Physicists determine how auroras are created

The Northern Lights, or Aurora Borealis, that fill the skies in high latitude regions have fascinated people for thousands of years. But how they are created, although theorized, had not been conclusively proven.

In a new study, a team of physicists led by the University of Iowa reports definitive evidence that the brightest auroras are produced by powerful electromagnetic waves during geomagnetic thunderstorms. The phenomena, known as Alfven waves, accelerate electrons back to Earth, causing the particles to produce the familiar atmospheric light show.

The study, published online June 7 in the journal Nature Communication, concludes a decades-long quest to experimentally demonstrate the physical mechanisms of electron acceleration by Alfven waves under conditions corresponding to Earth’s auroral magnetosphere.

“The measurements revealed that this small population of electrons undergoes a ‘resonant acceleration’ by the electric field of the Alfven wave, similar to a surfer catching a wave and being continuously accelerated as the surfer moves with the wave,” explains Greg Howes, associate professor in the Department of Physics and Astronomy at Iowa and co-author of the study.

Scientists know that energized particles that emanate from the sun, such as electrons that run at about 45 million kilometers per hour, rush along the Earth’s magnetic field lines into the upper atmosphere, where they enter. collide with molecules of oxygen and nitrogen, throwing them into an excited atmosphere. State. These excited molecules relax by emitting light, producing the colorful hues of dawn.

The theory was supported by space missions which frequently found Alfven waves traveling to Earth above the aurora, presumably accelerating electrons along the way. Although space measurements supported the theory, limitations inherent in measurements of spacecraft and rockets had precluded a definitive test.

Physicists led by the University of Iowa report definitive evidence of the creation of the aurora. In experiments, physicists have demonstrated the physical mechanisms of the acceleration of electrons by Alfven waves under conditions corresponding to the Earth’s auroral magnetosphere. Illustration by Austin Montelius.

Physicists were able to find confirmatory evidence in a series of experiments conducted at the Large Plasma Device (LPD) in the Basic Plasma Science Facility at UCLA, a national collaborative research center jointly supported by the US Department of Energy and the 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 that we can definitively confirm that it works,” says Craig Kletzing, professor in the Department of Physics. and Astronomy at Iowa and study co-author. “These experiments allow us to make the key measurements that show that spatial measurements and theory explain, indeed, a major way in which auroras are created.”

The phenomenon of electrons “surfing” on the electric field of a wave is a theoretical process known as Landau damping, first proposed by the Russian physicist Lev Landau in 1946. Thanks to numerical simulations and to mathematical modeling, the researchers showed that the results of their experiment were consistent. with the predicted signature for Landau’s amortization.

The experiment, simulation and modeling agreement provides the first direct evidence that Alfven waves can produce accelerated electrons, causing aurora, says Troy Carter, professor of physics at UCLA and director of the UCLA Plasma Science and Technology Institute.

“This difficult experiment required a measurement of the very small population of electrons moving through the LPD chamber at almost the same speed as Alfven waves, counting less than one in a thousand electrons in the plasma,” says Carter.

James Schroeder, assistant professor of physics at Wheaton College and corresponding author of the study, received a doctorate from Iowa. Frederick Skiff, professor in the UI Department of Physics and Astronomy, is a co-author of the study. Stephen Vincena, a physics researcher at UCLA, and Seth Dorfman, of the Space Science Institute and a visiting scholar at UCLA, are contributing authors.

More detailed information is available at homepage.physics.uiowa.edu/~ghowes/research/aurora.html.

The United States National Science Foundation, the United States Department of Energy, and NASA funded the research.


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Gail Mena

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