An unmanned space craft has shed new light on one of our Sun's enduring mysteries: why its outer atmosphere – or corona – is much hotter than the surface of the star itself. The first results from Japan’s Hinode mission point to a special kind of magnetic wave as the main mechanism in heating the corona. Data from Hinode show that the corona is swarming with these "Alfvén waves", which could also be responsible for the solar wind – the origin of which has been another long-standing mystery.

The corona is a region of ionized gas – or plasma – that extends millions of kilometres from the surface of the Sun. Physicists have known for nearly 70 years that it has a temperature of several million Kelvin, while the solar surface is a relatively mild 6000 K.

Although there is no shortage of energy in the solar interior to heat the corona to such high temperatures – only about 0.01% of the total solar output is needed – the mechanism by which energy is transferred from the interior of Sun and to the corona has eluded physicists. A related mystery is the origin of the solar wind, which is a stream of charged particles that flows at very high velocity out of the open parts of the corona into interplanetary space. One of the prime candidates for coronal heating and the solar wind are Alfvén waves – torsional transverse magnetic oscillations that are believed to propagate at very high speeds along the magnetic field lines that run out of the surface of the Sun and into the corona. However, these waves have proved very difficult to see.

Now, in this week's issue of Science, Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and colleagues in the US, Norway and Japan have shown that the surface of the Sun is apparently swarming with Alfvén waves, making it even more likely that they are responsible for heating the corona. Using data acquired by Hinode’s Solar Optical Telescope (SOT), they also concluded that Alfvén waves are responsible for accelerating the solar wind to hundreds of kilometres per second.

Wiggling spicules

The team focussed the SOT on the Sun’s chromosphere – the relatively thin region between the surface and the corona -- where they observed thin, short-lived jets of hot gas, known as "spicules", that shoot out of the chromosphere and into the corona at over 100,000 km/h. “Our observations show that many of these jets wiggle sideways while they form”, De Pontieu told physicsworld.com. According to De Pontieu, these wiggles are caused by the transverse motion of the magnetic field – which occurs in Alfvén waves.

The team came to this conclusion by performing advanced computer simulations of the surface and atmosphere of the Sun, which produced waves similar to those that were wiggling the spicules. By comparing the simulations to the Hinode observations the team were able to conclude that the waves were Alfvén waves.

De Pontieu and colleagues were also able to make direct observations of the amplitudes of the Alfvén waves. Then by using computer simulations of how this wave energy leaks into the corona, they concluded that it is sufficient to power the solar wind.

The team is less sure whether the Alfvén waves are sufficient to heat the corona to its very high temperature because, according to De Pontieu, their current models of process are not sufficiently detailed to allow them to draw this conclusion.

Polar prominences

In the same issue of Science, Takenori Okamoto of Japan’s National Astronomical Observatory and colleagues in Japan and the US report the first ever evidence for Alfvén waves in solar prominences – large structures of relatively cool plasma that form in the corona. These prominences were known to contain thread-like features that support the continuous flow of material. Using the SOT, Takenori and colleagues were able to observe oscillations in these threads and conclude that they are consistent with Alfvén waves propagating along the threads. The team also concluded that such Alfvén waves could be responsible for heating the corona.

Meanwhile, Jonathan Cirtain of the Harvard-Smithsonian Center for Astrophysics and colleagues in the US and Japan have published the first evidence of Alfvén waves in X-ray jets -- fast-moving eruptions of hot plasma that occur near the solar poles. The team studied thousand of jets and found that many of them moved at about 3 million km/h – which is the speed at which Alfvén waves are believed to propagate. The team concludes from this that Alfvén waves are responsible for high-speed elements of the solar wind.

Robertus Erdélyi of the UK’s University of Sheffield told physicsworld.com that the Alfvén waves seen by De Pontieu and colleagues are important because they are a very plausible mechanism for transferring huge amounts of energy into the corona. However, Erdélyi cautions that Hinode is only capable of gathering 2D images of the corona, whereas proving that the oscillations are Alfvén waves – rather than other magnetic waves called “kink” waves – requires either 3D images or spectroscopic observations are needed. He therefore believes that the results need to be verified using, for example, using data from NASA’s STEREO mission, which involves two spacecraft that work together to obtain 3D images of the sun.