Why is the sun hotter outside than inside?

The mystery of why temperatures in the sun's outer atmosphere soar to several million degrees, far hotter than temperatures near the sun's surface, has been solved. New observations made with instruments aboard Japan's Hinode satellite reveal the hotter outer atmosphere is due to nanoflares. Nanoflares are small, sudden bursts of heat and energy. 'They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop,' said James Klimchuk, an astrophysicist at the Goddard Space Flight Centre's Solar Physics Lab in Maryland. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun's corona. Scientists previously thought steady heating explained the corona's million degree temperatures. Observations from the NASA-funded X-Ray Telescope (XRT) and Extreme-ultraviolet Imaging Spectrometer (EIS) instruments aboard Hinode reveal that ultra-hot plasma is widespread in solar active regions. The XRT measured plasma at 10 million degrees Kelvin, and the EIS measured plasma at five million degrees Kelvin; 273 degrees Kelvin equal zero degree Celsius. 'These temperatures can only be produced by impulsive energy bursts,' said Klimchuk, who presented the findings at the International Astronomical Union General Assembly meeting in Rio de Janeiro, Brazil.