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Out of the shadows: an illuminating eclipse.

From the mountaintop vantage point of Mauna Kea, Hawaii, some of the world's largest and most powerful telescopes witnessed July 11's total eclipse of the sun. Using infrared, visible-light and radio-wave detectors, scientists took advantage of the event to study features of the solar atmosphere that show up clearly only when the moon blocks the brilliant light of the solar disk.

While the results remain preliminary, they have already yielded several new insights.

One set of Mauna Kea observations, made with a spectrograph attached to NASA's infrared telescope, pinpointed the location of a key spectral line of magnesium at one edge of the sun. Scientists use this infrared emission line, which splits into three components in the presence of a strong magnetic field, to gauge magnetic activity in the solar atmosphere. But uncertainties regarding the line's exact altitutde have limited its usefulness, notes Donald Jennings of NASA's Goddard Space Flight Center in Greenbelt, Md. Ever since asttronomers discovered the line in the early 1980s, they have debated whether it lies in the upper photosphere--a region at or just above the visible surface of the solar disk--or in a region just beyond the photosphere, called the lower chromosphere.

Two weeks ago, as the moon swept across the face of the sun like a giant shutter, Jennings and his colleagues measured the time interval between the moment when the moon first blocked the solar disk and when it later obscured the magnesium line as it moved across the sun. They then used the known velocity of the moon to calculate the altitude of the magnesium emission. The team, headed by Drake Deming of Goddard, discovered that the bulk of the emission occurs in the upper photosphere, just a few hundred kilometers above the solar surface, Jennings told SCIENCE NEWS. The finding provides a reference point that will help scientists "stitch together" the magnetic activity on the sun's surface with that in the solar atmosphere.

Other research teams at Mauna Kea examined solar features with wide-field infrared cameras, capturing the first infrared pictures of the sun during an eclipse. Astronomers had hoped the images would reveal whether a ring of dust or rocky material circles the sun thousands of kilometers above the solar surface. Such a ring might arise from dust shed by comets passing near the sun or from rocky material left over from the solar system's formation, says Eric Tollestrup of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Tje preliminary images show no evidence of a ring, but this doesn't necessarily rule out its existence. The brightness of the corona--the outermost layer of the solar atmosphere -- may have obscured the relatively dim glow of sun warmed rocks or vaporized dust, Tollestrup says. If so, further processing to subtract the coronal background from the images might yet reveal the ring.

The preliminary pictures do show other key features of the solar atmosphere. Tollestrup says. These include bulges in the region surrounding the eclipsed solar disk, which depict long extensions of the corona called streamers; and white areas that indicate solar prominences -- elongated gas clouds held intact by magnetic fields.

Some Mauna Kea observers are collaborating with researchers who imaged the sun at the same time from other sites. In one such project, astronomers plan to match visible-light features in the solar atmosphere observed from Hawaii with X-ray images taken simultaneously above White Sands, N.M., where the eclipse had not yet occurred. Comparing the X-ray images of the full solar disk with the eclipse photographs of the corona should help astronomers trace the path of magnetic field lines from the surface of the sun to its outer atmosphere, explains Leon Golub of the Harvard-Smithsonian Center for Astrophysics, who made the White Sands observations with colleagues from IBM in Yorktown Heights, N.Y. The X-ray images themselves, he says, may feature the highest resolution of any such photos of the sun (SN: 9/30/89, p.233). The bright areas of these pictures reveal elongated, magnetically confined regions of hot plasma -- structures that did not show up clearly in previous X-ray images, Golub says.
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Title Annotation:solar atmosphere studied during the July 11, 1991 total eclipse
Author:Cowen, Ron
Publication:Science News
Date:Jul 27, 1991
Words:682
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