Solar supergranulation revealed by granule tracking

Abstract

Context. Supergranulation is a pattern of the velocity field at the surface of the Sun, which has been known about for more than fifty years, however, no satisfactory explanation of its origin has been proposed. Aims. New observational constraints are therefore needed to guide theoretical approaches which hesitate between scenarios that either invoke a large-scale instability of the surface turbulent convection or a direct forcing by buoyancy. Methods. Using the 14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained a 7.5 h-long sequence of high resolution images with unprecedented field size. Tracking granules, we have determined the velocity field at the Sun's surface in great detail from a scale of 2.5 Mm up to 250 Mm. Results. The kinetic energy density spectrum shows that supergranulation peaks at 36 Mm and spans on scales ranging between 20 Mm and 75 Mm. The decrease of supergranular flows in the small scales is close to a -power law, steeper than the equipartition Kolmogorov one. The probability distribution function of the divergence field shows the signature of intermittency of the supergranulation and thus its turbulent nature. Conclusions. © 2008 ESO.