Mysterious Pulsating Auroras Caused by Showers of Electrons

Aurora, or northern lights, on display in September 2017. Olivier Morin/AFP/Getty Images

If you've been lucky enough to catch sight of an aurora, it probably looked like a glowing green curtain dancing across the sky. But there's a second, rarer type of aurora—and now, scientists have finally figured out the secret to how they form.

These auroras pulsate, appearing to dim and brighten every five to 40 seconds without moving. The pulsating variety occurs much higher in the atmosphere, making it harder to see, but happens closer to the equator. Normal auroras are trapped around the poles, at about the latitude of Alaska and Scandinavia.

Their secret is a second, equally mysterious phenomenon called chorus waves, according to a new paper published in the journal Nature. Chorus waves are one of a few different types of plasma waves. Plasma is a fancy term for charged particles, like those that constantly flow out from our sun.

When plasma streams enter the Van Allen Belts, doughnut-shaped zones that envelop Earth, they get tangled in the belts' magnetic fields, forming plasma waves. Chorus waves occur when the plasma is fairly warm and electrons are forced toward the side of Earth currently turned away from the sun.

For decades now, scientists have suspected that chorus waves and pulsating auroras might be connected, but they had no way to prove it. That has now changed, thanks to a Japanese spacecraft called the Exploration of Energization and Radiation in Geospace, which launched in December 2016. The spacecraft was designed to track how electrons are born and die in the Van Allen Belts as the sun's plasma beats down on Earth.

Read more: Giant solar storms battered Earth in 1770, causing nearly constant aurora

The spacecraft was armed with a special sensor the team behind the research had designed that can actually distinguish different types of electrons hitting it. That allowed the team to finally connect the chorus waves to the electrons to the pulsating aurora. They found that the connection really does hold up: As chorus waves happen, they create small showers of electrons, which triggered the pulsating aurora.

The researchers behind the paper want to keep studying the phenomenon to better understand the consequences of plasma waves. They also think the stunning phenomenon may not be limited to Earth—it could also cause the light shows spacecraft have captured circling Jupiter and Saturn, which also experience chorus waves.