Close observations of two gigantic jets in September 2010 enabled researchers to map out the electrical-charge imbalances that spawned this particular type of lightning.

“For the first time, these observations give us details of what lightning channels are doing when they form gigantic jets,” said lightning scientist Steven Cummer of Duke University, co-author of research published May 5 in Geophysical Research Letters.

Lightning occurs when a spark travels from either a negatively charged or positively charged area of a thundercloud to an area of opposite charge. Thunderclouds tend to accumulate positive charges in their upper portions, while the middle and lower levels accumulate negative charges.

Most lightning travels from a negative to a positive area, and can discharge within the same cloud, in another cloud, on the ground or in the upper atmosphere.

As a spark starts to travel, it opens up a pathway for electricity to move through, becoming an elongated spark.  If the elongated spark, called a lightning leader, can’t find an area in the cloud with a strong enough electrical field to discharge into, “it can actually punch through the side of a thundercloud and hit the ground,” said Cummer.

These “bolts from the blue” can be very dangerous, because the area they strike can be miles away from the actual storm.

Lead author Gaopeng Lu of Duke University thinks that bolts from the blue and gigantic jets of lightning form in similar conditions within a thundercloud.

When Lu, Cummer and their colleagues studied the signals emitted by a gigantic jet of lightning off the coast of Florida, as well as a second one in Oklahoma, “what we saw in these two lightning flashes were attempted bolts from the blue,” Cummer said.

Each bolt that failed to escape its thundercloud triggered a second lightning leader. Those secondary leaders then shot through the tops of the thunderclouds and produced gigantic jets of lightning.

“It seems two things might happen in sequence,” Lu said. The initial leader that failed to develop into a bolt from the blue depleted the upper regions of their thunderclouds of positive electrical charge.

When the second lightning leader developed, there wasn’t enough positive charge in the tops of the thunderclouds to discharge the leader and stop it from leaving. “So the lightning developed upward,” said Cummer. When it escaped the top of the thundercloud the leader produced a gigantic jet.

“The authors did well in pulling together and examining multiple data sets,” said Richard Blakeslee, a lightning researcher with NASA Marshall Space Flight Center who wasn’t involved in the research. It’s important to look at all the data you can get when dealing with such rare events, he said.

By using both high and low frequency detectors, the scientists could study both the structure of the lightning leaders in space and time, as well as the charge imbalances and transfers during the production of a gigantic jet.

“We’re lucky that the gigantic jets occurred near a [very high frequency] lightning mapping array,” Lu said. Now that the researchers know what kinds of signals gigantic jets give off, they plan to look back through the lightning mapping data to see if they can spot more.

Images and video: Steven Cummer, Duke University.