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

TL;DR

This study introduces a novel segmentation algorithm to analyze solar photospheric granules, revealing size-dependent dynamical behaviors and a cascade process from convection to turbulence across different granule scales.

Contribution

A new granule segmentation method was developed, enabling detailed analysis of granule sizes and motions, uncovering a size-based dynamical transition in solar photospheric granules.

Findings

Large granules (>1420 km) are non-intermittent and convection-dominated.

Medium granules (265-1420 km) exhibit mixed convection and turbulence.

Mini granules (<265 km) are intermittent and turbulence-dominated.

Abstract

Granules observed in solar photosphere are believed to be convective and turbulent, but the physical picture of 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-meter New Vacuum Solar Telescope (NVST) and the 1.6-meter Goode Solar Telescope (GST). 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 km 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 non-linearity in flatness function, which reveals that they are in the mixing motions of convection and turbulence. Mini granules with sizes below 265 km follow 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 transport 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.