While experts have noted that lightning can indeed strike twice at the same location, the reason the lightning channel is “reused” remained unclear. But now an international research group led by the University of Groningen may have finally solved this mystery.
Using the Low Frequency Antenna Array (LOFAR), a Dutch radio telescope made up of thousands of antennas scattered across Northern Europe, researchers were able to observe the development of lightning flashes in unprecedented detail.
Research has shown that negative charges inside a thundercloud are not all discharged in a single flash. Instead, some of them are stored inside structures that experts call needles. The researchers determined that a negative charge through these needles could cause a second discharge to the ground.
“This discovery is in stark contrast to the current picture, in which the charge flows through plasma channels directly from one part of the cloud to another or to the ground,” said Professor Olaf Scholten.
The study's first author, Dr. Brian Hahr, explained that the reason the needles have never been seen before is because of LOFAR's “superior abilities”.
"These needles can be 100 meters long and less than five meters in diameter, and are too small and too short-lived for other lightning detection systems."
For their observations, scientists used only the Dutch LOFAR stations, which cover an area of 3200 square kilometers. They analyzed raw time trails measured in the 30-80 MHz range.
“These data allow us to detect lightning propagation at a scale where, for the first time, we can distinguish the primary processes. Furthermore, the use of radio waves allows us to look inside the thundercloud, where most of the lightning resides,”said Dr. Hare.
High-resolution 3D images of the lightning clearly showed a break in the discharge channel at a location where needles are formed. These appear to discharge negative charges from the main channel, which then re-enter the cloud. As a result, the reduction of charges in the channel causes the break.
But then, once the charge in the cloud is built up again, the flow through the channel is restored and there is a second discharge of lightning. Based on this finding, lightning can strike in the same area repeatedly.
"VHF emissions along the positive channel are caused by fairly regularly repeating discharges along the previously formed side channels-needles," explained Professor Scholten. "These needles seem to be draining the charges in a pulsed manner."
“From these observations, we can see that part of the cloud is being recharged, and we can understand why the lightning strikes the Earth can be repeated several times,” said Dr. Hare.