light show

Physicists solved a longstanding mystery of the aurora borealis

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The aurora borealis is one of nature’s greatest wonders. As ethereal as it seems, this magical sight is the result of physics, and it begins with solar activity.

NASA/Scott Kelly

Every so often, the Sun burps out solar flares, which in turn cause violent geomagnetic storms.

NASA Goddard Space Flight Center

These storms ripple out and can hit the Earth’s magnetic field — these ripples are known as Alfven waves.

As the storms hit our magnetic field, they bring streams of electrons along for the ride. The charged particles collide with atoms and molecules in our atmosphere, exciting them in turn.

Expedition 11 Crew

These excited atoms and molecules then release their newfound energy in the form of visible light — the aurora borealis.

But physicists have long been baffled by two questions:

1. How do electrons make the journey through Earth’s atmosphere?

2. How do they reach speeds of 45 million miles per hour?

saravutvanset/RooM/Getty Images

Alfven waves may be the key to the electrons’ success and their speed.

Lev Savitskiy/Moment/Getty Images

In a recent paper in Nature Communications, physicists detail how these waves enable electrons to surf their way into Earth’s atmosphere.

NASA via Giphy

To come to their conclusions, the physicists put to the test a 40-year-old hypothesis that electrons surf Alfven waves. They did this by creating a simulation in UCLA’s Large Plasma Device, which is essentially 20 meter-long tunnel full of sensors.

The physicists found that streams of charged particles hitch a ride on Alfven waves, illustrated here by the yellow dots. The waves then travel along the magnetic fields in Earth’s atmosphere.

“Understanding the dynamics of how all of this all fits together, the aurora is just one part of that,” Greg Howes, a physicist at the University of Iowa, tells Inverse.

Electrons (yellow balls) accelerate toward the Earth along the magnetic field lines. Alfven waves follow (blue lines) and collide with errant atoms and molecules in the atmosphere (white balls).

Austin Montelius, College of Liberal Arts and Sciences, University of Iowa

The collision excites these molecules, which then expel energy in the form of light.

They found that electron speeds increase with Alfven waves in a process called electron acceleration. The electrons sped up as the waves undulated, akin to how surfers gain speed as they ride a wave’s underside.

Garrett McNamara

This research will help us better understand and predict space weather, says Howes. That includes its impact on our outer-space technology, like satellites and GPS.

NASA/Johnson Space Center

Read more physics stories here.

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