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Physicists Think They Know Exactly What Causes Auroras

The process wasn't easy but we now have proof.

In a new study led by the University of Iowa, physicists have gathered definitive evidence for the mechanism that underpins the formation of auroras. The study, published in the journal Nature Communications, finally provides reliable evidence that auroras, like the famous aurora borealis, are formed by powerful electromagnetic waves during geomagnetic storms through a phenomenon known as Alfven waves

These waves accelerate electrons towards the Earth which, as a result, causes the particles to produce the light show that we are familiar with. This research is not only interesting in and of itself, but it finally draws to a close a decades-long campaign to demonstrate experimentally the physical mechanisms of Alfven waves. 

According to one of the study's author Greg Howes (associate professor in the Department of Physics and Astronomy at Iowa), "measurements revealed this small population of electrons undergoes 'resonant acceleration' by the Alfven wave's electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave."

Prior to this study, it has been widely thought that energized particles from the Sun interact with the Earth's magnetic field lines to excite oxygen and nitrogen molecules. When these excited molecules "relax", they release visible light to produce the colorful and exciting visual displays of auroras. 

While some evidence has been gathered using space-based measurements, these are often fraught with limitations preventing satisfactorily definitive results. 

what causes the auroras
Auroras have fascinated humans for millennia, but we may now have definitive proof as to the cause. Source: Mark Wilson/Flickr

Electrons are able to "surf" the Earth's magnetic field

During the study, researchers were able to gather data from experiments using the Large Plasma Device (LPD) in UCLA's Basic Plasma Science Facility. This is a national collaborative research facility supported jointly by the U.S. Department of Energy and National Science Foundation.

"The idea that these waves can energize the electrons that create the aurora goes back more than four decades, but this is the first time we've been able to confirm definitively that it works," explained Craig Kletzing, professor in the Department of Physics and Astronomy at Iowa and a study co-author. "These experiments let us make the key measurements that show that the space measurements and theory do, indeed, explain a major way in which the aurora is created."

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The idea that electrons could "surf" through something called Landau damping on the electric field of a wave is nothing new, however. It was first proposed by Russian physicist Lev Landau way back in 1946 -- hence the name. 

What is new is that by using numerical simulations and mathematical modeling, the research team was able to demonstrate that the results of their experiment agreed with the predicted signature for Landau damping.

This is very exciting indeed, and the agreement of results from experimentation, simulation, and modeling provides the first direct evidence that Alfven waves can produce accelerated electrons — resulting in the formation of the auroras.

"This challenging experiment required a measurement of the very small population of electrons moving down the LPD chamber at nearly the same speed as the Alfven waves, numbering less than one in a thousand of the electrons in the plasma," explained says Troy Carter, professor of physics at UCLA and director of the UCLA Plasma Science and Technology Institute.

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You can view the original study in the journal Nature Communications

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