Nanoflares responsible for superhot corona of the Sun

Published in The Hindu on August 15, 2009

The Sun’s corona is known to be very hot. In fact, it is actually much hotter than the Sun’s surface. But the reason why the corona, the Sun’s outer atmosphere, is extremely hot — several million degrees Kelvin — was not known.

Scientists have apparently found the answer: nanoflares.

The very hot temperatures of the Sun were previously attributed by scientists to a steady-heating phenomenon. According to the steady-heating model, the coronal loop of a given length and temperature should in turn have a specific density. But measurements showed otherwise. The coronal loops have much higher density.

Until now scientists did not find evidence of nanoflares, though the bigger solar flares are well known and studied.

The NASA’s X-Ray Telescope and the Extreme-ultraviolet Imaging Spectrometer, which are on board Japan’s Hinode satellite have provided the first ever proof of nanoflares.

These two instruments were able to measure ultra-hot plasma of 10 million degree K and 5 million degree K respectively, notes NASA

According to James Klimchuk, an astrophysicist at the Goddard Space Flight Center’s Solar Physics Laboratory in Greenbelt Maryland, U.S., the very high temperature measured by the instruments can only be produced by “impulsive energy bursts” — the nanoflares.

Dr. Klimchuk presented his findings recently at the International Astronomical Union general Assembly in Rio de Janeiro, Brazil.

The nanoflares begin to heat the coronal loop in a very strange way. And wherever the density of the loop is low, the brightness is less and vice versa. About one million degree K is sufficient enough to make the corona bright.

According to a theoretical model used by Dr. Klimchuk and his team, the coronal loop is a collection of 5 million -10 million degree K high-density low-brightness strands and one million degree K low-density but brighter strands.

“For the first time we have detected this 10 million degree K plasma, which can only be produced by the impulsive energy burst of nanoflares,” Dr. Klimchuk was quoted as saying in the NASA release.

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