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Moss on the sun found by NASA's Trace probe
Posted: Dec. 17, 1999

Trace image
Solar image from TRACE. Photo: NASA

A new feature near the surface of the Sun, termed "solar moss" because its weird, sponge-like appearance resembles the plant, has been discovered by astronomers using NASA's Transition Region and Coronal Explorer (TRACE) spacecraft.

"With this discovery, we are beginning to resolve the Sun's mysterious transition region, a thin region in the solar atmosphere where the temperature soars from ten thousand to millions of degrees," said Dr. Thomas Berger of the Lockheed-Martin Solar and Astrophysics Lab (LMSAL), Palo Alto, Calif. "We are excited because this discovery offers us a new way to study the mass and energy flows in this region. It also helps us understand how the large magnetic loops in the Sun's outer atmosphere, the corona, form out of the highly intermittent magnetic fields on the Sun's surface. Studying the solar moss may ultimately shed light on the long-standing problem of how the corona is heated to million-degree temperatures."

Solar moss occurs at the base of certain coronal loops, immense magnetic arches of hot gas that are anchored in the Sun's visible surface and could span several dozen Earths laid end to end. It appears only below high pressure coronal loops in active regions, typically persisting for tens of hours, but has been seen to form rapidly and spread in association with loops that arise after a solar explosion, called a flare.

The moss consists of hot gas at about two million degrees Fahrenheit which emits extreme ultraviolet light observed by the TRACE instrument. It occurs in large patches, about 6,000 - 12,000 miles in extent, and appears between 1,000 - 1,500 miles above the Sun's visible surface, sometimes reaching more than 3,000 miles high. It looks "spongy" because the patches are composed of small bright elements interlaced with dark voids in the TRACE images. These voids are caused by jets of cooler gas from the Sun's lower atmosphere, the chromosphere, which is at about 10,000 degrees Fahrenheit. The bright moss elements move around and can vary in brightness over very short periods of time -- 30 seconds or less.

"The TRACE observations of solar moss show how the transition region is much more complex and dynamic than previously observed," said Dr. Bart De Pontieu of LMSAL. "We are getting a glimpse of how the Sun's magnetic field changes from a chaotic jumble at its visible surface to the well-organized magnetic field present in coronal loops. This transition is complicated by the presence of the dynamic and relatively cold jets from the chromosphere. These jets sometimes interact with and push around the much hotter plasma at the base of the coronal loops."

The coronal loops, at up to nine million degrees Fahrenheit, probably heat the moss by thermal conduction, similar to the way heat moves along an iron bar if one end is placed in a fire. However, there are indications that brief increases in brightness in the moss, corresponding to temporary million-degree temperature rises, may be due to input from local energy sources whose nature is unknown at present. The moss observations are thus helping to locate the as yet elusive energy source that is responsible for heating the Sun's outer atmosphere.

High resolution extreme ultraviolet images from TRACE allowed the solar moss to be observed for the first time, and the rapid cadence with which TRACE takes pictures provided unprecedented details about its behavior.

"However, the use of multiple instruments was key in determining the physical characteristics of the moss," according to Dr. Lyndsay Fletcher of LMSAL. "The Yohkoh/SXT telescope demonstrated the association of moss with the hotter coronal loops, and the Solar and Heliospheric Observatory CDS spectrometer allowed us to determine the moss temperature and density." Additionally, the extremely high resolution chromospheric imaging capability of the Swedish Vacuum Solar Telescope on the island of La Palma, Spain, led to the discovery that dark voids seen in the moss were actually cooler chromospheric gasses interacting with the moss.

TRACE is a NASA mission operated jointly by NASA's Goddard Space Flight Center, the Lockheed Martin Solar and Astrophysics Lab, the Smithsonian Astrophysical Observatory and Montana State University.

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