For a long time, scientists could not explain why lightning hits the same place twice. Now this mystery has been solved. An international team of researchers led by the University of Groningen used the LOFAR radio telescope to study lightning flashes in great detail. Their work proved that negative charges in a rain cloud are not discharged all at once. They are partially preserved. This process takes place inside structures that scientists have called needles. Through them, a negative charge can send a second discharge to the ground. The results were published on April 18 in the scientific journal Nature.
Needles
"This discovery is very different from the current picture, in which the charge flows through the channels of the plasma directly from one part of the cloud to another or to the ground," - explained a professor of physics at the University of Groningen.
It was not possible to conduct such a study earlier, since there was no necessary equipment. But thanks to the latest LOFAR radio telescope, it was possible to detect the needles and determine their sizes. They are 100 meters long and 5 meters in diameter.
Low Frequency Array (LOFAR) manufactured in Holland. It consists of several thousand antennas that are scattered throughout Northern Europe. They are connected to the central computer using optical fiber and can work as one unit.
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Low Frequency Array is designed for radio astronomy observations, but the functionality allows it to be used for studying lightning.
Inside the cloud
To observe lightning, scientists used only antennas located in Holland, the total area of which is 3200 square meters. The data obtained with the help of them helped to see what happens inside the cloud during a thunderstorm.
Lightning occurs when strong updrafts create a special static electricity in large clouds, one part of which becomes negatively charged and the other positively.
When this division of charge reaches a certain rate, a strong discharge appears, which is commonly called lightning. Typically, it starts in a small area of hot, ionized air, the so-called plasma.
This small part is converted into a branched plasma channel. It can be several kilometers long. The positive channel tips accumulate negative charges from the cloud. Then they pass through the channel to the negative tip and the charge is discharged.
New algorithm
Researchers have developed a new scheme for receiving data from the LOFAR radio telescope. It allows them to visualize the radiation from two flashes of lightning. A very accurate time stamp on all data and an antenna array allowed scientists to identify radiation sources with high resolution.
“Close to the central zone of the radio telescope, where the antenna density is highest, the spatial accuracy was about one meter,” says Professor Scholten. In addition, the data obtained was able to localize 10 times more VHF sources than other 3D imaging systems, with time resolution in the nanosecond range. This resulted in a high resolution 3D image of a lightning strike.
Break
Thanks to research, it became clear why there are breaks in the outlet channel at the place where the needles are formed. Most likely, they discharge negative charges from the main channel, which then again falls into the thundercloud. It turned out that a decrease in the charges in the channel provokes an open circuit. But when the charge in the cloud becomes high again, the flow through the channel is restored, which leads to a repeated lightning discharge. It is thanks to this principle that lightning will repeatedly strike the same area.
Scholten commented on the discovery as follows: “VHF radiation along the positive channel is caused by regularly repeating discharges along the previously formed side channels, needles. These needles seem to deplete charges in a pulsed manner. " “This is a completely new phenomenon,” adds Professor Joe Dwyer of the University of New Hampshire (USA), the third author of the article. "Our new surveillance methods capture a huge number of needles in the lightning that weren't seen before." And Brian Hare concludes: "Thanks to these observations, we see that part of the cloud is recharged, and you can understand why the lightning bolt into the ground is repeated several times."
Tatiana Andreeva