Time Series of Solar Granulation Images. II. Evolution of Individual Granules

Abstract

The properties of the evolution of solar granulation have been studied using an 80 minute time series of high spatial resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma. An automatic tracking algorithm has been developed to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic procedure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good Ðt to the distribution of granular lifetimes, T . Our estimated mean lifetime is about 6 minutes, which is at the lower limit of the ample range of values reported in the literature. We note a linear increase in the timeaveraged granular sizes and intensities with the lifetime. T \ 12 minutes marks a sizeable change in the slopes of these linear trends. Regarding the location of granules with respect to the meso-and supergranular Ñow Ðeld, we Ðnd only a small excess of long-lived granules in the upÑows. Fragmentation, merging, and emergence from (or dissolution into) the background are the birth and death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these families leads to the following conclusions : (1) fragmentation is the most frequent birth mechanism, while merging is the most frequent death mechanism ; (2) spontaneous emergence from the background occurs very rarely, but dissolution into the background is much more frequent ; and (3) di †erent granular mean lifetimes are determined for each of these families ; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minutes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a granular evolution : 0.8 Mm2 and 1.3 Mm2 These values mark limits charac-77). terizing the birth mechanism of a granule, and predict its evolution to some extent. The Ðndings of the present work complement the earlier results presented in this series of papers and reinforce with new inputs, as far as the evolutionary aspects are concerned, the conclusion stated there that granules can be classiÐed into two populations with di †erent underlying physics. The boundary between these two classes could be established at the scale of d g \ 1A .4.

Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE

“…Interestingly, the lifetime distribution of our Ca ii H BPs (ranges between 167 and 1507 s) is comparable with that found by Hirzberger et al (1999) for granules, who showed a range of 168-1800 s for granular lifetimes with only a relatively small number of longlived granules with lifetimes longer than 1200 s. However, they found a mean lifetime of about 360 s for the granules which is smaller than the lifetime of our Ca ii H BPs. In addition, the existence of horizontal convective supersonic flows at the boundaries between granules and intergranular lanes (e.g., Cattaneo et al 1989;Solanki et al 1996;Nordlund et al 2009;Bellot Rubio 2009;Vitas et al 2011) may explain our observations of fast motions BPs and supersonic pulses which can be due to the impact of fast granular flows on the flux tubes.…”

Modelling solar irradiance variability on time scales from minutes to months

“…The key features of our model are: 1) the main birth and death mechanisms of granules are fragmentation (birth and death) and emergence from (birth), or dissolution into (death) the background (e.g., Mehltretter 1978;Kawaguchi 1980;Hirzberger et al 1999); 2) the brightness value of each granule is assigned randomly within a given range; 3) the lifetime distribution of all granules follows an exponential law (Fig. 3a).…”

Super-diffusion versus competitive advection: a simulation

“…Wang & Sheeley 1993;Schrijver et al 1996;Berger et al 1998;Hagenaar et al 1999;Chae et al 2008;Abramenko et al 2011;Chitta et al 2012;Giannattasio et al 2013Giannattasio et al , 2014aJafarzadeh et al 2014) at all accessible spatiotemporal scales, ranging from granulation (diameter 1 Mm and lifetime few minutes; Muller 1999;Hirzberger et al 1999;Del Moro 2004) to super-granulation (diameter 30 Mm and lifetime a day; Raju et al 1999;Srikanth et al 2000;Del Moro et al 2004). …”