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The Source of the Aurora Borealis: Electrons Surfing on Alfvén Waves

New experiments have shown the source of the aurora borealis. Researchers have demonstrated Alfvén waves accelerating electrons under conditions that correspond to Earth’s magnetosphere. The magnetosphere around the Earth contains ionized charged particles or plasma, one of the four types of matter along with solid, liquid, and gas. Plasmas are similar to both fluids and gases but also contain magnetic and electric fields. In 1942, Hannes Alfvén predicted that plasmas could support waves. These waves are today called Alfvén waves. The new experiments show that electrons “surf” on the electric field of the Alfvén wave in a phenomenon known as Landau damping. This means the energy of the wave is transferred to the accelerated electrons, like a surfer catching a wave and being continually accelerated as the surfer moves along with the wave. These electrons are the ultimate source of the light we call the aurora borealis.

The shimmering displays of the aurora borealis have always fascinated humanity. Now scientists have demonstrated how electrons associated with the aurora accelerate towards the Earth, where they collide with molecules in the upper atmosphere. These collisions cause the emission of auroral light. The new research reproduced the conditions in space above the aurora in the laboratory. It shows that Alfvén waves launched by storms in the earth’s magnetic field can accelerate the electrons that cause the bright glow of the aurora. These results definitively confirm this previously unproven hypothesis.

The spectacular displays of the aurora borealis have always fascinated humankind. More recently, they have captured the attention of scientists curious to explain this phenomenon. Scientists know that electrons from space precipitate into the upper atmosphere and collide with atoms and molecules to produce auroral light. The cause of this electron precipitation has been an area of ongoing research. One theory is supported by the fact that scientists regularly find powerful electromagnetic waves called Alfvén waves traveling toward the Earth above auroras. According to this theory, Alfvén waves accelerate electrons toward Earth, causing them to precipitate and produce auroras. Although space-based measurements provide strong support of this theory, the limitations of these measurements have prevented a definitive test. To overcome these limitations, researchers performed laboratory experiments on the Large Plasma Device (LAPD) at University of California, Los Angeles (UCLA) Basic Plasma Science Facility, a national collaborative research facility supported jointly by the Department of Energy and the National Science Foundation. In a laboratory experiment that reproduced the conditions in Earth's magnetosphere, the team launched Alfvén waves down the LAPD chamber and then measured the electrons accelerated by the electric field of the Alfvén waves. Measurements revealed these particular electrons undergo resonant acceleration by the wave's electric field, similar to a surfer catching a wave and being continually accelerated by the wave. This phenomenon of the energy of a wave transferring to accelerated electrons is known as Landau damping. The research team combines the measurements of the wave's electric field and the electrons to generate a unique signature of the electron acceleration by Landau damping. Through simulations and modeling, they showed that the signature of acceleration measured in the experiment agreed with the predicted signature for Landau damping. The agreement of experiment, simulation, and modeling provides the first direct test showing that Alfvén waves can produce accelerated electrons that cause the aurora.

Gregory G. Howes     
University of Iowa
[email protected]

James W. R. Schroeder
Wheaton College, IL
[email protected]

This research was supported by the Department of Energy Office of Science, the National Science Foundation, and the National Aeronautics and Space Administration. The Basic Plasma Science Facility is supported by a Cooperative Agreement through the Department of Energy and a grant through the National Science Foundation.

Schroeder, J. W. R., et al.Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves, Nature Communications 12, 3103 (2021). [DOI: 10.1038/s41467-021-23377-5]

University of Iowa physics department page on aurora research.

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