The Dutch radio telescope LOFAR is able to observe the creation and propagation of lightning flashes. An international scientific team, with a leading role for the University of Groningen, in collaboration with, among others, CWI researchers, shows that LOFAR can detect the developing flashes at an unprecedented one-metre resolution. These high resolution observations may lead to better lightning protection. The results were published on 16 February in the Journal of Geophysical Research: Atmospheres.
You may think that lightning storms are nothing special, but scientists are still struggling to understand how the flashes come about. ‘As a matter of fact, we know very little about this process. That was a surprise to me too, when I discovered this a few years ago’, explains Olaf Scholten, professor of Astroparticle Physics at the University of Groningen. It is not easy to study lightning: you never know where it strikes, and when it does, it is quite destructive.
But an accurate registration of lighting is possible using radio antennas. If you ever listened to a VHF radio during a thunderstorm, you will know such a storm emits radio signals. So in different parts of the world, antenna arrays are dedicated to lightning research. Part of the LOFAR antenna arrays, of which the Dutch part is distributed over some 3,200 square kilometres, are quite similar and have now been used to study lightning at an unprecedented accuracy, revealing crucial dynamics of lightning.
Pulses
The scientists have analysed data collected by LOFAR on 12 July 2016 during a thunderstorm, early in the evening. ‘One scientist waited for a flash of lightning and then pushed a button to freeze the last 20 seconds of data collected by the array’, Scholten explains. This happens in the so called Transient Buffer Boards, which were mounted on the LOFAR array at the request of (and with money from) the Radboud University. They allow scientists to temporarily hold part of the huge data stream produced by LOFAR for a thorough inspection. Most of the data is usually discarded after in an automated selection process.
During this inspection, the data from one antenna were analyzed to determine the exact timing of the lightning flash. That one second, plus some seconds before and after, were then downloaded from the buffer boards at 24 different LOFAR stations used for the study. ‘This took half an hour, these antennas produce quite a bit of data.’
A lightning flash starts with a series of pulses which each ionize channels of some one meter wide and about 50 meters in length. Only when an ionization channel short circuits, either with the ground or another cloud, the well know flash of lightning appears, shooting through the channel. The LOFAR data makes it possible to determine the position of each pulse very accurately: ‘By determining to the nanosecond when each pulse hits the different antennas, we can determine its position in the sky with a resolution of about one metre.’
