The mystery of why temperatures in the Sun's atmosphere exceed those nearer its surface has finally been explained by tiny bursts of heat and energy called nanoflares.

The Sun's outer atmosphere, or corona, is made up of loops of hot gas that arch high above its surface. These loops contain bunches of smaller, individual magnetic strands that reach temperatures of millions of degrees Kelvin, even though the temperatures at the surface are mild in comparison, registering just 5,700 degrees.

The mystery of why the corona is significantly hotter than the surface has long puzzled scientists, but new observations made by Japan's Hinode satellite reveal the culprit as nanoflares, small sudden bursts of energy that occur within the magnetic tubes.

"Coronal loops are the fundamental building blocks of the corona," says James Klimchuk, an astrophysicist at the Goddard Space Flight Center's Solar Physics Laboratory in Greenbelt, who presented the research at the International Astronomical Union General Assembly meeting in Rio de Janeiro last week. "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."

Scientists previously thought steady heating explained the corona's high temperature, but this model requires that a coronal loop of a given length and temperature should have a specific density, whereas the new evidence shows that they have much higher density than the steady heating model predicts. By considering nanoflares as an alternative explanation, the observed density can be explained.

The observations are also supported by theoretical models. "We simulate bursts of heating and predict what the loop should look like when observed with a variety of instruments," says Klimchuk. The team used the NASA-funded X-Ray Telescope (XRT) and Extreme Ultraviolet Imaging Spectrometer (EIS) on Japan's Hinode spacecraft to test their model, which measured ten and five million degree plasma respectively. "These temperatures can only be produced by impulsive energy bursts," confirms Klimchuk.

The ultra-hot plasma cools very quickly, however, with energy being lost from the cooling conducted down to the relatively cool solar surface. There the gas is heated to about one million degrees Kelvin and expands upward to become the one million degree component of the corona that has been observed for many years.

"What we see is one million degree Kelvin plasma that has received its energy from the heat flowing down from the superhot plasma," says Klimchuk. "For the first time, we have detected this ten million degree plasma, which can only be produced by the impulsive energy bursts of nanoflares."

The observations also confirm that nanoflare activity is prevalent across the Sun's active regions. Further detail of nanoflare activity will be revealed by NASA's forthcoming Solar Dynamics Observatory.