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Auroras form when electrons in space mount waves in the Earth's magnetic field

By recreating the conditions for an aurora in the lab, the researchers confirmed how these bright, shiny curtains of light form.

The northern lights arise when electrons from space cascade into the upper atmosphere, where they collide with oxygen and nitrogen molecules to paint the sky red and green (SN: 2/7/20). But it was difficult to discern what exactly attracts those electrons to Earth. For decades, scientists have suspected that electrons mount ripples in the Earth's magnetic field, called Alfvén waves, into the atmosphere, like small surfers catching waves offshore. But no satellite directly observed this.

A new experiment that accelerated electrons with Alfvén waves provides the first direct evidence that these perturbations in the Earth's magnetic field can push electrons into the atmosphere to cause auroras, plasma physicist James Schroeder and colleagues reported online on June 7 in Nature Communications.

Ripples called Alfvén waves in the Earth's magnetic field (illustrated as blue doodles) can sweep electrons (yellow) from space into the Earth's atmosphere to illuminate auroras – similar to the way ocean waves push surfers to shore.Austin Montelius / University of Iowa

The team filled a tube 20 feet long and 1 meter wide with a soup of charged particles or plasma. The electric coils created a magnetic field that ran the length of the chamber. An antenna at one end of the instrument generated its own magnetic field, which essentially ripped the magnetic field lines inside the vessel to send Alfvén waves undulating through the plasma.

As expected, electrons in the plasma were dragged into the Alfvén waves and accelerated by the camera. What’s more, “the energy gained per electron per second was similar, in our experiment, to what would be needed in space to create an aurora,” says Schroeder of Wheaton College, Illinois.

These results not only confirm the physics behind atmospheric light shows on Earth, Schroeder says. "What is happening here can most likely be happening on Jupiter or anywhere else, like Saturn, where we see auroras."

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