The outer parts of the solar corona are millions of degrees hotter than the surface of the Sun – a fact that has puzzled astrophysicists for quite some time (CERN Courier June 2008 p8). Now David Jess of Queen's University Belfast and colleagues have made major progress in unravelling this mystery. Using the 1 m Swedish Solar Telescope on La Palma in the Canary Islands they observed a tiny conglomeration of highly magnetized bright points on the Sun's surface. Their observations show evidence for Alfvén waves, oscillations of magnetized plasma, moving up from the lower solar atmosphere, which may hold the key to the coronal heating.
Hannes Alfvén predicted such waves in his seminal paper of 1942. Their incompressible nature and ability to penetrate the solar atmosphere have made them likely candidates for heating the solar corona. However, Alfvén waves on the Sun had evaded unambiguous observation until now, owing to difficulties in getting clear-enough images of small parts of the Sun using Earth-based telescopes. The long-wavelength plasma oscillations that Jess and colleagues have observed appear to follow magnetic field lines towards the corona. They seem to be capable of carrying enough heat to the corona to explain its high temperature.