Study reveals needle-like structures in positively charged scout leaders

Contrary to popular belief, lightning often strikes twice, but the reason why a lightning channel is "reused" has remained a mystery. At present, an international research team led by the University of Groningen has used the LOFAR radio telescope to study the development of lightning with unprecedented details. The results were published April 18 in the scientific journal Nature.

The team used the LOFAR radio telescope to study the development of lightning with unprecedented details. Their work reveals that negative charges inside a storm cloud are not discharged at one time, but are partly stored along the leader's channel during interruptions. in structures that researchers call needles. This can cause a repeated spill on the ground.

The needles

"This discovery contrasts sharply with the current situation, in which the charge flows along plasma channels directly from one part of the cloud to another, or on the ground," says Olaf Scholten, professor of physics at KVI-CART institute of the University of Groningen. It was never possible to observe the needles before LOFAR's "supreme abilities", adds his colleague, Dr. Brian Hare, first author of the newspaper. "These needles can be 100 meters long and less than five meters in diameter and are too small and have a short life span for other lightning detection systems."

The Low Frequency Network (LOFAR) is a Dutch radio telescope composed of thousands of single antennas in northern Europe. These antennas are connected to a central computer via fiber optic cables, which means that they can function as a single entity. LOFAR is primarily designed for radio astronomy observations, but the antenna frequency range also makes it suitable for lightning research because discharges produce bursts in the VHF (very high frequency) radio band.

For current observations of lightning, the scientists used only the Dutch LOFAR stations, which cover an area of ​​3,200 square kilometers. This new study analyzed raw (nanosecond) time-domain traces measured in the 30-80 MHz band. Brian Hare said, "These data allow us to detect the spread of lightning on a scale where, for the first time, we can distinguish primary processes.In addition, the use of radio waves allows us to look at inside the storm cloud, where most lightning resides. "

Lightning occurs when strong updrafts generate a kind of static electricity in large cumulonimbus clouds. Parts of the cloud become positively charged and others negatively. When this charge separation is sufficiently large, a violent lightning discharge occurs. Such a discharge begins with a plasma, a small ionized air zone hot enough to be electrically conductive. This small area develops into a forked plasma channel that can reach several kilometers. The positive ends 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 was already known that significant VHF emission occurred at the ends of the negative channels, while positive chains only showed emissions along the channel, not at the top.

A new algorithm

Scientists developed a new algorithm for LOFAR data, allowing them to view two lightning VHF radio broadcasts. The antenna network and the very accurate timestamping of all the data allowed them to locate the emission sources with unprecedented resolution. "Near the central area of ​​LOFAR, where the antenna density is the highest, the spatial accuracy was about one meter," says Professor Scholten. In addition, the obtained data located 10 times more VHF sources than other three-dimensional imaging systems, with a temporal resolution of the order of one nanosecond. This resulted in a high resolution 3D image of the lightning discharge.

The results clearly show the presence of a rupture in the discharge channel at a place where needles are formed. These seem to discharge negative charges from the main channel, which then re-enters the cloud. Reducing the charges in the channel causes the pause. However, once the charge in the cloud becomes sufficiently high, the flow through the channel is restored, causing a second lightning discharge. By this mechanism, lightning will strike the same sector several times.

Scholten said: "The VHF emissions along the positive channel are due to fairly regular repeated discharges along the previously formed side channels, the needles, which seem to drain the charges in a pulsating manner."

"It's 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 an abundant amount of 39 needles in the flash, which had not been seen before. "

Brian Hare concludes: "From these observations, we see that part of the cloud is recharged, and we can understand why a lightning discharge on the ground can be repeated several times."