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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"

"Leon makes the most fantastically detailed x-ray pictures of the corona," said Pasachoff. "On his images, you can see loops of gas the size of the Earth. In our experiments, in Africa, we are studying how those loops shake, and whether that shaking heats the corona to millions of degrees."

Golub and Pasachoff are co-authors of a new book about the sun: "Nearest Star: The Exciting Science of Our Sun," which has just been published by Harvard University Press.

At Pasachoff's site in Lusaka, Zambia, the total eclipse will last 3 minutes, 14 seconds. This period of total coverage of the sun by the moon will begin at about 9:09 U.S. Eastern Time (the local time in Zambia is six hours ahead of this.)

Pasachoff's team is collaborating with scientists from Zambia, England, Slovakia, Indonesia, and India—and with the students from Williams College. "It's fun to be here with so many students," he said. "It is exciting for them and for me to have a chance to work in a foreign place and to anticipate the drama of the eclipse."

Two of the Williams College experiments in Zambia deal with the still-open question of how the corona can reach a temperature of millions of degrees, even though the everyday surface of the sun below it is only 6,000 degrees Celsius (about 11,000 degrees Fahrenheit).

Part of these experiments and a third experiment are being done in liaison with scientists conducting experiments on the Solar and Heliospheric Observatory (SOHO) spacecraft and the Transition Region and Coronal Explorer (TRACE) spacecraft.

Targeting the Corona

The Williams College team's first experiment is a search for rapid vibrations in the corona. A theory holds that extreme coronal heating is caused by loops of gas held in place by the sun's magnetic field. The question is whether the vibrations bring enough energy into the corona to heat it sufficiently.

The second experiment involves making a map of the temperature and polarization of the corona, using a technique of comparing electronic images of the corona taken at special ultraviolet wavelengths. Following theoretical work, these wavelengths are chosen to include two at which the difference between the shape of the everyday sun's spectrum and the corona's spectrum is especially striking.

The third experiment is to image the solar corona during the eclipse to compare with observations of the corona seen with the Extreme-ultraviolet Imaging Telescope (EIT) on board the SOHO, in collaboration with scientists at NASA's Goddard Space Flight Center and from the U.S. Naval Research Laboratories.

"We are all geared up and ready to go," said Pasachoff. "Our two tons of equipment arrived safely, and we have spent two weeks testing it and making sure it tracks the sun exactly across the sky. Everything is working well, and we hope for continued clear skies on Thursday afternoon."

Pasachoff filed a dispatch after his team observed the eclipse in Zambia on June 21: The weather was clear and the team gathered a lot of data. It was, he said, a "Spectacular Show." Go>>

Watch tonight's broadcast of National Geographic Today on cable television in the United States at 7 p.m. ET/PT for video of the eclipse.


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