Sprites may regulate the 300 thousand volt difference between the ground and the Earth's upper atmosphere. A new model hints that researchers have underestimated the reach and power of these high-altitude light flashes.
Sprites accompany powerful lighting strikes. They are about 50 km across and reach high into the upper atmosphere as far as the ionosphere - 80km above the ground.
Now a 3D computer model of these fleeting discharges suggests that, "they [also] extend as far down as the cloud-tops," says Victor Pasko, who built the model with colleagues at Penn State University in University Park.
Pasko's team began in the laboratory. They recreated the lightning-like "streamers" of electrical charge associated with sprites and then scaled-up their dimensions and behaviour to the size of an entire sprite in a computer model. The resulting simulation "agrees very well with certain observed sprites," says Pasko.
If sprites do indeed go lower into the atmosphere where the air is thick with gases like nitrogen and oxygen, they could play a role in atmospheric chemistry, Pasko points out. They may help produce high-level ozone and capture nitrogen, essential for life on Earth.
Researchers had suggested this before, "but no one could really back it up," says sprite researcher Chris Barrington-Leigh at the University of California, Berkeley. "This [model] will fuel further research into their role."
Much of the atmosphere between us and the edge of the ionosphere is too high for planes and too low for satellites. "Sprites provide windows to view that environment," says Umran Inan a physicist at Stanford University in California. Before sprites were discovered this region was best called the "ignorosphere," Inan jokes.
The ionosphere is important because this electric blanket bounces long distance radio communications around the globe. Fluctuations in the ionosphere interfere with GPS and other satellite signals.
Despite nearly a century of anecdotal reports from airline pilots, most scientists didn't really believe in sprites until about ten years ago when they were captured on high-speed video. Their position and beauty caused "a major frenzy of excitement" among scientists, recalls Barrington-Leigh.
Numerous images later, sprites remain poorly understood, because they disappear almost as soon as they form. The blink of an eye lasts 250 milliseconds; sprites often last only ten. Computer models freeze sprites in time.