Electrons surf on plasma waves: 85-year-old northern lights theory proven in the lab 

Electrons surf on plasma waves: 85-year-old northern lights theory proven in the lab 

Electrons surf on plasma waves: 85-year-old northern lights theory proven in the lab 

Solar particles surf on lightning-fast plasma waves before flying into our atmosphere and causing the northern lights. Scientists have proven that for the first time. Without that speed, we would not see the aurora. The researchers thus confirmed a theory from 1937, a puzzle piece that was still missing to understand the mysterious spectacle.

The Northern Lights (Aurora Borealis) and Aurora Borealis (Aurora Australis) appeal enormously to the imagination: green curtains of light billowing in the night sky above the poles. For centuries it has also fascinated scientists, who tried to find out the cause of the light.

The spectacle is created in the magnetic field around our earth. Outside our atmosphere hangs plasma, a mixture of tiny energy-rich particles that are continuously sent into space by the sun – also called the solar wind. Solar wind can also cause waves in the plasma, just like the wind at sea moves the water. In solar flares, explosions on the sun, such plasma can suddenly be thrown firmly into space. The waves then get bigger, like a storm at sea. 

An illustration of the origin of aurora: electrons (yellow) collide in the outer layer of our atmosphere against atoms (white). The energy released as a result is released as photons (here green and red) – the light we perceive.
Austin Montelius, College of Liberal Arts and Sciences, University of Iowa

Usually, those particles do not reach the earth, because the magnetic shield around our earth distracts them. But at our north and south poles, those particles – guided by that magnetic field – do flow straight to Earth and end up in the outer layer of our atmosphere. There they collided with air particles, such as oxygen molecules. 

Energy is released during the collision. So we perceive that as green (or red) undulating light, or when the electrons collide with nitrogen molecules, as blue light, at least when the particles collide with each other at extremely high speed. Very exceptionally, the light can even be seen far from the pole, for example in the Netherlands, as a result of a very strong solar flare.

The mechanism behind the light had therefore been known for a while, but it still contained a small mystery: it had never been demonstrated before how those electrons could travel so fast. In 1937, Nobel laureate Hannes Alfvén, a Swedish physicist, came up with a theory: the particles would surf on the plasma waves, just as surfers take advantage of the energy of a wave to make speed. But then up to 72 million kilometers per hour.

Scientists have now proved for the first time that this theory about the so-called “Alfvén waves” is correct. In a lab, they tested whether electrons can really pick up the energy of plasma waves. It did, and some of it went fast enough to cause light.

So, after 85 years, Alfvén’s theory has been proven. In reality, there are many other factors involved, the scientists say, but because the process is so fast in space, it is almost impossible to determine all this in the northern or southern lights themselves. In any case, a missing puzzle piece has now been put in place.

This article is based on a study from the scientific journal Nature Communications and an article by  New Scientist.

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