Although the saying goes ‘lightning never strikes the same place twice’, in fact it often does. Why it does so however, has long remained a mystery, but in 2019 a team of scientists led by the University of Groningen (RUG) used LOFAR to shed light on this matter. The radio telescope was able to chart lightning flashes in unprecedented detail, showing structures in the lightning channels the researchers dubbed needles. Through these needles, a negative charge may cause a repeated discharge to the ground.
Published by the editorial team, 29 May 2020
Lightning occurs when strong updrafts generate a kind of static electricity in large cumulonimbus clouds. Parts of the cloud become positively charged, others negatively. Once this charge separation is large enough, a violent discharge occurs: lightning. Such a discharge starts with a plasma, a small area of ionized air hot enough to be electrically conductive. This small area grows into a forked plasma channel that can reach lengths of several kilometres. The positive tips of the plasma channel collect negative charges from the cloud, which pass through the channel to the negative tip, where the charge is discharged. Lightning produces a large amount of VHF (very high frequency) radio bursts at the growing tips of the negative channels, while the positive channels show emissions only along the channel, not at the tip. Since LOFAR can detect signals in the VHF radio band, it is able to detect lightning propagation at an unprecedented scale and to ‘look’ inside a thundercloud, where most of the lightning resides.
The LOFAR study revealed the occurrence of a break in the discharge channel at a location where needles are formed. These needles appear to discharge negative charges from the main channel, which subsequently re-enter the cloud. 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 researchers published their results in the science journal Nature on April 18th.
The reason why the needles have never been seen before lies in the ‘supreme capabilities’ of LOFAR, says Dr 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.’
Although LOFAR is developed primarily for radio astronomy observations, this discovery proves that it is also very suited for lightning research.