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Warm Coronal Loops Offer Clue to Mysteriously Hot Solar Atmosphere
Coronal loops observed by NASA�s TRACE spacecraft.
Coronal loops observed by NASA’s Transition Region And Coronal Explorer (TRACE) spacecraft. Credit: NASA/TRACE
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Unlocking the mystery of the processes responsible for heating the sun’s outer atmosphere, called the corona, to million degree temperatures is no easy task. Scientists at NASA reveal a new understanding of the coronal heating enigma.

The corona is made up of loops of hot gas that arch high above the sun’s surface. These loops can have a wide range of temperatures, many reaching several million degrees Kelvin.

Warm coronal loops, those of intermediate temperature, have proven the most difficult to explain. Small, sudden bursts of energy bursts called nanoflares seem to be the key.

“Nanoflares can release their energy in different ways, including the acceleration of particles, and we now understand that the right mix of particle acceleration and direct heating is one way to explain the observations,” says Dr. James Klimchuk, an astrophysicist at NASA Goddard Space Flight Center’s Solar Physics Laboratory in Greenbelt, Md.

Another possibility is that energy release happens very gradually, but very close to the sun’s surface. In this case, a phenomenon called thermal nonequilibrium causes the loops to go through periodic fits of dynamic behavior. The latest computer simulations suggest that these solar temper tantrums may also be able to explain the observations associated with coronal heating.

X rays and ultraviolet (UV) radiation from the solar corona affect Earth and its atmosphere. As Earth’s atmosphere heats up, it expands and becomes denser at high altitudes. When this happens, satellites experience more drag, which changes their orbits. Accurately predicting this “space weather” gives satellite operators more time to respond to or avert problems that could potentially cause interruptions and outages.

In order for scientists to build realistic models of the corona, they must understand the root cause of the X-ray and UV radiation affecting Earth. And the source of the radiation is coronal heating.

The team’s findings suggest two possible explanations for the heating of warm coronal loops. “We have now discovered two possible solutions to this dilemma: energy is released in small, sudden bursts with the right mix of particle acceleration and direct heating, or energy is released gradually very close to the solar surface,” says Klimchuk.

A comparison of numerical simulations with imaging and spectral data from NASA missions led to the team’s findings, which will be presented on May 29 at the American Geophysical Union conference in Ft. Lauderdale, Fl.

Laura Layton
NASA's Goddard Space Flight Center