For researchers Victor Pasko and Mark Stanley, tossing a keenly sensitive digital video camera into their kit as they headed to Puerto Rico last September was an afterthought.
It turned out to be a good move. During the last hour of the last night they were in the field, the two men recorded an event that may help solve a long-standing mystery about earth's electrical system.
The duo was preparing to spend a month in Puerto Rico to track the effect of lightning on the earth's ionosphere using radar at the National Radiotelescope and Ionospheric Center at Arecibo.
But they also thought they might have a chance to capture the antics of blue jets and sprites - elusive fountains of light that lance upward from the tops of thunderheads or plunge from the bottom of the ionosphere during thunderstorms. Ever since these discharges were first captured on video in 1989, scientists have puzzled over the role they might play in the earth's electrical system.
As the two monitored a thunderstorm roughly 200 kilometers (124 miles) away, a blue jet erupted from the top of the thunderhead. In less than a second, the jet reached the ionosphere, then vanished. The camera, in the right place at the right time, captured the event.
"You could see this with the naked eye. It was one of the most impressive things I've ever seen in my life," says Dr. Stanley, an atmospheric physicist at the Los Alamos National Laboratory in Los Alamos, N.M.
Previous images taken by other scientists had suggested that blue jets and sprites reached altitudes of 42 kilometers. Last year's jet, however, reached 70 kilometers, leading the team to claim that their sequence of images represents the first conclusive evidence for a direct electrical path between a thundercloud top and the ionosphere - like the cloud-to-ground path lightning provides.
"This was one of the most shocking results we got on the trip," Stanley says.
With 45,000 thunderstorms daily around the globe, the team speculates that such discharges in tropical regions could play a role in charging the earth's "battery.".
The two terminals of this battery are the earth's surface and the ionosphere, explains Dr. Pasko, an associate professor of electrical engineering and a researcher at Penn State University's Communications and Space Sciences Laboratory.
He notes that at very low frequencies in the electromagnetic spectrum, the earth's surface acts like a metal shell. So does the ionosphere - a layer of the atmosphere filled with charged particles that begins at an altitude of 60 kilometers. Like the two terminals in a car battery, these two shells exhibit a difference in electrical potential, or voltage. In the car battery, the difference is 12 volts. In the atmosphere, it's 300,000 volts.
Pasko notes that the atmosphere acts as an insulator, preventing all but a weak current from flowing between the "terminals."
"There's a lot of interest in understanding how the battery is charged and maintained," Stanley says.
Thunderstorms are important in this process, he continues, because cloud-to-ground lightning delivers a net flow of charge to the surface. "But no one has seen charge flowing from a cloud to the ionosphere," he says. "There's been a big debate over whether this could happen."