Morphological variations of solar granules in the presence of magnetic fields

TL;DR

This study investigates how magnetic fields influence the morphology of solar granules, revealing that stronger magnetic fields suppress granule size and alter their shape and alignment, providing insights into plasma dynamics near the solar surface.

Contribution

It presents a detailed analysis of the relationship between magnetic field properties and granule morphology using high-resolution solar observations, highlighting new correlations and behaviors.

Findings

Granule area decreases with increasing magnetic field strength.

Highly elongated granules are found in both magnetic and non-magnetic regions.

Granule major axes tend to align with strong horizontal magnetic fields.

Abstract

Solar granulation consists of dynamic convective plasma cells that rise from the solar interior to the surface. The interaction between these plasma cells and the Sun's magnetic field provides valuable insights into plasma dynamics near the solar surface and how they evolve in the presence of magnetic fields. This study analyses the morphological characteristics of solar convective cells, investigating the relationship between magnetic field properties and granule dynamics - specifically how granule area, shape, and brightness vary under different magnetic field conditions. Observations of the active region NOAA 11768 were taken with the Swedish 1-m Solar Telescope (SST). A segmentation algorithm was applied to continuum intensity images to identify individual granules and determine their sizes, shapes, and mean brightness. The magnetic field vector and line-of-sight velocity were derived from CRISP spectropolarimetric data to investigate their role in shaping granule properties. We find that granular area decreases systematically with increasing magnetic field strength, with the largest granules occurring in non-magnetic regions and a mean granule area of approximately 1.58 arcsec$^2$ (effective diameter of 1.42 arcseconds). Both mean continuum intensity and granule size decrease with stronger fields, confirming the suppression of convective energy transport in magnetised regions. No correlation was found between mean granule brightness and mean up-flow velocity. Highly elongated granules appear in both magnetic and non-magnetic regions, while near-circular granules are exclusive to non-magnetic areas. An alignment between granule major axes and magnetic field azimuth is observed where the horizontal field component is strong, confirming that granules are highly sensitive to magnetic fields, which inhibit the lateral expansion of convective cells.