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Imaging elusive winter sprites

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Krista D. Zanolli, [email protected]

“Sprites,” or upper-atmospheric lightning that occurs high above thunderstorm clouds, are a natural phenomenon that have caught the attention of Colin Price, professor and head of the geophysics and planetary sciences department at Tel Aviv University.

Sprites are described as millisecond-long atmospheric flashes between 35 and 80 miles from the ground, much higher than the seven to 10 miles where regular lightning bolts typically occur.

“Lightning from the thunderstorm excites the electric field above, producing a flash of light called a sprite,” Price explained.

Although sprites were first photographed in 1989, Price and his colleagues have since focused their attention on sprites that occur only in the northern hemisphere’s winter months.

According to Price, who has been involved in sprite research since 1995, a major effort began aboard the Columbia space shuttle with the first Israeli astronaut, Ilan Ramon. Ramon captured images of sprites from space, which were downloaded to the researchers prior to the tragic end to the mission on Feb. 1, 2003, when the shuttle disintegrated upon re-entering Earth’s atmosphere. According to reports, another usable tape with images survived the crash and was later found on the ground.

In memory of Ramon, the team continued sprite measurements from the ground, calling its efforts the ILAN project.

“Sprites, which only occur in conjunction with thunderstorms, never occur on their own and are cousins to similar natural phenomena dubbed by atmospheric electricians as ‘elves,’ ‘goblins’ and ‘trolls,’ ” Price explained. The flashes are so named because they appear to “dance” in the sky. Although there is much debate over their cause, they may explain some bizarre reports of UFO sightings, he said.

The university’s research team, one of the leading global groups studying the phenomenon, is now working in collaboration with other Israeli scientists from The Open and The Hebrew universities to take three-dimensional pictures of sprites with the hope of better understanding their structure.

An image of a “sprite” (roughly 30 miles high by 30 miles wide) flashing above a distant thunderstorm is seen here. The sprite is about 175 to 250 miles away from the camera. Image courtesy of the ILAN Science Team.

Because of the unique vantage point in Israel, Price and his team are leading the world in the study of winter sprites. Using remote-controlled roof-mounted cameras, they can view sprite-producing thunderstorms as they hover over the Mediterranean Sea. Their recent efforts have revealed that the circular structure of the sprites actually resembles a birthday cake with candles. Using a triangulation process, Price calculates the dimensions of the sprites’ features. “The ‘candles’ in the sprites are up to fifteen miles high, with the cluster of candles forty-five miles wide – it looks like a huge birthday celebration.”

And, because of the high altitude of sprites, the researchers are studying the impact they might have on Earth’s ozone layer, although they believe the global impact to be minimal.

“Our next step is looking at sprites with specific optical filters in order to derive information about their energies,” Price said. He noted that most images of sprites are taken using broadband white-light cameras, but that his group is trying to use calibrated cameras to translate the light levels into energy levels.

Photonics Spectra
Apr 2009
atmospheric lightningelectric fieldgeophysicsResearch & TechnologyTech Pulse

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