The Puzzle of the Sun's Coronal Heating
A stalk-like prominence rises up above the Sun, then split into roughly four strands that twisted themselves into a knot. This image was taken by the Solar Dynamics Observatory on July 12, 2011. A new paper shows for the first time that coronal loops are comprised of many small, thin strands.
The corona of the sun is the hot (over a million kelvin), gaseous outer region of its atmosphere. The corona is threaded by intense magnetic fields that extend upwards from the surface in loops that are twisted and sheared by the convective stirrings of the underlying dense atmosphere. Understanding the corona and its physical processes is essential to the development of a long-range space weather prediction capability.
The mechanisms that heat the corona are poorly understood. Each magnetic loop is actually a tube containing hot gas, and one theory posits that the enclosed gas is heated by short pulses of energy - so-called nanoflares - generated by the way the field twists and braids around the loop and its connections to the turbulent gas motions in the stellar photosphere. It has never been possible to test this theory, however, in part because (it is thought) each of the nanoflares heats only a small, thin strand of the whole magnetic field loop - and a strand is too small to see clearly.
The Solar Dynamics Observatory (SDO) was launched in February of 2010 with a high spatial resolution ultraviolet camera, the Atmospheric Imaging Assembly (AIA); SAO was a major partner in building AIA. Paola Testa, Ed DeLuca and Leon Golub, together with three colleagues, used AIA to show for the first time that hot coronal loops do indeed contain very fine substructures, consistent with the nanoflaring hypothesis, that can heat the gas in the loops. This initial result still awaits more testing; time variability, for example, will be discussed in future papers. But this result helps to resolve an important outstanding puzzle about the heating of the solar corona.