Definitive evidence how auroras are formed

This challenging experiment required the measurement of a very small population of electrons.

An aurora is a natural phenomenon is characterized by a display of a natural-colored (green, red, yellow, or white) light in the sky. Sometimes, it is also known as polar light that appears most often in the regions of high altitudes like the Arctic and Antarctic.

The aurora borealis has fascinated people for thousands of years. But how they’re created, while theorized, had not been conclusively proven.

A new study by the University of Iowa has offered definitive evidence that powerful electromagnetic waves produce the most brilliant auroras during geomagnetic storms. Scientists dubbed this phenomenon as Alfven waves.

The Alfven waves speed up electrons towards Earth, causing the particles to produce the familiar atmospheric light show.

Greg Howes, associate professor in the Department of Physics and Astronomy at Iowa and study co-author, said, “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.”

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

Energized particles that emanate from the sun collide with particles from gases such as oxygen and nitrogen present in the Earth’s atmosphere. The solar wind that carries the electrically charged particles disrupts the Earth’s magnetosphere. This kicks the particles into an excited state. These excited molecules relax by emitting light, producing the colorful hues of the aurora.

Many spacecraft missions support this theory. However, those missions have frequently found Alfven waves traveling Earthward above auroras, presumably accelerating electrons along the way. Even though space-based estimations had upheld the theory, limitations intrinsic to spacecraft and rocket measurements had prevented a conclusive test.

In this new study, physicists now found confirmatory evidence in a series of experiments conducted at the Large Plasma Device (LPD) in UCLA’s Basic Plasma Science Facility; a national collaborative research facility supported jointly by the U.S. Department of Energy and National Science Foundation.

Craig Kletzing, professor in the Department of Physics and Astronomy at Iowa and a study co-author, said, “This is the first time we’ve been able to confirm definitively that it works. 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.”

For this study, physicists performed some numerical simulations and mathematical modeling. They demonstrated that the results of their experiment agreed with the predicted signature for Landau damping.

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

“This challenging experiment required a measurement of the tiny 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.”

Journal Reference:
  1. Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves, Nature Communications (2021). DOI: 10.1038/s41467-021-23377-5

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