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Upward Lightning

Sunday, August 23, 2009 at 10:07 PM EDT

An ordinary lightning storm can be a pretty impressive event. The electrical potential difference, between the cloud and the ground, can be on the order of 100 million volts, and the lightning’s path can be 4-5 miles long. The passage of lightning superheats the air through which it passes, turning it into a plasma at 30,000° C. The resulting shock wave is what produces thunder. The energy carried in a typical lightning strike is on the order of 500 megajoules.

Less well know is the fact that lightning can travel upward, too, from the top of the cloud up to the lower layers of the ionosphere. Although these lightning events, sometimes called gigantic jets, are less common than ordinary lightning, they are quite a bit bigger. They typically exhibit multiple channels (”lightning bolts”), and can travel 40 miles or so upwards. Given that they occur above the clouds of a storm, they are difficult to observe. An article at PhysOrg reports that a team from Duke University has managed to capture images of a gigantic jet, mainly by serendipity:

Images of gigantic jets have only been recorded on five occasions since 2001. The Duke University team caught a one-second view and magnetic field measurements that are now giving scientists a much clearer understanding of these rare events.

The charge transferred from the cloud, in the troposphere, to the ionosphere, is comparable to that transferred in an ordinary lightning strike.

Because of the difficulty of observing these events, scientists are still working out what sets of conditions are likely to cause them. The Duke research team has published a paper online, at the journal Nature Geoscience [abstract here] in which they attempt to generate some preliminary answers to these questions.