Heating Coronal Loops
http://antwrp.gsfc.nasa.gov/apod/ap050814.html
http://antwrp.gsfc.nasa.gov/apod/ima...loop_trace.jpg

Explanation: Why is the corona of the Sun so hot? Extending above the
photosphere or visible surface of the Sun, the faint, tenuous solar
corona can't be easily seen from Earth, but it is measured to be
hundreds of times hotter than the photosphere itself. Astronomers have
long sought the source of the corona's heat in magnetic fields which
loft monstrous loops of solar plasma above the photosphere. Detailed
observations of coronal loops from the orbiting TRACE satellite are
pointing more closely to the unidentified energy source. Recorded in
extreme ultraviolet light, this and other TRACE images indicate that
significant heating occurs low in the corona, near the bases of the
loops as they emerge from and return to the solar surface. This
tantalizing TRACE image shows clusters of the majestic, hot coronal
loops which span 30 or more times the diameter of planet Earth.

See: http://antwrp.gsfc.nasa.gov/apod/ap050814.html