Small, sudden bursts of heat and energy, called nanoflares, are
responsible for the million-degree temperature of the sun's tenuous atmosphere,
a new study reveals.
The mystery of why temperatures in the sun's outer atmosphere,
or corona, soar to several million degrees Kelvin (K) — much hotter than
temperatures nearer the sun's
surface — has puzzled scientists for decades.
"Why is the sun's corona so darned hot?" said
study member James Klimchuk of NASA's Goddard Space Flight Center in Greenbelt, Md.
To answer this question, Klimchuk and colleagues constructed
a theoretical model of the nanoflares, which are components of the loops of hot
gas that arch high above the solar surface to make up the corona.
"Coronal loops are the fundamental building blocks of
the corona," Klilmchuk said. "Their shape is defined by the magnetic
field, which guides the hot flowing gases called plasma."
These loops are made up of bundles of smaller, individual
magnetic tubes or strands that can have temperatures reaching several million
degrees Kelvin (K), even though the sun's surface is only 5,700 degrees K
(9,800 Fahrenheit). (One million degrees K would be about 1.8 million degrees
Fahrenheit.)
Nanoflares are small, sudden bursts of energy that happen
within these thin magnetic tubes in the corona.
Unlike the bigger solar
flares, which can be viewed through satellites and ground-based telescopes
and can disrupt electronics and communications networks on Earth, nanoflares
are so small that they cannot be resolved individually, so until now, no direct
evidence of nanoflares was seen. Only see the combined effect of many of them
occurring at about the same time is visible.
Klimchuk's model tries to pin down exactly what happens when
these nanoflares erupt.
"We simulate bursts of heating and predict what the
loop should look like when observed with a variety of instruments,"
Klimchuk said.
To test their model, the team observed gas emissions in the
solar corona using the NASA-funded X-Ray Telescope and Extreme Ultraviolet
Imaging Spectrometer on Japan's Hinode
spacecraft.
"The 10 million degree temperatures we detected in the
corona can only be produced by the impulsive energy bursts," Klimchuk
said.
The ultra-hot plasma cools very quickly, however, which
explains why it is so faint and has been so difficult to detect until now.
The energy lost from the cooling conducts down to the comparatively
cooler solar surface. The gas there at the surface is heated to about 1 million
degrees K and expands upward to become the 1 million degree component of the
corona that has been observed for many years.
Klimchuk presented the findings on August 6 at the International
Astronomical Union General Assembly meeting in Rio de Janeiro, Brazil.
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