Radio telescope LOFAR shows how lightning grows

The Dutch radio telescope LOFAR, which fans out over a large part of Europe from the Northern Netherlands, is able to observe the creation of lightning flashes. An international scientific team, with a leading role for University of Groningen physicists Brian Hare and Olaf Scholten, 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 KVI-CART research institute, 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.’

In this manner, the scientists could watch the lightning grow right up to the moment of discharge. University of Groningen post doc Brian Hare analysed the data, which was then used by prof. dr. Stijn Buiting (Free University Brussels, Belgium) to make a 3D reconstruction. This shows a kind of growing tree with branches elongating and dividing. And this is only the beginning, says Scholten: ‘With some extra effort, we can get even more information from the data.’ His hopes are that further analysis will tell us what sets off a the ionisation preceding a flash, and what exactly causes the discharge. ‘This could help us in the design of better lightning protection.’

The new article is part of a wider research project aimed at really understanding lightning flashes. This work is done by an interdisciplinary collaboration of the KVI-CART at the University of Groningen, astronomers from Radboud University Nijmegen, the department of high energy physics at the Free University Brussels and t he Dutch national research institute for mathematics CWI in Amsterdam. Earlier research from this project showed that lightning flashes could be set off by cosmic particles entering the Earth atmosphere. Furthermore, it produced a new method to use cosmic rays to study charge distribution in thunder clouds.

Schools

The lightning bolt analysed by Brian Hare was positioned some forty kilometres from the heart of the LOFAR radio telescope, the place with the highest antenna density, and some thirty kilometre from the nearest antenna field. Data have also been collected from a flash right above an antenna field, which could reveal even more details. ‘Just one bolt of lightning provides us with a huge amount of information. We are now talking to other lightning scientists, to discuss placing their dedicated antennas at the LOFAR sites, thus combining our observations.’ Also, he is developing a project for schools with the University of Groningen science centre Science LinX. Schools will receive a teaching program and their very own antenna to help collecting new data on lightning.

Reference: B. M. Hare, O. Scholten, A. Bonardi, S. Buitink, A. Corstanje, U. Ebert, H. Falcke, J.R. Hörandel, H. Leijnse, P. Mitra, K. Mulrey, A. Nelles, J. P. Rachen, L. Rossetto, C. Rutjes, P. Schellart, S. Thoudam, T. N. G. Trinh, S. ter Veen, T. Winchen: LOFAR Lightning Imaging; Mapping Lightning with Nanosecond Precision. Journal of Geophysical Research: Atmospheres. 16 februari 2018, DOI 10.1002/2017JD028132