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 di rectly observed on the solar surface, with a hierarchy of four size scales. The smallest observed convection cells, called granules, have typical hori zontal sizes of 1,000-2,000 km and have been successfully reproduced by num erical simulations (Spruit 1997; Stein & Nordlund 1998). Cells at three lar ger scales are also detected (Leighton et al. 1962; November et al. 1981; B eck et al. 1998), but these have so far not been amenable to numerical mode lling, 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 me sogranules. In addition, our simulation demonstrates that the observed meso granulation is driven close to the solar surface and therefore rules out th e text-book explanation of mesogranulation as cellular convection driven by superadiabaticity in the deeper layer where neutral helium ionizes. By pro xy, this result also casts doubt on the traditional explanation of supergra nulation, even larger convection cells with diameters of 20,000-30,000 km, as being driven by the yet deeper second ionization of helium.