The northern lights are caused by electrons hurtling toward Earth — and now we know how they get there

June 7, 2021

Surfing electrons from space shed new light on the northern lights. (Unsplash/Vincent Guth)

Space-borne electrons surf Alfvén waves on their way to Earth after being blasted from the sun, researchers found, proving once and for all how these particles cause nature's glimmering nighttime show — the northern lights.

Published Monday in Nature Communications, the computational work addresses the long-standing question of how these particles are pushed into Earth's atmosphere to generate the fascinating displays, marking the first concrete documentation of the trajectory of these aurora-producing electrons.

"This is an idea that's existed — this type of interaction between Alfvén waves and electrons — but we've managed to actually make that measurement and show that it does actually happen," corresponding author Jim Schroeder, an assistant professor in the Department of Physics at Wheaton College, told The Academic Times.

Alfvén waves are commonly found throughout the universe wherever there are magnetic fields, ions, plasma or any type of high-energy particle. They're even found in laboratories working with plasma-based experiments.

"If you look out into the solar wind, or around the sun, or between stars," Schroder said, "Alfvén waves are all over in plasmas throughout the universe."

Schroeder works in a field referred to as laboratory astrophysics, in which studies simulate phenomena of the cosmos to better understand the mechanisms of space without needing to travel there. For this particular project, the researcher decided to tackle the question: How do electrons that escape from the sun behave on their way to Earth?

"We can take a question from space and bring it down into the laboratory," he said. "People have been most commonly trying to answer this question in space: They fly satellites above auroras or fly rockets through auroras and make measurements."

The difficulty with such a technique in studying these electrons is that vehicles in space are constantly moving; satellites must be floating around while collecting data. Instead, Schroeder's lab-centric simulation of the aurora's formation more easily uncovered that the electrons behind the glowing show behave like surfers. 

"A surfer at the right speed can be picked up by an ocean wave and carried along, [and] can speed up as they ride the wave," he said. "We found that there are electrons — in the experiment — that do exactly the same thing."

It's already known that the northern lights are produced when magnetic fields from the sun — an ocean of plasma — erupt the charged gas from its surface and send it outward. As the field holding the high-energy particles, including stray electrons, extends further into space, it resembles something like a rubber band protruding from the star's side.

"When I arrived at grad school and was looking around for research topics and learned that there was this really fundamental question of, 'Where do the electrons come from that cause auroras?'" Schroeder recalled, "I was hooked."

Eventually, the "rubber band" stretches out so far that it snaps, so to speak. The section unconnected to the sun catapults toward Earth. While humans remain safe from the particles crashing toward the planet due to its own protective magnetic fields, after a bit of push-and-pull as well as coupling between both bodies' fields, some electrons remain in Earth's atmosphere, leading to auroral displays.

"Alfvén waves can push electrons forward, can give them additional speed, can transfer energy to the electron — which is kind of the missing piece of the puzzle," Schroeder said.

Additionally, the researcher noted that the team's simulations found that though Alfvén waves are typically above all auroras, some of the brightest auroras were especially associated with them.

"It's not just that we found that Alfvén waves can push electrons towards Earth," he said. "We also have some greater insight into some of the most exciting auroral displays."

Understanding how these high-energy particles work could also shed light on how space weather operates — knowledge that could be vital in ensuring safety of astronauts and efficiency of Earth-based electricity.

"There are consequences for understanding how space weather affects our global network of satellites, understanding how space weather affects the power grid," Schroeder said. "This kind of goes into the category of understanding a region of space that we're increasingly dependent on."

In the future, Schroeder hopes to pivot to some other questions of space physics that laboratory simulations and experiments can study better than space travel can. In particular, he hopes to explore questions of energetic electrons found in other areas of space, such as in the Van Allen radiation belts. 

"We've become really dependent, in our everyday lives, on space — the satellites that fly through there that we use for communication, that we use for navigation," Schroeder said.

That could prove an especially timely research topic as physicists are preparing to send satellites near the belts in medium Earth orbit due to crowding in the more accessible and safe low Earth orbit. 

The paper, "Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves," published June 7 in Nature Communications, was authored by J. W. R. Schroeder, Wheaton College; G. G. Howes, C. A. Kletzing and F. Skiff, University of Iowa; T. A. Carter and S. Vincena, University of California, Los Angeles; and S. Dorfman, Space Science Institute. 

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