Investigations of Sizes and Dynamical Motions of Solar Photospheric Granules by a Novel Granular Segmenting Algorithm

Abstract

Granules observed in the solar photosphere are believed to be convective and turbulent, but the physical picture of the granular dynamical process remains unclear. Here we performed an investigation of granular dynamical motions of full length scales based on data obtained by the 1 m New Vacuum Solar Telescope and the 1.6 m Goode Solar Telescope. We developed a new granule segmenting method, which can detect both small faint and large bright granules. A large number of granules were detected, and two critical sizes, 265 and 1420 km, were found to separate the granules into three length ranges. The granules with sizes above 1420 km follow Gaussian distribution, and demonstrate flat in flatness function, which shows that they are non-intermittent and thus are dominated by convective motions. Small granules with sizes between 265 and 1420 km are fitted by a combination of power-law function and Gauss function, and exhibit nonlinearity in flatness function, which reveals that they are in the mixing motions of convection and turbulence. Mini granules with sizes below 265 km follow the power-law distribution and demonstrate linearity in flatness function, indicating that they are intermittent and strongly turbulent. These results suggest that a cascade process occurs: large granules break down due to convective instability, which transports energy into small ones; then turbulence is induced and grows, which competes with convection and further causes the small granules to continuously split. Eventually, the motions in even smaller scales enter in a turbulence-dominated regime.

Length Scale of Photospheric Granules in Solar Active Regions

“…In this work, we applied the granular segmenting algorithm (Liu et al 2021) to extract and identify granules in an active region. The algorithm extracts granules by searching the local intensity valleys as edges.…”

“…We segment and classify granules by using the method described in Liu et al (2021) which is proved to be accessible to both large bright and small faint granules. Three steps are included to extract and identify granules, namely granular edge detection, separation of closely connecting granules and distinguishing granule cells from none-granule features.…”

“…Three steps are included to extract and identify granules, namely granular edge detection, separation of closely connecting granules and distinguishing granule cells from none-granule features. Here, we briefly describe these steps, and for more details we refer to Liu et al (2021).…”

“…Hereby, Abramenko et al (2012) concluded that large granules are not intermittent but the small granules are highly intermittent. Liu et al (2021) did the same work by using a new granule segmenting method. They fitted the size distribution by Gauss, the combination of Gauss and power-law, and power-law separately in three different length ranges, and detected two critical points to separate the granules into three regimes.…”

“…Actually the size distributions and flatness applied in Abramenko et al (2012) and Liu et al (2021) was originally used in hydrodynamics. The fluid motions in hydrodynamics include laminar flows and turbulent flows.…”