Battles Still Rage at Megiddo
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Sprites and Elves in the Heavens
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TAU Research Briefs
Sprites and Elves in the Heavens Above
Although lightning and thunderstorms have been studied scientifically ever since Benjamin Franklin, surprising new discoveries are still being made. Airplane pilots had long reported sighting strange flashes of light far above thunderstorms, but their sightings were dismissed as fantasy, due to the lack of hard evidence. Then in, 1989, while testing a low-light telescope camera, scientists at the University of Minnesota accidentally captured one of the phantoms on video. Mounting major efforts, scientists subsequently discovered a whole host of elusive new optical phenomena, which they named sprites, elves, jets and trolls. Such unusual flashes occur at high altitudes, 50-100 km above the earth's surface, above active thunderstorms in conjunction with very intense lightning flashes.
Sprites are red (see accompanying NASA photograph) and have lifetimes of only a fraction of a second. Typically, a sprite accompanies only about one in every 200 regular lightning flashes. The red light is produced by electrons in the upper atmosphere colliding with and exciting air molecules, mainly nitrogen. The molecules de-excite by emitting red light (light is produced similarly by the electrically excited neon atoms in a fluorescent light bulb).
The powerful lightning that produces the red sprites also emits strong radio waves of very low frequency. These radio waves travel around the earth, bouncing off the ionosphere and the earth's surface, and can travel all the way around the globe before dissipating. In order to detect these weak radio waves, special equipment must be deployed at very quiet locations. Israel's Negev Desert is an ideal location for such a task.
Over the past few years
Dr. Colin Price, Prof. Zev Levin and Dr. Yoav Yair
of the TAU Department of Geophysics and Planetary Sciences have built two such field stations at Mitzpe Ramon and Sde Boker. Their sensitive detectors can measure changes in the electric and magnetic fields in the atmosphere produced by lightning all over the planet. During the summer of 1999, while researchers in the U.S. were photographing sprites, the TAU team sat in the Negev desert "listening" to the radio waves arriving from more than 10,000 km away. Due to recent advances in time-synchronization technology - all the groups were equipped with GPS clocks -the teams could correlate the exact time of photographing the sprites in the U.S. and detecting their signals in Israel.
The majority of sprites viewed in the U.S. produced easily detected electromagnetic signals, a half-second later, in the TAU receiving station at Mitzpe Ramon (photograph above). The various detectors provided information regarding the direction of arrival of the signals, as well as the distance of the lightning causing them. The investigators could estimate the position of the sprite-producing lightning to within 10%. The TAU researchers are now studying the global distribution and variability of sprites from their Negev field site.
This TAU research will be part of an upcoming U.S. Space Shuttle mission with the first Israeli astronaut. His primary mission will be a TAU experiment to study atmospheric dust over the Mediterranean. However, during nighttime hours, the Israeli astronaut will look for sprites above tropical thunderstorms. He will use a special charge coupling device (CCD) camera with six spectral channels: two in the ultraviolet (UV) and four in the visible and near infrared. At the same time, the TAU ground team will be listening in the Negev for the signals produced thousands of kilometers away.
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