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Nothing can ruin our joy in the aurora borealis or the northern lights, the bands of blue, green and violet light that cascades from the sky. Not even knowing for sure what is causing them.
Physicists have long wondered what gives rise to this very specific light phenomenon that occurs in the polar regions of the earth.
Now they are safe.
An article published in the journal Nature communication this week suggests that the natural light show starts when disturbances on the sun pull on the Earth’s magnetic field. It creates cosmic waves known as Alfvén waves that fire electrons at high speeds into the Earth’s atmosphere, where they create the Northern Lights.
“It was kind of theorized that this is where the energy exchange takes place,” said Gregory Howes, an associate professor of physics and astronomy at the University of Iowa. “But no one had ever come up with a definitive demonstration that the Alfvén waves actually accelerate these electrons under the appropriate conditions that you have in space above the Northern Lights.”
How the aurora is shaped
The sun is unstable, and violent events that, like geomagnetic storms, can echo into the universe.
In some cases, the sun’s disturbances are so strong that they move out onto the Earth’s magnetic field like a rubber band and pull it away from our planet.
But like a tight elastic when released, the magnetic field clicks back, and the force of that recoil creates powerful ripples known as Alfvén waves about 80,000 miles from the ground. As these waves get closer to Earth, they move even faster thanks to the planet’s magnetic traits.
On the same orbit, there are electrons that also travel towards Earth, but not as fast as the Alfvén waves.
Sometimes electrons travel on these super-fast Alfvén waves and reach speeds as high as 45 million miles per hour when they throw downward.
“Think about surfing,” said Jim Schroeder, a physics professor at Wheaton College and lead author of the article. “To be able to surf, you have to paddle up to the right speed for an ocean wave to pick you up and accelerate you, and we found that electrons were surfing. If they were moving at the right speed relative to the wave, they would be retrieved and accelerated. “
Austin Montelius / College of Liberal Arts and Sciences, University of Iowa
When the electrons reach the Earth’s thin upper atmosphere, they collide with nitrogen and oxygen molecules and send them into an excited state. The excited electrons eventually fall down and release light, which is what we see as the aurora.
What the experiment showed
While scientists for decades had assumed that Alfvén waves were responsible for accelerating electrons, this laboratory experiment has produced the only conclusive evidence.
“No one had actually ever measured this before between electrons and Alfvén waves,” Schroeder told NPR.
The researchers used what is known as the Large Plasma Device at UCLA’s Basic Plasma Science Facility to recreate the interaction between Alfvén waves and electrons.
Such a study would have been impossible in space, as scientists can not predict when aurora will occur and would not be able to account for other factors in the cosmos, they said.
The researchers suggested that their findings could help understand more broadly how particles get energy and also give them a clearer picture of how events on the sun affect space near Earth as well as the technological infrastructure we have there, such as satellites.
For Schroeder, there is another much simpler advantage to this type of research.
“This appeals to our sense of awe and wonder,” he said. “We have been fascinated by the Northern Lights for thousands of years, looking at the night sky and appreciating their beauty. And I have always found that understanding how something is created enhances my understanding of that beauty.”