LOFAR radio telescope shows how lightning grows

The Dutch LOFAR radio telescope, which fans out over a large part of Europe from the Northern Netherlands, can observe the creation of lightning flashes. An international team of scientists, with a leading role for University of Groningen physicists Brian Hare and Olaf Scholten, has shown that LOFAR can detect the developing flashes at a one-meter 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 it a few years ago’, explains Olaf Scholten, Professor of Astroparticle Physics at the KVI-CART research institute at the University of Groningen. It is not easy to study lightning: you never know where it will strike, and when it does, it is quite destructive.

But an accurate registration of lighting is possible with radio antennas. If you have ever listened to a VHF radio during a thunderstorm, you will know such a storm emits radio signals. There are antenna arrays in different parts of the world that are dedicated to lightning research. As part of the LOFAR antenna array, the Dutch part of which is distributed over some 3,200 square kilometres, is quite similar to these arrays, the team used it to study lightning.

Pulses

The scientists analyzed data collected by LOFAR during a thunderstorm early in the evening of 12 July 2016. ‘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 what are known as the Transient Buffer Boards, which were mounted on the LOFAR array at the request of (and with funding from) Radboud University. These allow scientists to temporarily hold part of the huge data stream produced by LOFAR for thorough inspection. Most of the data is usually discarded after an automated selection process.

During such an inspection, the scientists analyzed the data from one antenna to determine the exact timing of the lightning flash. They then downloaded that one second, plus a few seconds before and after, from the Buffer Boards at 24 different LOFAR stations. ‘This took half an hour. These antennas produce quite a bit of data.’

A lightning flash starts with a series of pulses, each of which ionizes channels of about one meter wide and 50 metres long. Only when an ionization channel short-circuits, either with the ground or another cloud, does the familiar flash of lightning appear, 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 postdoc Brian Hare analyzed the data, which Prof. Stijn Buiting (Free University Brussels, Belgium) then used 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.’ He hopes that further analysis will tell us what triggers the ionization preceding a flash, and what exactly causes the discharge. ‘This could help us design better lightning protection.’

The new article is part of a wider interdisciplinary research project aimed at fully understanding lightning flashes. KVI-CART, astronomers from Radboud University Nijmegen, the department of high-energy physics at the Free University Brussels and the national research institute for mathematics and computer science in the Netherlands (CWI) are collaborating on the project. Earlier research from the project has shown that lightning flashes can be set off by cosmic particles entering the Earth’s atmosphere . It has also produced a new method to use cosmic rays to study charge distribution in thunder clouds.

Schools

The lightning bolt that Brian Hare analyzed was some 40 kilometres from the heart of the LOFAR radio telescope, the place with the highest antenna density, and some 30 kilometres from the nearest antenna field. Data was also collected from a flash right above an antenna field, which may 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.’ He is also developing a project for schools with the University of Groningen Science LinX science centre. Schools will receive a teaching programme and their very own antenna to help collect 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