Surface convection of the Sun: granules and supergranules

Convection appears at the surface of the Sun under the appearance of granulation which was discovered most probably by Herschel at the beginning of the nineteenth century. Granulation looks like small convective cells of typically 1000 km wide and paves the surface of the Sun. The lifetime of granules isi, in the mean, less than 10mn. In 1956 another scale of motion was discovered by Hart using Doppler pictures of the surface of the Sun. These pictures show some horizontal motions at a scale of 15000 to 30000 km, hence the name supergranulation. The origin of supergranulation is still a matter of debate; we propose that it come either from a large-scale instability of the granular flow piped by strong stratification or that it is a scale of convection driven by the fixed-flux boundary conditions and fixed by the magnetic field intensity.

To investigate the dynamics of the plasma at the Sun's surface, one may follow the displacement of granules. The technics are described in Roudier et al. 1999, Rieutord et al. 2007 and Tkaczuk et al. 2007. It may seem strange that granule indeed trace the plasma flow as they are vortices (rising vortex rings) and therefore far from passive scalars. Actually they do so provided the scale considered are large enough. In Rieutord et al. 2001, we proved with numerical simulations that this is indeed the case when the scale of motion is larger than 2500km and the time scale is longer than 30mn.