Image: VICTOR PASKO/Penn State University

Most people know lightning as the jagged bolts that jump between clouds and the ground. But lightning flickers above thunderstorms too, where the ghostly flashes have such fanciful names as elves and sprites. Now new evidence, published today in the journal Nature, suggests that one of these especially ephemeral above-storm forms, blue jets--cones of sapphire light that last for up to several hundreds of milliseconds--may connect the cloud top to a layer of electrically charged air known as the ionosphere.

Penn State University researcher Victor Pasko and his colleagues captured the elusive phenomenon on videotape last September from the roof of the Arecibo Observatory's Lidar laboratory, deep in the Puerto Rican jungle. Earlier measurements suggested that blue jets could only reach an altitude of 40 kilometers or so, but the one documented by Pasko's team rose to about 70 kilometers--the lower edge of the ionosphere. Flashes of this magnitude could explain the 300,000-volt difference between the electrical charge of the ground and that of the oppositely charged ionosphere. "Until now," the authors note, "no experimental data related to sprites or blue jets have been reported which conclusively indicate that they establish a direct path of electrical contact between a thundercloud and the lower ionosphere."

Considering that this large blue jet was produced by a type of relatively small thunderstorm that occurs frequently around the world, the team writes, "such cloud-to-ionosphere discharges may be very common in the tropics and may constitute an important, but as yet unaccounted for, component of the global electric circuit."

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