Downflows in sunspot umbral dots

TL;DR

This study investigates the velocity and magnetic properties of umbral dots in sunspots using high-resolution spectropolarimetric data, revealing strong upflows and transient downflows that support magnetoconvection models.

Contribution

It provides detailed observational evidence of flow dynamics and magnetic features of umbral dots at high spatial resolution, validating theoretical models.

Findings

Most umbral dots are associated with strong upflows in deep photospheric layers.

Some umbral dots exhibit transient downflows at their edges with velocities up to 1000 m/s.

Downflows have short lifetimes of only a few minutes.

Abstract

We study the velocity field of umbral dots at a resolution of 0.14". Our analysis is based on full Stokes spectropolarimetric measurements of a pore taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630 nm lines. In addtion, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most umbral dots are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0.25", velocities of up to 1000 m/s, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields.