TIME-SERIES OF SOLAR GRANULATION IMAGES .1. DIFFERENCES BETWEEN SMALLAND LARGE GRANULES IN QUIET REGIONS

A 90 minute time series of high spatial resolution white-light images of solar granulation, obtained at the Swedish Vacuum Solar Tower (Obse rvatorio del Rogue de los Muchachos, La Palma), was analyzed to study how the physical properties of the granules changed with size. The obs ervational material was corrected for global motions and for the instr umental profile, and a subsonic filter was applied. A definition of gr anular border was adopted using the inflection points of the intensity of tile images, and the granular cells were defined as areas includin g, in addition to the granules, one-half of their surrounding intergra nular lanes. Using time series to investigate the average behavior of solar granulation has three strong advantages: the first is the possib ility of removing the acoustic waves; second, the possibility of estim ating the effect of the variability of seeing on our results; and, thi rd, the opportunity to attain high statistical significance in the ana lysis as a result of the large number of extracted granules (61,138). It is shown that the granules of the sample can be classified accordin g to their mean and maximum intensities and their fractal dimension in to two regimes, with diameters smaller than and larger than 1.'' 4, re spectively. A broad transition region in which both regimes coexist wa s found. The resolved internal brightness structure of both the granul es and the intergranular lanes shows a linear increase of the number o f substructures with the granular and intergranular areas. The diamete rs of these substructures range between our effective resolution limit (similar to 0.'' 3) and similar to 1.'' 5, with preferential sizes at 0.'' 65 and 0.'' 55, respectively. Moreover, it seems that large and small granules are unevenly distributed with respect to the large-scal e vertical flows. Thus smaller granules are more concentrated along do wndrafts whereas larger ones preferentially occupy the updrafts. Final ly, a physical scenario compatible with the existence of these two gra nular populations is discussed.