Is solar mesogranulation a surface phenomenon?

Convection is the main form of energy transport in the subsurface layers of the sun and other cool stars. The imprint of cellular convection can be directly observed on the solar surface, with a hierarchy of four size scales. The smallest observed convection cells, called granules, have typical horizontal sizes of 1,000-2,000 km and have been successfully reproduced by numerical simulations (Spruit 1997; Stein & Nordlund 1998). Cells at three larger scales are also detected (Leighton et al. 1962; November et al. 1981; Beck et al. 1998), but these have so far not been amenable to numerical modelling, so that their formation scenarios remain untested. Here we present a numerical simulation which resolves both the granular and the next larger, mesogranular, scale. The mesogranules have horizontal extents of 5,000-10,000 km. Our 2D simulation reproduces key properties of both granules and mesogranules. In addition, our simulation demonstrates that the observed mesogranulation is driven close to the solar surface and therefore rules out the text-book explanation of mesogranulation as cellular convection driven by superadiabaticity in the deeper layer where neutral helium ionizes. By proxy, this result also casts doubt on the traditional explanation of supergranulation, even larger convection cells with diameters of 20,000-30,000 km, as being driven by the yet deeper second ionization of helium.