Gigantic jets blast electricity into upper atmosphere
The gigantic jet observed by Steven Cummer and his team. The thunderstorm that produced this jet was over 300 kilometres away, below the visible horizon (Image: Steven Cummer)
See YouTube video: Gigantic jets
by Michael Marshall, New Scientist, August 23, 2009
The ancient Greeks might have thought Zeus was furious with heaven itself. The power of lightning strikes that shoot upwards from storm clouds has been measured for the first time – and they turn out to be every bit as powerful as normal lightning.
First caught on camera in 2003, "gigantic jets" shoot upwards from thunderclouds and can reach altitudes above 80 kilometres. But it wasn't until 21 July last year that Steven Cummer at Duke University in Durham, North Carolina, and his colleagues managed to measure the electrical discharge from a single gigantic jet, released from tropical storm Cristobal.
"No one had been very close to one with the right radio instrumentation before," Cummer says. "So we didn't know whether they just petered out without doing anything much, or whether they actually took some charge and dumped it somewhere."
Electric jet
The jet came out of a high storm cloud, beginning at an altitude of about 14 kilometres, and shot upwards for a further 75 kilometres.
At those heights, the atmosphere is a much better electrical conductor than at ground level because of ionising radiation from space. As a result, the jet was able to discharge 144 coulombs of charge into the upper atmosphere in about 1 second.
This is comparable to the charge transferred by a large cloud-to-ground lightning strike.
"It's fantastic that they see such a high charge transfer between the thundercloud and the ionosphere," says Victor Pasko of Pennsylvania State University in University Park.
Changing weather
Gigantic jets could be important for our ability to predict lightning strikes.
"There is this newly identified path for discharging the thunderstorm, and a lot of charge can be moved," says Cummer. "In storms that can produce gigantic jets, it might influence what other lightning is happening in the storm."
This time, however, the team found no difference in the rate of ordinary lightning strikes around the time of the gigantic jet. "I'm surprised they saw no drop in lightning rates before or after the jet – but that might be because of the sheer size of the storm," says Pasko.
Gigantic jets are one of a host of new atmospheric phenomena discovered in recent years. Other examples are sprites
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo607 (in press)
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