Fractal models of blue jets, blue starters show similarity, differences to red sprites

Blue jets and blue starters form from multiple streamers of electrical energy rather than as a single glowing column, according to Penn State researchers who modeled the formation of these atmospheric phenomena.

"In 1999, Petrov and Petrova predicted that blue jets and blue starters were made up of a large number of small scale channels or streamers," says Dr. Victor P. Pasko, associate professor of electrical engineering. "We used a modification of a fractal model, a type of model originally developed to study corona streamers in gas insulation systems, to model a blue jet."

Pasko, working with Jeremy J. George, undergraduate in electrical engineering, applied both a two-dimensional and three-dimensional version of the model to replicate a blue jet.

Blue jets were first documented by University of Alaska researchers in 1994, but were probably seen by pilots long before formal documentation occurred. Red sprites, blue jets more familiar cousins, have only been documented for about 10 years. While sprites are more common, forming almost always in a certain size lightning storm, they are much shorter lived, lasting only a few milliseconds. Blue jets should be easier to see because their duration is hundreds of milliseconds to a second long, but they are much rarer and their blue light is difficult to see from the ground.

"Both the two-dimensional and three-dimensional models agreed and produced a realistic reproduction of a blue jet," Pasko told attendees at the fall meeting of the American Geophysical Union today (Dec. 11) in San Francisco. "The charge distribution always creates a cone shape."

The model assumes an internal streamer structure and while there are differences in the two- and three- dimensional models, seem to confirm that blue jets are made of many small streamers and not a solid glowing column.

Since 1994, researchers have studied blue jets to see how they compare to the better-known red sprites. One difference is that while red sprites always follow a substantial lightning strike, blue jets are not directly triggered by lightning. They are, however, related to strong hail activity in thunderstorms.

"Hail is indicative of a large charge separation and hail is negatively charged," says Pasko. "When hail falls from the sky, a very positively charged cloud remains overhead."

Blue jets propagate from the tops of clouds toward the ionosphere 12 to 30 miles high. They are a mile or two at the base and 5 or 6 miles at the top and are always cone shaped and blue. Blue jets propagate slowly from bottom to top, but extinguish simultaneously. Blue starters have the same properties, but travel a shorter distance into the atmosphere.

The researchers did find that using the same fractal models, with a low charge as input, produced short blue jets, or blue starters.

"Blue starters appear to be blue jets that never quite make it," says Pasko.