by W. Wayt Gibbs
from ScientificAmerican Website
recovered through WayBackMachine Website
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 lightning-like emissions shooting upward from cloud-tops, 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 (below image) of
these extraordinarily brief flashes, playfully named sprites.
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.
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 photo-detectors, each in its own 45-centimeter (18-inch) barrel and each pointing to a different part of the sky.
COLOR IMAGE of a Red Sprite on July 4, 1994 at 04:00:20 UTC,
which reached an altitude of over 85 km.
The tendrils beneath the sprite are as low as 60 km.
The bright area beneath the sprite is an over-exposure of normal lightning
occurring in the top portion of an active thunderstorm complex located in the Texas panhandle.
Cloud tops in were about 18 km.
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.
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.
Zeus himself might be impressed.