Steven Montross, age 9, of Sioux City, Iowa, for his question:
What is the solar corona?
A few weeks ago, Andy's young readers just said hello to the solar corona. Lots of readers asked for more about it. The solar corona deserves a story of its own and Andy is eager to tell you what he knows about it.
Scientists learned many new things about the solar corona only a few years ago. This was at the very beginning of the Space Age, when they started sending up manmade satellites to get some closer information about the sun. The word "solar," as you well know, means something, that belongs to the sun or something to do with the sun. "Corona" is created from an older word for "crown." The dazzling round face of the sun sits in the midst of a pale hazy crown, like a halo. At least, this is what it looked like to astronomers of long ago.
Naturally, these sensible people did not stare straight at the sun to see its corona. Its dazzling face would have damaged their eyesight and most likely blinded them. But there are times when the brilliant face of the daytime sun is blotted out of sight completely by the solid, round moon. This happens during, a total solar eclipse. For a few minutes, the dark moon gets between us and the sun and exactly fits over its face. But it does not cover the filmy halo around the sun. In early days, astronomers had to wait for a total eclipse to study this solar corona.
At last they invented a special gadget to fit over the sun's glaring face so that they could study the corona whenever they wanted. It is called the "coronagraph" and it has helped a lot. Astronomers learned that the thin, pearly crown was a part of the sun's atmosphere. It was the top layer of gases that fold around the sun, somewhat as the airy layers of our atmosphere enfold the earth. Its streamers reach millions of miles out into space. From side to side, it may be 30 times wider than the sun.
When the Space Age began, astronomers thought this hazy halo was rather cool, at least cooler than the seething face of the sun. The orbiting satellites, however, had special gadgets to take its accurate temperature. And what they reported was astonishing. It was hard to believe that the temperature of the solar corona is more than a million degrees. This is more than 10 times hotter than the seething face of the sun. Other satellites were sent aloft to check the first report and they gave the same answer. They also sent back pictures and a lot of other information about what goes on in the corona. Some of the details do not seem to make sense and we are waiting for the experts to figure them out.
We know that the gases of the corona are very, very thin. They are so filmy that the brilliant sun out dazzles its pale, hazy halo. This is why we see it only when the moon hides the sun. We know that gaseous atoms of iron, nickel and calcium in the corona are highly charged with electricity. This may be caused by the sun's blazing radiation. But at present, no one knows why the solar corona is so hot and no one can explain how it became hotter than the seething sun itself.