ON THE NATURE OF EXPLODING GRANULES AND GRANULE FRAGMENTATION

The morphological evolution of solar granulation is dominated by granule expansion and fragmentation. ''Exploding'' granules undergo these processes in a particularly vigorous manner, rapidly expanding to a large size, darkening in the center, and splitting by the formation of dark interior radially directed lanes. We argue that such events can be better understood if granulation is viewed as downflow-dominated surface-driven convection rather than as a collection of more deeply driven upflowing thermal plumes. Regions of maximum granular upflow lie not in the centers of the granules but along their sides, immediately adjacent to the intergranular downflow lanes. These upflows occur primarily in response to the buoyancy and pressure gradient forces induced in proximity to the strongly driven downflow plumes. The upflows are thus dynamically linked to the downflow sites, and granular expansion results in a weakening to the central flow. Radiative losses can then exceed the advected heat supply in the granule center, with the fluid cooling until buoyancy forces becomes sufficient to trigger the formation of a new downflow plume there. Lateral propagation proceeds as neighboring flows are disturbed, with propagation preferentially occurring in directions predisposed to weak upflow by the strength and shape of the downflows defining the granule boundary. Thus the radially oriented structures seen in observations of some fragmenting granules may be formed. Finally, the strong downflow plumes initiated in the solar photosphere entrain surrounding material as they descend. With depth this more weakly downflowing material establishes a connectivity which is strikingly of mesogranular scale. This may help to explain the observed correlation between the spatial distribution of exploding granules and mesogranular flows, and suggests that both mesogranulation and supergranulation are secondary manifestations of granulation itself.