Surfing particles solve aurora origin mystery

Electrons undergo resonant acceleration by Alfven wave’s electric field

Published - July 11, 2021 03:56 pm IST

The aurora borealis as seen in England in  2014. A long-standing mystery about the auroras has now been solved.

The aurora borealis as seen in England in 2014. A long-standing mystery about the auroras has now been solved.

Auroras have been mentioned in these pages in the last few months. Aurora borealis or northern lights near the north pole and aurora australis or southern lights near the south pole are fascinating natural light shows that have captivated human imagination for centuries.

We learnt how these are caused by the burping sun as the electrons emitted this way race through space towards the Earth. When they are funnelled down Earth’s magnetic field lines, they go on to collide with oxygen and nitrogen molecules in the upper atmosphere. When these molecules re-radiate the energy thus gained as light, auroras are formed.

Electrons catch Alfven waves

While this much has been well known for a long time now, one mystery about these auroras that has baffled scientists is how exactly these electrons accelerate to extremely high speeds on the last leg of their journey to Earth. Researchers from the University of Iowa have found the answer to this question and have published their findings in June 2021.

The scientists have come up with the first direct evidence that the electrons catch a wave, especially Alfven waves (a type of wave that occurs in a plasma as a result of the interaction between the magnetic fields and electric currents within it), that travel Earthward along the magnetic field lines. By simulating conditions of the Earth’s aurora magnetosphere in a chamber and then launching Alfven waves, researchers were able to find out if the electrons inside were affected.

Like a surfer

In the process, they noted that the electrons underwent resonant acceleration by the Alfven wave’s electric field. The scientists likened this acceleration to that of a surfer catching a wave and being continuously accelerated while moving along with the wave.

By measuring the resonant acceleration, they were able to show that electrons could reach the speeds necessary for aurora displays. Through their experiments and measurements, scientists have thus proven a theory known as Landau damping that talks about surfing electrons.

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