RUG researchers look inside lightning

Using LOFAR radiotelescopes, RUG scientists have succeeded in getting a look at the inside of a lightning bolt.
By Christien Boomsma / Translation by Sarah van Steenderen

Four years ago, astrophysicist Olaf Scholten didn’t know anything about lightning, but today he is one of leading scientists in the ‘lightning community’. He and fellow researcher Brian Hare managed to capture 3D images inside a thunderstorm cloud. The images are a hundred times more detailed than was previously possible. The scientists published their results in the prestigious magazine, Nature, on 18 April.

The results were entirely different from what they expected to find: ‘During a thunderstorm, the air ionises into a kind of plasma channel. The electrical field in a thunderstorm then gives it a negative and a positive side’, says Scholten. ‘We were expecting to see the charge going straight into the ground, travelling through that channel.’

Instead, the researchers saw ‘needles’ sticking out of the channel. These needles were at least one hundred metres long and five metres wide. ‘The current goes from the “stem” to the “tip” of the needle, says Scholten. ‘After a short break, the current is repeated. This can happen ten times in a row, easily.’

The video shows the development of the lightning in slow motion. In real time, the duration is less than 0.2 seconds and towards the end the discharge, spans about five kilometres in all
directions. / Video Stijn Buitink en Brian Hare

Nanosecond

When that happens, the charge that the plasma channel collects electrifies the cloud. As soon as the voltage difference becomes big enough, another bolt shoots down to the ground.

The needles, which are only visible for a fraction of a second, have never been observed before, because researchers usually observe the phenomenon from several kilometres away.

Using the LOFAR radiotelescopes, Scholten and Hare managed to get a closer look. With telescopes, they can observe electrical pulses emanating from the lightning down to the nanosecond. This data can be used to calculate where in the atmosphere they were created. By combining the data from various telescopes, the researchers managed to create an image.

The video shows a zoom-in on the positively charged channel. In real time, the development
takes only 0.1 seconds and spans a distance of about 400 metres. / Video Stijn Buitink en Brian Hare

Mystery

Lightning is considered one of the most dangerous natural phenomena, but it’s still a scientific mystery. Scholten became a lightning scientist by chance four years ago, when he was using the LOFAR telescope for a study into high energy cosmic particles. He discovered that strange, anomalous measurements coincide with thunderstorms. This discovery led to a completely new way of studying how charges were spread out across clouds; scientists previously used weather balloons for this.

The current method has changed over the past four years. Scholten doesn’t just use the antennae on the LOFAR ‘super terp’. ‘The data from the stations farther away is essential. We’re studying the lightning from various angles, which gives us a 3D picture.’

Because they now know what to look for, they can tell the telescope which events to record. ‘As soon as there’s a thunderstorm in the north of the Netherlands, LOFAR shuts down everything it’s doing for other studies and gives us priority’, Scholten says. ‘Fortunately, normal astronomy has no use for the kind of weather we’re looking for.’

Nederlands

17 April 2019