The spectacularly colorful aurora borealis — or northern lights — that fills the sky in high-latitude regions has fascinated people for thousands of years. Now, a team of scientists has resolved one of the final mysteries surrounding its origin.

Scientists know that electrons and other energized particles that emanate from the sun as part of the «solar wind» speed down Earth’s magnetic field lines and into the upper atmosphere, where they collide with oxygen and nitrogen molecules, kicking them into an excited state. These molecules then relax by emitting light, producing the beautiful green and red hues of the aurora.

What has not been well understood is precisely how groups of electrons accelerate through the magnetic field on the last leg of their journey, reaching speeds of up to 45 million mph. In a study published today in the journal Nature Communications, that question is answered by physicists from UCLA, Wheaton College, the University of Iowa and the Space Science Institute.

A popular theory has been that electrons hitch a ride on Alfven waves — a type of electromagnetic wave that spacecraft have frequently identified traveling Earthward along magnetic field lines above auroras. While space-based research has provided strong support for the theory, limitations inherent to spacecraft measurements have prevented a definitive test.

To overcome these limitations, the physicists conducted laboratory experiments on the Large Plasma Device at UCLA’s Basic Plasma Science Facility, a national collaborative research site supported jointly by the U.S. Department of Energy and National Science Foundation.

After reproducing conditions that mimicked those in Earth’s auroral magnetosphere, the team used specially designed instruments to launch Alfven waves down the plasma device’s 20-meter-long chamber. Because Alfven waves are thought to collect only a small portion of electrons in the plasma of space, the physicists focused on determining whether there were electrons that appeared to be traveling at a rate comparable to the electric field of the waves.

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