Time series of solar granulation images. II. Evolution of individual granules

The properties of the evolution of solar granulation have been studied usin g an XO minute time series of high spatial resolution white-light images ob tained with the Swedish Vacuum Solar Telescope at the Observatorio del Rogu e de los Muchachos, La Palma. An automatic tracking algorithm has been deve loped to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic proc edure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good fit to the distribution of granular lifetimes, T. Our estimated mean lifetime is abou t 6 minutes, which is at the lower limit of the ample range of values repor ted in the literature. We note a linear increase in the time-averaged granu lar sizes and intensities with the lifetime. T = 12 minutes marks a sizeabl e change in the slopes of these linear trends. Regarding the location of gr anules with respect to the meso- and supergranular flow field, we find only a small excess of long-lived granules in the upflows. Fragmentation, mergi ng, and emergence from (or dissolution into) the background are the birth a nd death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these famili es leads to the following conclusions: (1) fragmentation is the most freque nt birth mechanism, while merging is the most frequent death mechanism; (2) spontaneous emergence from the background occurs very rarely, but dissolut ion into the background is much more frequent; and (3) different granular m ean lifetimes are determined for each of these families; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minu tes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a gr anular evolution: 0.8 Mm(2) (d(g) = 1."39) and 1.3 Mm(2) (d(g) = 1."77). Th ese values mark limits characterizing the birth mechanism of a granule, and predict its evolution to some extent. The findings of the present work com plement the earlier results presented in this series of papers and reinforc e with new inputs, as far as the evolutionary aspects are concerned, the co nclusion stated there that granules can be classified into two populations with different underlying physics. The boundary between these two classes c ould be established at the scale of d(g) = 1."4.