The aurora borealis, or northern lights, that fill the sky in high-latitude
regions have fascinated people for thousands of years. But how they're
created, while theorized, had not been conclusively proven.
In a new study, a team of physicists led by University of Iowa reports
definitive evidence that the most brilliant auroras are produced by powerful
electromagnetic waves during geomagnetic storms. The phenomena, known as
Alfven waves, accelerate electrons toward Earth, causing the particles to
produce the familiar atmospheric light show.
The study, published online June 7 in the journal Nature Communications,
concludes a decades-long quest to demonstrate experimentally the physical
mechanisms for the acceleration of electrons by Alfven waves under
conditions corresponding to Earth's auroral magnetosphere.
"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," says Greg Howes, associate professor in the Department
of Physics and Astronomy at Iowa and study co-author.
Scientists have known that energized particles that emanate from the
sun—such as electrons racing at approximately 45 million miles per
hour—precipitate along the Earth's magnetic field lines into the upper
atmosphere, where they collide with oxygen and nitrogen molecules, kicking
them into an excited state. These excited molecules relax by emitting light,
producing the colorful hues of the aurora.
The theory was supported by spacecraft missions that frequently found Alfven
waves traveling Earthward above auroras, presumably accelerating electrons
along the way. Although space-based measurements had supported the theory,
limitations inherent to spacecraft and rocket measurements had prevented a
definitive test.
The physicists were able to find 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.
"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," says 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
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 Russian
physicist Lev Landau in 1946. Through numerical simulations and mathematical
modeling, the researchers demonstrated that the results of their experiment
agreed with the predicted signature for Landau damping.
The agreement of experiment, simulation, and modeling provides the first
direct evidence that Alfven waves can produce accelerated electrons, causing
the aurora, says Troy Carter, professor of physics at UCLA and director of
the UCLA Plasma Science and Technology Institute.
"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," Carter says.
Reference:
Laboratory measurements of the physics of auroral electron acceleration by
Alfvén waves, Nature Communications (2021). DOI:
10.1038/s41467-021-23377-5