(September 26, 2000)
Giant fountains of fast-moving, multimillion-degree gas in the outermost atmosphere of the Sun have revealed an important clue to a long-standing mystery -- the location of the heating mechanism that makes the corona 300 times hotter than the Sun's visible surface.
Scientists discovered an important clue while observing immense coils of hot, electrified gas, known as coronal loops. These fiery, arching fountains now appear in unprecedented detail with NASA's Transition Region and Coronal Explorer (TRACE) spacecraft.
Scientists are interested in the corona, which appears as a halo of light seen by the unaided eye during a total solar eclipse, because eruptive events in this region can disrupt high-technology systems on Earth. Astronomers also hope to use the solar corona studies to better understand other stars.
"The mysterious energy source that makes the Sun's atmosphere so incredibly hot has been an enigma for more than 70 years, and before we discover what it is, we needed to learn where it is," said Dr. Markus Aschwanden of the Lockheed-Martin Solar and Astrophysics Laboratory (LMSAL), in Palo Alto, CA.
Aschwanden is lead author of a paper describing this research to be published in the Astrophysical Journal. "Locating the source of coronal heating is a key piece of this puzzle, and we are excited that solar observatories like TRACE are allowing us to resolve the hidden events occurring in the atmospheres of stars."
The new observations reveal the location of the unidentified energy source, showing that most of the heating occurs low in the corona, within about 10,000 miles from the Sun's visible surface. The gas fountains form arches, hundreds of thousands of miles high, capable of surrounding 30 Earths. As gas emerges from the solar surface, it's heated and rises, then cools and crashes back to the surface at more than 60 miles per second.
Millions of different-sized coronal loops comprise the corona, and a 30-year-old theory assumes the loops are heated evenly throughout their height. The TRACE observations show that instead, most of the heating must occur at the base of the loops, near where they emerge from and return to the solar surface.
The old theory of uniform heating predicted that the loops would be substantially hotter at their tops because gas at the top of the loops is thinner, and does not radiate heat away as efficiently as the dense gas near the bottom. If the loop were heated evenly over its entire height, the top, which can't lose heat as well, would become hotter than the rest. Earlier, less- detailed observations of the coronal loops could not confirm nor invalidate the uniform heating theory because they could not reveal that the loop tops were really about the same temperature as the bases.
However, the high-resolution TRACE pictures show that, just as a thick piece of rope consists of many thin fibers, what was thought to be one coronal loop is actually a bundle of thin, individual loops. Although some thin loops in the bundle are hotter than other spirals, precise measurements by TRACE show that, over its height, each separate, thin loop varies much less than the uniform heating theory predicts.
"Since a loop loses heat most rapidly from its bases, most of the heat must also be going in at the bases for the loop to be at a uniform temperature," said Dr. Karel Schrijver, a member of the research team, also of LMSAL. "If this were not so, the lower parts would have been much cooler than the tops, which do not lose heat as quickly."
NASA Administrator Daniel S. Goldin unveiled the new TRACE images today, along with Ellen Futter, president of the American Museum of Natural History, at the museum's Rose Center for Earth and Space, New York.
TRACE, launched in April 1998, is training its powerful telescope on the "transition region" of the Sun's atmosphere, a dynamic region between the relatively cool surface and lower atmosphere regions of the Sun, about 10,000 degrees Fahrenheit, and the extremely hot upper atmosphere, which burns up to 3 million degrees Fahrenheit.
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