|Since ancient times, lightning has both awed and fascinated
people with its splendor and might. The early Greeks, for instance, associated
the lightning bolt with Zeus,
their most powerful god. And even after a modern understanding of the electrical
nature of lightning developed, certain mysteries persisted. Many observers
described luminous displays flickering through the upper reaches of the
night sky. Some of these curiosities could be explained as auroras
or weirdly illuminated clouds, but others were more baffling. In particular,
pilots flying through the darkness occasionally observed strange flashes
above thunderstorms. But the scientific community largely regarded these
reports as apocryphal--until 1990, when John R. Winckler and his colleagues
at the University of Minnesota first
captured one of these enigmatic phantoms using a video camera. Their images
revealed lightning of a completely new configuration.
Winckler's achievement ushered in a flurry of activity to document such high-altitude electrical phenomena. And hundreds of similar observations--from the space shuttle, from aircraft and from the ground--have since followed. The result has been a growing appreciation that lightninglike effects are not at all restricted to the lower atmospheric layers sandwiched between storm clouds and the ground. Indeed, scientists now realize that electrical discharges take place regularly in the rarefied air up to 90 kilometers above thunderclouds.
It is remarkable that these events, many of which are visible to the naked eye, went undiscovered for so long. In retrospect, the existence of some form of lightning high in the atmosphere should not have come as a surprise to scientists. They have long known that well above the turbulent parts of the atmosphere, ultraviolet rays from the sun strike gas molecules and knock electrons loose from them. This process forms the ionosphere, an electrically conductive layer that encircles the earth. Large differences in voltage can exist between storm clouds and the ionosphere, just as they do between clouds and the ground. Impelled by such enormous voltages, lightning can invade either zone when the air--which is typically an electrical insulator--breaks down and provides a conductive path for electric currents to follow.
Because the atmosphere becomes less dense with increasing altitude, the lightning that happens at greater heights involves fewer air molecules and produces colors not seen in typical discharges. Usually they appear red and are only faintly visible. Thus, researchers must employ sensitive video cameras to record these events against the backdrop of the darkened night sky. The feebleness of the light given off and the transient nature of such emissions combine to present severe technical challenges to the researchers involved in studying these ghostly atmospheric events. Nevertheless, in just a few years investigators have made considerable progress in understanding them.
The newly discovered electrical events of the upper atmosphere fall into four categories. Two types of high-level lightning, termed sprites and elves, appear (despite their fanciful names) to be manifestations of well-understood atmospheric physics. The causes for the other two varieties, called blue jets and gamma-ray events, remain more speculative. But our research group and many others around the world are still amassing our observations in hopes of deciphering the physical mechanisms driving these strange occurrences as well. Until that time, we must admit something like the ancient sense of awe and wonder when we contemplate these curious bursts of energy that dance through the ethereal world between earth and space.
DISCOVERY OF INTENSE GAMMA-RAY FLASHES OF ATMOSPHERIC ORIGIN. G. J. Fishman, P. N. Bhat, R. Mallozzi, J. M. Horack, T. Koshut, C. Kouveliotou, G. N. Pendleton, C. A. Meegan, R. B. Wilson, W. S. Paciesas, S. J. Goodman and H. J. Christian in Science, Vol. 264, pages 1313-1316; May 27, 1994.
PRELIMINARY RESULTS FROM THE SPRITES94 AIRCRAFT CAMPAIGN, 1: RED SPRITES. D. D. Sentman, E. M. Wescott, D. L. Osborne, D. L. Hampton and M. J. Heavner in Geophysical Research Letters, Vol. 22, No. 10, pages 1205-1208; May 15, 1995.
PRELIMINARY RESULTS FROM THE SPRITES94 AIRCRAFT CAMPAIGN, 2: BLUE JETS. E. M. Wescott, D. Sentman, D. Osborne, D. Hampton and M. Heavner in Geophysical Research Letters, Vol. 22, No. 10, pages 1209-1212; May 15, 1995.
ELVES: LIGHTNING-INDUCED TRANSIENT LUMINOUS EVENTS IN THE LOWER IONOSPHERE. H. Fukunishi, Y. Takahashi, M. Kubota, K. Sakanoi, U. S. Inan and W. A. Lyons in Geophysical Research Letters, Vol. 23, No. 16, pages 2157-2160; August 1, 1996.
STEPHEN B. MENDE, DAVIS D. SENTMAN and EUGENE M. WESCOTT have spent much of their time during recent years investigating curious electrical activity of the upper atmosphere. Mende received a Ph.D. in physics from Imperial College at the University of London in 1965. From 1967 to 1996 he worked for Lockheed Palo Alto Research Laboratory. Mende is currently a fellow at the space sciences laboratory of the University of California, Berkeley. Sentman studied space physics under James Van Allen at the University of Iowa, where he earned his doctorate in 1976. After 14 years at the University of California, Los Angeles, Sentman joined the physics department at the University of Alaska-Fairbanks, where he now serves on the faculty. Wescott received a Ph.D. in geophysics from the University of Alaska-Fairbanks in 1964. He worked for three years at the National Aeronautics and Space Administration Goddard Space Flight Center in Maryland before returning to the University of Alaska-Fairbanks as a professor of geophysics.