The evolution of solar granules deduced from 2-D simulations

The evolution of solar granules is investigated on the basis of two dimensional numerical solutions of the hydrodynamic equations describing a compressible, radiatively coupled and gravitationally stratified medium representative of the solar surface layers. The simulation covers 17 Mm on the solar surface and was run for over 5h of solar time, hence allowing the evolution of over 400 granules to be followed. A statistical investigation of the temporal evolution of granules therefore becomes feasible. Two types of granules can be distinguished by their means of death: fragmenting and dissolving granules. Properties and average evolutionary histories of these two types of granules are considered. It is found that fragmenting granules are in general large at birth and expand further with time. It is confirmed that fragmentation into two (or more) parts is produced by buoyancy braking, which in turn is initiated by the stronger horizontal flows in larger granules. This last property, finally, is due to mass conservation. The expansion, however, is due to a pressure excess relative to neighbouring granules. The pressure excess is particularly marked if the neighbours are dissolving granules. In contrast, dissolving granules are born small and shrink before finally disappearing. The shrinkage is caused by their neighbours which generally posses excess gas pressure and larger horizontal flows. In summary, according our findings the fate of a granule is decided by its properties at birth and the company it keeps. Evidence is presented suggesting that the evolution of both types of granules is driven by events near the solar surface.