Double rainbows are one thing, but a double aurora（极光 )? They can happen, though some kinds are rare. One particular red-and-green aurora is especially confusing. Now, images taken by amateur astronomers may help reveal its secrets.
Citizen scientists living in Canada and Finland used cameras to document an unusual combination of auroras in the night sky. Physicists have studied those images to learn what may have caused the unusual light show.
Alan Dyer was outside his home in Strathmore, Canada, when the dancing Northern Lights caught his attention. He grabbed his camera and started filming. "I knew I had something interesting," says Dyer. At first glance, the double aurora looks like a floating slice of watermelon. The green aurora is well-understood. The fruity looking red st rip is more mysterious. Scientists have known about these red auroras for decades. But there's no agreement on how they form. One idea is that Earth's magnetic field can heat up certain areas of the atmosphere. That heat could knock particles( 粒子） around, like in proton（质子） rain.
Researchers had seen these two types of auroras occur together before. But it was always mysterious, says Toshi Nishimura, a space physicist at Boston University. After looking at satellite observations along with the images taken by Dyer and others, Nishimura and his team concluded how the two phenomena might be related. The key is the thin rays in the red aurora. They mark the paths of electrons falling through the Earth's magnetic field. That means electron rain appears to cause the red aurora, just as proton rain causes the green one. Both get powered by the solar wind. But the electrons carry less energy than the protons, so they make for a more reddish color.
Electron rain might not be the only way to produce the stable red auroras, though. The results show that what's going on is more complicated than researchers had thought. The auroras Dyer saw could be danger zones for radio communication and GPS systems, says Nishimura.
The citizen scientists who took the photos were a critical part of the new findings, Nishimura says.
