In contrast to popular belief, lightning often does strike twice, but the reason why a lightning channel is ‘reused’ has remained a mystery. Now, an international research team led by the University of Groningen and including scientists from DESY has used the LOFAR radio telescope to study the development of lightning flashes in unprecedented detail. Their work reveals that the negative charges inside a thundercloud are not discharged all in a single flash, but are in part stored alongside the leader channel at Interruptions. This occurs inside structures which the researchers have called needles. Through these needles, a negative charge may cause a repeated discharge to the ground. The results are published in the journal Nature.
“This finding is in sharp contrast to the present picture, in which the charge flows along plasma channels directly from one part of the cloud to another, or to the ground”, explains Olaf Scholten, Professor of Physics at the University of Groningen. The reason why the needles have never been seen before lies in the ‘supreme capabilities’ of LOFAR, adds his colleague Brian Hare, first author of the paper: “These needles can have a length of 100 metres and a diameter of less than five metres, and are too small and too short-lived for other lightning detections systems.”
The Low Frequency Array LOFAR is a decentralized radio telescope consisting of thousands of simple antennas. Related antenna fields are located in many European countries, in Germany, for example, at Forschungszentrum Jülich and in Norderstedt near Hamburg. These antennas are connected to each other via fiber optic networks and connected to high-performance computers. This interconnection allows the use of the antennas as a single, giant virtual telescope.
LOFAR is primarily used for astronomical observations. However, the system is very flexible so that it is also suitable for measuring radiation bursts emitted in the very high frequency (VHF) band by lightning in Earth’s atmosphere. “We measure frequencies from 30 to 80 megahertz, that is between the short wave and the ultra-short wave range,” reports Hare. “This data allows 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.”
Lightning occurs when internal turbulence electrically charges different parts of large cumulonimbus clouds against each other. The effect is comparable to the static charge familiar from everyday life. If the voltage difference between positive and negative cloud parts becomes too large, a sudden discharge occurs, which we can see as lightning. In the process, a plasma, that is an electrically conductive gas, is first created in a small, punctiform area, which can then spread to channels. The tip of such a plasma channel can be positively or negatively charged. It was already known that a large amount of VHF emissions is produced at the growing tips of the negative channels while the positive channels show emissions only along the channel, not at the tip.
LOFAR allows the radio waves emitted by a flash to be stored unprocessed in their original form. This in turn makes it possible to develop new imaging techniques that can draw a three-dimensional image of a flash from the raw data – ten times better than previous measurements, up to one meter accurate and thanks to radio waves inside a cloud that can be up to 20 kilometers away from the telescope.
“The measurements originally came from our research group dealing with cosmic rays,” reports co-author Anna Nelles of DESY. “At the interface between particle physics and astronomy, this area was already quite exotic for a radio telescope. LOFAR was built mainly for astronomy. The fact that we are now the best lightning interferometer in the world came as a surprise to everyone and shows the exciting possibilities that can result from basic research with an outstanding infrastructure.”
The observations clearly show the occurrence of a break in the discharge channel, at a location where needles are formed. The needles appear to store negative charges from the main channel, which subsequently re-enter the cloud. “These needles accumulate charge, which then does not flow into the negative channels as expected, but is pumped back into the cloud via the needles. This recharges the cloud,” explains Hare. The reduction of charges in 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 discharge of lightning. By this mechanism, lightning will strike in the same area repeatedly.
“The VHF emissions along the positive channel are due to rather regularly repeated discharges along previously formed side channels, the needles. These needles appear to drain the charges in a pulsed manner,” explains Scholten. This is a totally new phenomenon, adds co-author Joe Dwyer of the University of New Hampshire (US): “Our new observation techniques show copious amounts of needles in the lightning flash, which have not been seen before.” And Brian Hare concludes: “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.”