Coronal Holes: The (Rarely Round) Gaps In The Sun's Atmosphere

By Joe Palca

Published September 5, 2013 at 2:38 PM CDT

NASA's Solar Dynamics Observatory captured this picture of the sun on June 18. The dark blue area in the upper left quadrant of the sun is a huge coronal hole more than 400,000 miles across. Coronal holes are areas of the sun's outermost atmospheric layer — the corona — where the magnetic field opens up and solar material quickly flows out.

There's a hole in the sun's corona. But don't worry — that happens from time to time.

"A coronal hole is just a big, dark blotch that we see on the sun in our images," says Dean Pesnell, project scientist for NASA's Solar Dynamics Observatory. "We can only see them from space, because when we look at them [through] a regular telescope, they don't appear."

That's because you have to look at wavelengths of light that the human eye can't see. As the name suggests, coronal holes are holes in the sun's corona, not the sun itself. The corona is a hot and glowing outer layer of atmosphere that surrounds the sun. It extends millions of miles into space.

Pesnell says scientists aren't really sure where coronal holes come from. "Some people claim that they are the skeleton of old sunspots," he says.

Sunspots are also dark blotches, but they're on the surface of the sun, below the corona. They're caused by strong magnetic fields that cause cool regions — which look dark — to form on the sun's surface. The idea is that when sunspots fade away, they leave behind a coronal hole.

One thing scientists do know for sure about coronal holes is that they're not round. "They have all kinds of cool shapes," Pesnell says. "We've seen [some] that look like rubber chickens. My favorite is the one that looks like Kokopelli, a flute player from Native American cultures in the Southwestern U.S."