RELEASE: 00-146

FOUNTAINS OF FIRE ILLUMINATE SOLAR MYSTERY 

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. 

For images and more information on the Internet, visit: 

http://www.gsfc.nasa.gov/GSFC/SpaceSci/sunearth/tracecl.htm