Comapring Earth and the Sun

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    Dolores Beasley

    Headquarters, Washington, DC

    (Phone: 202/358-1753)

    Bill Steigerwald

    Goddard Space Flight Center, Greenbelt, MD

    (Phone: 301/286-5017)

    RELEASE: 00-146

    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 about 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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