Why Sun's Atmosphere Is 'So Darned Hot'

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. link...