Time Series of Solar Granulation Images. I. Differences between Small and Large Granules in Quiet Regions

Abstract

A 90 minute time series of high spatial resolution white-light images of solar granulation, obtained at the Swedish Vacuum Solar Tower (Observatorio del Roque de los Muchachos, La Palma), was analyzed to study how the physical properties of the granules changed with size. The observational material was corrected for global motions and for the instrumental proÐle, and a subsonic Ðlter was applied. A deÐni-tion of granular border was adopted using the inÑection points of the intensity of the images, and the granular cells were deÐned as areas including, in addition to the granules, one-half of their surrounding intergranular lanes. Using time series to investigate the average behavior of solar granulation has three strong advantages : the Ðrst is the possibility of removing the acoustic waves ; second, the possibility of estimating the e †ect of the variability of seeing on our results ; and, third, the opportunity to attain high statistical signiÐcance in the analysis as a result of the large number of extracted granules (61,138).It is shown that the granules of the sample can be classiÐed according to their mean and maximum intensities and their fractal dimension into two regimes, with diameters smaller than and larger than 1A .4, respectively. A broad transition region in which both regimes coexist was found. The resolved internal brightness structure of both the granules and the intergranular lanes shows a linear increase of the number of substructures with the granular and intergranular areas. The diameters of these substructures range between our e †ective resolution limit and with preferential sizes at and (D0A .3) D1A .5, 0A .65 0A .55, respectively. Moreover, it seems that large and small granules are unevenly distributed with respect to the large-scale vertical Ñows. Thus smaller granules are more concentrated along downdrafts whereas larger ones preferentially occupy the updrafts. Finally, a physical scenario compatible with the existence of these two granular populations is discussed.

High‐Resolution Proper Motions in a Sunspot Penumbra

“…7 November (1989) and Hirzberger et al (1997) employ a larger window to select mesogranulation, whereas Wang et al (1995b) do not provide units for the divergence of the horizontal velocities. 8 The finding of low vertical velocities should not depend on the specific observation we use.…”

Three-dimensional simulations of near-surface convection in main-sequence stars

“…We find a correlation between area and mean brightness for the smaller granules, whereas for the larger ones the brightness saturates and correlation is lost. This is consisitent with solar observations (see, e.g,., Hirzberger et al 1997). At least for large granules, there is negative correlation between size and mean vertical velocity (cf.…”

Fine structure and dynamics in a light bridge inside a solar pore

“…We used a filter with the cutoff phase velocity set to 5 km s −1 (see Hirzberger et al 1997 for a discussion).…”

“…After describing the structure and the time evolution of the granulation pattern in quiet regions in Hirzberger et al (1997Hirzberger et al ( , 1999a and the dynamics of fine structures in an active region in Sobotka et al (1999a), the present work is dedicated to the analysis of the internal structure and kinematics of a granular light bridge observed inside a large pore (a sunspot lacking the penumbra).…”