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MELISSA BLOCK, HOST:

This summer, to fill small holes in our program, NPR science correspondent Joe Palca has been doing a series of short stories about holes. Well, Dean Pesnell heard one of them and wrote to Joe, suggesting he do a story on coronal holes. Pesnell is project scientist for NASA's Solar Dynamics Observatory. That's a spacecraft that studies the sun. Joe said, sure, why not? So he called Pesnell, and asked a crucial question.

JOE PALCA, BYLINE: What is a coronal hole?

DEAN PESNELL: Well, a coronal hole is just a big, dark blotch that we see on the sun in our images. We can only see them from space because when we look in a regular telescope, they simply don't appear.

PALCA: That's because you have to look at wavelengths of light that the human eye can't see. Now, as the name suggests, coronal holes are holes in the sun's corona, not the sun itself. The corona is a hot glowing layer that surrounds the sun and extends millions of miles into space. Pesnell says scientists aren't really sure where coronal holes come from.

PESNELL: Some people claim that they are the skeleton of old sun spots.

PALCA: 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 to form on the sun's surface that look dark. The idea is that when sunspots fade away, they leave behind a coronal hole - at least that's one idea.

One thing scientists do know for sure about coronal holes, they're not round.

PESNELL: Oh no. They have all kinds of cool shapes. We've seen them that look like rubber chickens.

(LAUGHTER)

PESNELL: We've seen them - my favorite is the one that looks like Kokopelli, a flute player from New Mexican Indians.

PALCA: I have to ask, is this your favorite kind of hole?

PESNELL: Well, I guess my - this is my favorite hole at this point in my life.

BLOCK: Coronal holes brought to us by NPR's Joe Palca and Dean Pesnell of NASA. To see pictures of coronal holes, you can go to our website, NPR.org.

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