That annoying crackle on an AM radio station might not be due to a lightning strike hitting the ground. Researchers have discovered that bolts jumping from the tops of thunderclouds all the way to the ionosphere, some 90 kilometers above Earth's surface, can be just as powerful as conventional ground strikes, though they form slower. The findings, published online this week in Nature Geoscience, suggest that thunderstorms can discharge electricity into the entire atmosphere, from Earth's surface to the edge of space.
Researchers have known since 2001 that lightning can jump upward. However, these gigantic jets, as researchers call them, have remained mysterious because they are difficult to observe. So electrical engineer Steven Cummer of Duke University in Durham, North Carolina, got pretty lucky last summer. His team has been investigating gigantic jets and other atmospheric electrical disturbances for the past 10 years. The team had set up a ground-based, low-light video camera on the university campus to record disturbances emanating from Tropical Storm Cristobal, which skirted the North Carolina coast. Armed with the camera and an array of instruments designed to detect lightning discharges at long distances, the team caught a gigantic jet flashing upward from a cloud top on 21 July 2008.
The bolt settled a fundamental question, Cummer explains. "We weren't really sure whether gigantic jets were like modest sparks that just tickled the upper atmosphere," he says, "or if they were really like lightning that dumps a lot of electric charge from a thunderstorm onto the lower edge of space." It's clearly the latter, Cummer says, adding that the instrument readings showed that the electrical charge rivaled the very biggest conventional lightning bolts.
Gigantic jets take a lot longer to reach maximum strength, however. According to previous research, a conventional lightning bolt powers up in about 1 microsecond, whereas the new findings show a gigantic jet takes 30,000 times longer. The reason seems to be that because the atmosphere is much less dense above the cloud tops, it takes longer for a discharge to form.
Cummer says his team will attempt to capture gigantic jets with faster cameras so that they can track the bolts' full development. They also want to study what happens to the upper atmosphere when the bolts release their electric charge.
It's the first time a paper has made a direct comparison between gigantic jets and ordinary lightning, says electrical engineer Victor Pasko of Pennsylvania State University, University Park. The measurements will "provide lots of information for experiments and further study," says Pasko, whose team originally discovered the phenomena.