The aurora borealis, or as it is known as the northern lights, has fascinated astronomy enthusiasts for thousands of years. But this dancing glow in high latitudes has puzzled scientists for centuries because of the mystery of its causes.
In a new study, a team of physicists led by the University of Iowa reported conclusive evidence that the brightest aurorae are caused by strong electromagnetic waves during geomagnetic storms.
This phenomenon, known as Alfvén waves, speeds electrons toward Earth, causing the particles to produce typical atmospheric light exposure.
The study, published online June 7 in Nature Communications, concludes in a decades-long quest to prove the physical mechanisms of electron acceleration by Alfvén waves under conditions consistent with Earth’s auroral magnetosphere.
“Measurements revealed that this small group of electrons undergoes resonant acceleration by the electric field of an Alfvén wave, similar to a surfer holding a wave and being continuously accelerated as a rider moves,” said Greg Hawes, associate professor in the Iowa Department of Physics and Astronomy and co-author of the study. The waves are along with the wave.”
Scientists knew that energetic particles emitted from the sun, such as electrons racing at 45 million miles per hour, are deposited along the Earth’s magnetic field lines in the upper atmosphere, where they collide with oxygen and nitrogen molecules, causing them to be pushed into an excited state. These excited molecules relax by emitting light, producing the colorful shapes of the aurora borealis.
This theory was supported by spacecraft missions that repeatedly found that Alfvén waves travel toward Earth above the aurora, presumably accelerating electrons along the way.
Although space measurements supported the theory, limitations inherent in spacecraft and rocket measurements prevented definitive testing.
The physicists were able to find definitive evidence in a series of experiments conducted in the Large Plasma Device (LPD) at the University of California, Los Angeles (UCLA) Plasma Science Core Facility, a collaborative research facility jointly supported by the US Department of Energy and the National Science Foundation.
“The idea that these waves can activate the electrons that create auroras dates back more than four decades, but this is the first time we’ve been able to conclusively confirm their success,” says Craig Klitzing, a professor in the Iowa Department of Physics and Astronomy and a co-author of the study. …these experiments allow us to make key measurements that show that space measurements and theories do indeed explain the main way in which the aurora is formed.”
The phenomenon of electrons “surfing” in 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 showed that the results of their experiment agreed with the likely signature of Landau damping.
The agreement of experiment, simulation and modeling provides the first direct evidence that Alfvén waves can produce accelerated electrons, causing auroras, says Troy Carter, professor of physics at UCLA and director of the Institute of Plasma Science and Technology at UCLA.
Source: Science Daily