Why rays often strike twice



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In contrast to popular belief, lightning strikes usually hit twice, but the reason a lightning channel is "reused" remains a mystery. Now an international research team led by the University of Groningen has used the LOFAR radio telescope to study the development of lightning in unprecedented detail. His work reveals that negative charges within a storm cloud are not discharged into a single flash, but are partly stored alongside the leading Interruptions channel. This occurs within structures that the researchers called needles. Through these needles, a negative charge can cause repeated discharge into the ground. The results were published on April 18 in the journal Nature.

Needles

"This finding is in stark contrast to the current image, where charge flows along the plasma channels directly from one part of the cloud to another, or to the ground," explains Olaf Scholten, a professor of physics at KVI-CART of the University of Groningen. The reason why needles have never been seen is in LOFAR's "supreme capabilities," adds his colleague Brian Hare, first author of the article, "These needles may be 100 meters long and less than five in diameter." meters, and are too small and too short for other lightning detection systems. "

The Low Frequency Array (LOFAR) is a Dutch radio telescope consisting of thousands of fairly simple antennas spread across northern Europe. These antennas are connected to a central computer via fiber optic cables, which means that they can operate as a single entity. LOFAR is mainly developed for radio astronomy observations, but the frequency range of the antennas also makes it suitable for lightning search, since the discharges produce bursts in the VHF (very high frequency) radio band.

Inside the cloud

For current lightning observations, the scientists used only the Dutch LOFAR stations, which cover an area of ​​3,200 square kilometers. This new study looked at chronometric (accurate to nanosecond) tracings measured in the range of 30-80 MHz. Brian Hare: "These data allow us to detect lightning propagation on a scale where, for the first time, we can distinguish primary processes "In addition, the use of radio waves allows us to look into the cloud, where most of the ray resides"

A lightning strike occurs when strong upward currents generate a kind of static electricity in large cumulonimbus clouds. Parts of the cloud are positively charged and others are negatively charged. When this separation of charge is large enough, a violent discharge occurs, which we know as lightning. Such discharge begins with a plasma, a small area of ​​ionized air hot enough to be electrically conductive. This small area grows in a bifurcated plasma channel that can reach lengths of several kilometers. The positive points of the plasma channel collect negative charges from the cloud, which pass through the channel to the negative end, where the charge is discharged. It has already been known that a large amount of VHF emissions is produced at the growing tips of negative channels, while positive channels show emissions only along the channel, not at the tip.

A new algorithm

Scientists have developed a new algorithm for LOFAR data, allowing you to view VHF radio emissions from two lightning strikes. The arrangement of antennas and the very precise time recording in all the data allowed to identify the sources of emission with an unprecedented resolution. "Near the central area of ​​LOFAR, where antenna density is highest, the spatial accuracy was about one meter," says Professor Scholten. In addition, the data obtained were able to locate 10 times more VHF sources than other three-dimensional imaging systems with a time resolution in the nanosecond range. This resulted in a high resolution 3D image of the electric discharge.

Break

The results clearly show the occurrence of a rupture in the discharge channel at a location where the needles are formed. These appear to discharge negative charges from the main channel, which subsequently re-enter the cloud. The reduction of loads on the channel causes the break. However, once the charge in the cloud becomes high enough again, the flow through the channel is restored, leading to a second lightning discharge. By this mechanism, a radius will hit the same area repeatedly.

Scholten: "VHF emissions along the positive channel are due to regularly repeated discharges along preformed lateral channels, needles. These needles seem to drain the charges in a pulsed manner." This is a totally new phenomenon, adds Professor Joe Dwyer of the University of New Hampshire, USA, third author of the paper: "Our new observation techniques show copious amounts of needles in the radius that have not been seen before." And Brian Hare concludes: "From these observations we see that a part of the cloud is recharged and we can understand why a lightning strike on the ground can repeat itself a few times."

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