Nick Carne
Lightning strikes twice in the same place surprisingly often and now, thanks to a Dutch radio telescope network called the Low Frequency Array (LOFAR), scientists have a better idea why.
An international team used the array to study the development of lightning flashes in unprecedented detail, and discovered that the negative charges inside a thundercloud are not discharged in a single flash.
Some are stored inside structures the researchers have called needles, making a repeated discharge to the ground quite possible.
“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”, says Olaf Scholten, from the University of Groningen in The Netherlands.
And it hasn’t been noticed before because there wasn’t equipment powerful enough to do so.
“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,” says Brian Hare, also from the University of Groningen.
LOFAR comprises thousands of simple antennas that are spread over Northern Europe but connected with a central computer through fibre-optic cables, allowing them to operate as a single entity. It is used primarily for radio astronomy observations, but the frequency range of the antennas also makes it suitable for lightning research, because discharges produce bursts in the very high frequency (VHF) radio band.
In the recent study, the researchers used only LOFAR stations in The Netherlands, covering an area of 3200 square kilometres, and analysed the raw time-traces – which are accurate to one nanosecond – measured in the 30-to-80 megaherz (MHz) band.
They developed a new algorithm for the data, allowing them to visualise the VHF radio emissions from two lightning flashes. The antenna array and the very precise time stamp on all the data allowed them to pinpoint the emission sources with unprecedented resolution.
The results, they say, clearly show the occurrence of a break in the discharge channel, at a location where needles formed. These 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 VHF emissions along the positive channel are due to rather regularly repeated discharges along previously formed side channels, the needles,” says Scholten. “These needles appear to drain the charges in a pulsed manner.”
The findings are published in a paper in the journal Nature.
