The evolution of solar granules deduced from 2-D simulations

The evolution of solar granules is investigated on the basis of two dimensi onal numerical solutions of the hydrodynamic equations describing a compres sible, radiatively coupled and gravitationally stratified medium representa tive of the solar surface layers. The simulation covers 17 Mm on the solar surface and was run for over 5 h of solar time, hence allowing the evolutio n of over 400 granules to be followed. A statistical investigation of the t emporal evolution of granules therefore becomes feasible. Two types of granules can be distinguished by their means of death: fragmen ting and dissolving granules. Properties and average evolutionary histories of these two types of granules are considered. It is found that fragmentin g granules are in general large at birth and expand further with time. It i s confirmed that fragmentation into two (or more) parts is produced by buoy ancy braking, which in turn is initiated by the stronger horizontal flows i n larger granules. This last property, finally, is due to mass conservation . The expansion, however, is due to a pressure excess relative to neighbour ing granules. The pressure excess is particularly marked if the neighbours are dissolving granules. In contrast, dissolving granules are born small and shrink before finally d isappearing. The shrinkage is caused by their neighbours which generally po sses excess gas pressure and larger horizontal flows. In summary, according our findings the fate of a granule is decided by its properties at birth a nd the company it keeps. Evidence is presented suggesting that the evolution of both types of granul es is driven by events near the solar surface.