--usually an especially fierce one--strikes down toward earth, and they almost always appear in groups.
Investigators now think sprites probably occur when an unusually potent stroke of lightning creates an intense electrostatic field above the cloud from which it emanates. Ions (electrically charged atoms) and electrons floating about the atmosphere are heated by this field and glow red in response, scientists speculate. Blue jets may be formed when such conditions happen to occur just as a cosmic ray (an extremely fast moving particle kicked out of a supernova or some other energetic cosmic event) collides with an air molecule in that region. The collision produces a shower of fast electrons; the upward-pointing electrostatic field above the cloud can accelerate these electrons further, to energies at which they emit blue light.
When researchers pointed their cameras to pick up sprites, they were surprised to discover that other bizarre light shows also illuminate the high altitudes. Vast blue jets, rising from clouds at 300 times the speed of sound in air, form cones of light that stand 40 kilometers tall. Then in mid-December a group of Stanford University researchers, led by Umran Inan of Stanford University, reported at the American Geophysical Union meeting in San Francisco that they had clocked yet another form of stratospheric lightning, one that, paradoxical though it may seem, propagates faster than the speed of light. These halos of red light, dubbed elves, were first conclusively recorded by Japanese scientists in 1995. But little was known about their structure and movement until Inan's group imaged the phenomenon with The Fly's Eye--a custom-built instrument that chains together 12 highly sensitive photodetectors, each in its own 45-centimeter (18-inch) barrel and each pointing to a different part of the sky.
The Stanford researchers managed to get a good look at 10 elves. All started just above a groundstroke of normal lightning but expanded into rings up to 300 kilometers (200 miles) across in less than a thousandth of a second. Although the elves appear to spread faster than light, analysis of the physics behind elves demonstrates that no particles actually move that fast, so Einsteinian relativity is not violated. The faster-than-light illusion seems to be caused by successively distant air molecules lighting up in rapid-fire sequence, like the strobe lights running along an airport runway.
Inan and Yuri Taranenko of Los Alamos National Laboratory have developed mathematical models which they think may explain what causes elves. According to their simulation, the key is the electromagnetic pulse produced by lightning strokes. This pulse expands like a balloon, upward and away from the groundstroke. If the pulse is strong enough, the theory goes, it energizes the ions and free electrons at the border between the stratosphere and ionosphere enough to make the charged particles shine red.
So, although the electromagnetic pulse expands at exactly the speed of light, Inan explains, the ring of shining particles formed at the lower edge of the ionosphere grows faster--just as a balloon released underwater will, as it breaks the surface, create a ripple that moves much faster outward than the rate at which the balloon rises upward. Zeus himself might be impressed.
--W. Wayt Gibbs in San Francisco
Red Sprites and Blue Jets from the University of Alaska
Sprites and Jets information from NASA
and Elf research at Pennsylvania State University
MOVIES OF SPRITES AND ELVES: