Mystery of Sun’s corona solved

Solar scientists have solved a riddle that has puzzled astronomers for decades – why the Sun’s outer atmosphere is millions of degrees hotter than its surface.

This outer atmosphere, called the corona, is formed of loops of hot gas arching thousands of miles high. It is so delicate that from Earth, it can only be seen during a total solar eclipse.

Now physicists at NASA have confirmed that tiny and sudden bursts of heat and energy, called nanoflares, are responsible for making temperatures soar within these coronal loops.

The loops are made up of bundles of smaller, individual magnetic tubes or strands that can reach temperatures of several million degrees Kelvin. However, the sun’s surface, or photosphere, is “only” 5,700 degrees K.

Scientists at NASA’s Goddard Space Flight Center, at Greenbelt, Maryland, found the energetic nanoflares were occurring within these thin tubes. They had evaded discovery before because, unlike solar flares, which can be seen by satellites and telescopes on Earth, they are too small to be resolved individually. Astronomers see only their combined effect when many occur at about the same time.

James Klimchuk, an astrophysicist at Goddard, announced the breakthrough at the International Astronomical Union’s General Assembly in Brazil. He said: “Coronal loops are the fundamental building blocks of the corona. Their shape is defined by the magnetic field, which guides the hot flowing gases called plasma.

“The magnetic field is also the source of the nanoflare energy. We believe that stresses in the field are released when thin sheets of electric current become unstable.”

Klimchuk and his colleagues created a simulation to see how nanoflares might occur and how plasma within the tubes would respond to skyrocketing temperatures. They then tested their model by observing real gas emissions in the solar corona using a NASA-funded instrument called the X-Ray Telescope and Extreme Ultraviolet Imaging Spectrometer on Japan’s Hinode spacecraft.

Klimchuk said: “The 10 million degree temperatures we detected in the corona can only be produced by the impulsive energy bursts. 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 relatively cold solar surface. The gas there 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.”

In March, Skymania News told how a team led by scientists from Belfast, Northern Ireland, had discovered giant, twisting waves of magnetism spreading upwards in the solar corona. Medical-type X-ray scans had earlier been used to map this outer atmosphere.

Now the scientists are looking forward to unlocking more secrets about the corona with NASA’s upcoming mission to study the Sun, the Solar Dynamics Observatory, due for launch from Florida in November. Like Hinode and the twin Stereo satellites it will tell us more about our fascinating home star.

Picture: The sun’s ghostly corona as seen during a total solar eclipse. (Credit: NASA/JPL-Caltech).

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