The Sun’s eleven-year cycle of magnetic activity is well known, but scientists are unsure what drives this magnetic dynamo. Now the results of a six-year study of the propagation of sound waves in the Sun have provided some clues. Michael Thompson of Imperial College in the UK and colleagues have found that flow patterns on the surface of the Sun run much deeper than previously thought – and that this could shed light on the processes that give rise to the magnetic behaviour of our local star (S Vorontsov et al 2002 Science 296 101).
Scientists have long known that the Sun’s surface rotates faster near the equator, where it rotates about once every 25 days, than it does near the poles, where it rotates about once every 33 days. Previous studies have also shown that the higher-latitude bands of rotation drift towards the poles, while the lower-latitude bands drift towards the equator, as the eleven-year cycle proceeds. These flow patterns could therefore shed light on the processes that give rise to the Sun’s magnetic cycle.
Thompson and co-workers used the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory (SOHO) satellite to measure oscillations of sound waves inside the Sun. This technique is known as helioseismology and provides a picture of the internal structure of the Sun. By analysing measurements taken every five minutes between May 1996 and January 2002, Thompson’s team was able to build up a picture of how this structure changed over a significant portion of the solar cycle.
Earlier studies had suggested that the bands of faster and slower flow penetrated the Sun’s surface to a depth of about 10% of its radius. But Thompson’s group has shown that the whole of the Sun’s ‘convective’ region – the turbulent outer shell of the Sun, which occupies about 30% of its radius – is swept along by these flows.
The Sun’s magnetic field is thought to be generated in and below this convective region, and according to Thompson, the new results suggest that the flow patterns are a response of the plasma in the convective region to the magnetic fields.
The researchers also found that the migration of the flow patterns follows a cycle that lasts about 3.5 years, together with its link with the well-known eleven-year cycle. “These findings provide new and stringent conditions on models of the Sun’s magnetic dynamo,” Thompson told PhysicsWeb. “We are still far from understanding, let alone predicting, the solar interior. But our results are a step towards a better grasp of the complex processes in the deep inside the Sun.”
