3.4 Horizontal patterns and evolution

On the scale of granulation, the shape and evolution of the temperature and velocity patterns also reflects the strong density stratification. The rapid decrease of density with height forces the plasma in each granule to spread outwards (Figure 8View Image). The plasma that reaches the surface looses energy very rapidly by radiative cooling, and as it collides with similar outflows from other granules it forms dark intergranular lanes where the fluid is deflected downwards and along the lanes towards cell vertices. Along the lanes, and particularly at the cell vertices, it forms turbulent downdrafts with low entropy (Figure 9Watch/download Movie).
View Image

Figure 8: Velocity and temperature structure of a granule with a corner cutout. A granule resembles a fountain with warm fluid rising up near the center, diverging horizontally, cooling, being pulled back down by gravity in surrounding intergranular lanes (from Stein and Nordlund, 1998Jump To The Next Citation Point).

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Figure 9: mov-Movie (123751 KB) Entropy fluctuation from a solar convection simulation. Horizontal scale is 6 Mm and the vertical scale range from the temperature minimum to 2.5 Mm below the surface.

Mass conservation takes care of itself by acting through the pressure. Large granules require a large pressure perturbation to drive their horizontal flows. If there is insufficient pressure to push enough mass out horizontally, the density builds up over the granule until the pressure is raised sufficiently to expel it. The excess pressure also decelerates the upflow and thus reduces the energy transport to the surface, in particular near the granule center, which then cools (Massaguer and Zahn, 1980). Hence, as granules grow, the upflow velocity near their center decreases and they develop an edge brightened appearance.

The horizontal flow, driven by the excess pressure in the interiors of the granules, must be halted when it reaches an intergranular lane, by which time it has cooled, become denser and is being pulled down by gravity. This requires an excess pressure also in the intergranular lanes (Hurlburt et al., 1984).

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