Total Eclipse May Help Solve Mystery of Sun's "Halo"

News Flash (6/21/01): Eclipse a "Spectacular Show"

David Braun
National Geographic News
June 20, 2001

The sun's surface temperature is about 6,000 degrees Celsius (11,000 degrees Fahrenheit). So how is it that the corona, the outermost layer of the sun's atmosphere, can reach 2 million degrees (4 million degrees Fahrenheit)?

Jay Pasachoff, in Africa on a National Geographic expedition to study this Thursday's total solar eclipse, explains, "The sun is hot, but it isn't a stove. If you go away from a stove, you feel cooler. But if you go up from the sun's surface, the temperature rises."

Pasachoff, an astronomer from Williams College in Massachusetts, is in Africa this week with a dozen students from the college, to try to figure out why that happens.

He and other scientists have swarmed to Africa to observe the eclipse—and the rare opportunity to see the corona and learn more about what causes it to heat up so dramatically.

Observations are possible from Earth only during the brief minutes of a total solar eclipse, when the everyday sun is hidden by the moon, allowing the faint corona to be observed from Earth. The corona is usually hidden by the blue sky, because it is about a million times fainter than the layer of the sun seen shining every day, the photosphere.

"We are incredibly lucky on Earth to have the celestial coincidence that the sun and moon appear to be the same size in the sky," Pasachoff said. "Actually, the sun is 400 times bigger than the moon, but it is also 400 times farther away. Nowhere else in the solar system would we be able to see such beautiful solar eclipses."

The ground observations in Zambia are funded in part by the U.S. National Science Foundation and the National Geographic Society's Committee for Research and Exploration. They will be augmented by observations from spacecraft and by high-resolution x-ray images made from a rocket that will be launched from New Mexico a few hours after the eclipse, as soon as it becomes daylight in the United States.

Leon Golub of the Harvard-Smithsonian Center for Astrophysics will launch the rocket from White Sands, New Mexico, to make very detailed x-ray images of the solar corona.

"We're flying a sounding rocket, which goes up above the atmosphere to take a 'sounding,' but then falls back to Earth," said Golub. "The rocket will carry our new XUV imaging telescope above the atmosphere for five minutes of solar observing, about the same length of time as a total eclipse."

Golub said a new feature of the telescope for this experiment is that, in addition to the ability to see the x-ray corona at high resolution, scientists will also be able to adjust the extreme ultraviolet wavelength in-flight. "This allows us to see the different components of the corona, which are at different (high) temperatures," said Golub.

Earth-Size "Loops of Gas"

Continued on Next Page >>


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