Comparison of different populations of granular features in the solar photosphere

Abstract

Context. The granulation is the most visible manifestation of convective motions occurring in the uppermost layers of the solar convection zone. Strong magnetic fields hinder these motions, but the appearance of bright structures such as umbral dots (UDs) and light bridges (LBs) in sunspots also shows that in strong magnetic field regions, the convection is not completely suppressed. Aims: We aim to investigate the properties of the granules identified by a new segmentation algorithm in regions characterized by different magnetic field strength, in order to improve the current knowledge of the mechanism behind the appearance of the different bright structures in sunspots. Methods: We analyzed data acquired by the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope on 6 August 2011 relevant to a large sunspot with a LB observed in NOAA AR 11263. We applied a new segmentation algorithm to the data acquired along the Fe I 630.15 nm line. Results: We found that the granules in the LB have a diameter between 0.̋22 and 0.̋99, that is, smaller than the granules in a nearby plage region (PL) and similar to those of the UDs. We observed values of the mean continuum intensity between 0.42 Ic and 0.98 Ic for the LB granules, which are similar to those of the UDs. PL granules have higher values, probably reflecting different conditions of the plasma convection. Mean Doppler velocity and mean magnetic field strength have been studied and even for these physical parameters we found similar values between LB granules and UDs. Conclusions: Different values for the physical properties analyzed have been found between the granules of the PL and LB granules of the three analyzed solar regions. In particular, we show that the granules in PL and sunspot regions have different physical properties. This clearly depends on the different physical conditions of the regions where these two kind of granular structures are embedded. We also confirm the recent findings on the similarity between granules in PL and quiet Sun regions. We show values of the various physical quantities analyzed in PL granules in agreement with those reported in the literature for quiet Sun granules. Finally, a noteworthy result is that the granules observed in the faint LB have physical properties similar to those found for UDs.