University Park, Pa. – Deep in the Puerto Rican tropical jungle surrounding the Arecibo Observatory, a team of stalwart researchers captured an elusive blue jet on video tape and found the first evidence of a connection between the ionosphere and cloud top in these events. "The first recorded image of an optical flash above a thunderstorm was obtained serendipitously on July 5, 1989," says Dr. Victor P. Pasko, associate professor of electrical engineering at Penn State. "However, pilots and others reported observations of sprites and blue jets long before the first one was captured on video, and numerous undocumented reports of similar phenomena have appeared in scientific literature for over a century."
Over the past 10 years many researchers have looked for sprites and blue jets from the ground, the air and from space. Many more have been seen and captured during campaigns in North, Central and South America, Europe, Australia and Japan. The effort in Puerto Rico last year was a small campaign organized by Pasko and John D. Mathews, professor of electrical engineering at Penn State; Mark A. Stanley, postdoctoral fellow at New Mexico Tech; and Umran S. Inan, professor and Troy G. Wood, graduate student at Stanford University.
In the latest issue of Nature, the researchers report, "Until now, 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." In describing the event they saw, they noted that the blue jet exhibited some common sprite features above 26 miles altitude.
Sprites usually propagate from 60 down to about 25 miles, are predominantly red and last for only milliseconds. They have a characteristic fluffy, mushroom shape. Blue jets develop at cloud tops at 12 to about 26 miles. They are blue, last for up to several hundreds of milliseconds and are cone-shaped. The September event seems to have characteristics of both blue jets and sprites. On September 14, 2001, in the evening, the researchers set up their equipment on the roof of the Lidar laboratory on the grounds of the Arecibo Observatory. A lightning intense, but relatively small, thunderstorm was situated about 125 miles out to sea.
The reported event was recorded using a monochrome low-light video system, but the researchers all agree that the phenomena was seen visually to be blue in color. The videotape shows the blue jet propagating from 10 miles at the cloud top in a branching, conical manner upward toward the ionosphere, the way blue jets propagate in computer models and in other videotapes, but then the images change. The top of the jet appears to look much more like a sprite than a blue jet, with hot spots and a fuzzy diffused appearance.
"This is perhaps a hybrid event that may be specific to the tropics," says Pasko. The researchers report, "The transition between the upper region dominated by hot spots and the lower region dominated by relatively smooth filamentary structure . . . is estimated at an altitude of 26 miles. This is similar to the normal upper terminal altitude of blue jets and the lower terminal altitude of sprites." Whether this is a newly captured atmospheric phenomenon or simply a variation on a blue jet is not yet known. The researchers have some explanations for the behavior of the jet in its initial stages, but cannot currently explain the rest of the phenomenon.
The researchers use a GEN III intensifier, provided by ITT Night Vision Industries attached to a video camera to film the event. This research was supported by the National Science Foundation. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, a national research center operated by Cornell University under a cooperative agreement with NSF.
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