Study reveals how lightning often strikes twice

Lightning often strikes twice, contrary to popular belief, and a new international study shows how.

It comes down to what the research team call ‘needles’, which store a negative charge along the main lightning channel that can cause repeated strikes.

The research team led by the University of Groningen in the Netherlands and involving a scientist from The Australian National University (ANU) used a low frequency radio telescope to crack the long-running lightning mystery.

Lightning strikes when electricity built up in a cloud is strong enough to break through the ionised air. The lightning bolt hits the ground within a fraction of a second after passing through a channel. According to the Bureau of Meteorology, lightning causes up to 10 deaths every year in Australia.

ANU Emeritus Professor Harvey Butcher said the team studied the development of lightning flashes in unprecedented detail, creating extremely high definition 3D images.

“The reason why a lightning channel is reused had been a mystery until now,” said Professor Butcher, a former Director of the ANU Research School of Astronomy and Astrophysics.

“This new research shows the negative charges inside a thundercloud are not drained all in one flash but are, in part, stored alongside breaks in the main lightning channel.

“This happens in structures called needles that have never been described before. Through these needles, a negative charge may cause a repeated discharge to the ground.”

Professor Butcher led the development of the telescope used by the team. The radio telescope, LOFAR (LOw Frequency ARray), is made of thousands of simple antennas spread across Northern Europe.

These antennas are connected with a central computer through fibre-optics, which means they can operate together to form a radio camera able to focus on both the distant Universe and on phenomena in the Earth’s atmosphere.

The findings are published in Nature.

Lead author Dr Brian Hare from the University of Groningen, said the data collected by LOFAR allowed the team to detect lightning activity within clouds and the surrounding environment at a scale where, for the first time, scientists can distinguish the primary processes.

“From these observations we see that a part of the cloud is re-charged, and we can understand why a lightning discharge to the ground may repeat itself a few times,” he said.

“The use of radio waves allows us to look inside the thundercloud, where most of the lightning resides.”

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For the present lightning observations, the scientists used antennas spread over an area of 3,200 square kilometers. The raw time-traces (accurate to one nanosecond) as measured in the 30-80 megahertz radioband were analysed in this new study to yield 3D images of lightning flashes with one-metre resolution.

Lightning occurs when strong updrafts generate static electricity in large clouds. Parts of the clouds become positively charged and others negatively charged. When this charge separation is large enough a violent discharge occurs, which we know as lightning. Such a discharge starts with a plasma, a small piece of ionised air hot enough to be electrically conducting. This small piece grows into a forked plasma channel that can reach lengths of several kilometers. The positive tips of the plasma channel collect negative charges from the cloud, which pass through the channel to the ultimate negative tip, where the charge is drained. It was already known that much very high frequency radio emission is produced at the growing tips of the negative channels, while the positive channels show emission only along the channel, not at the tip. 

The team said the new results clearly showed the occurrence of a break in the discharge channel, at a location where the needles are formed. These appear to drain negative charges from the main channel, which are then dumped back into the cloud. Once the dumped charge is high enough, the flow through the channel is restored leading to a second discharge of lightning. By this mechanism, lightning will strike repeatedly in the same area. 



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