Similarities of magnetoconvection in the umbra and in the penumbra of sunspots

TL;DR

This study compares the physical properties of magnetoconvective features in sunspot umbrae and penumbrae, revealing smooth transitions and suggesting brightness differences are due to convection efficiency rather than element density.

Contribution

It provides the first detailed comparison of umbral dots and penumbral filaments, showing their properties are continuous across the boundary and linking brightness changes to convection effectiveness.

Findings

Convective features in umbra and penumbra are physically similar and smoothly transition.

Brightness increase in penumbra is due to more effective magnetoconvection.

Penumbral filaments are larger and brighter than umbral dots.

Abstract

Context. It is unclear why there is a rather sharp boundary in sunspots between the umbra and the penumbra. Both regions exhibit magnetoconvection, manifesting in penumbral filaments in the penumbra and in umbral dots in the umbra. Aims. Here we compare the physical properties of umbral dots and penumbral filaments. Our goal is to understand how the properties of these convective features change across the boundary between the umbra and the penumbra and how this is related to the rapid increase in brightness at the umbra-penumbra boundary. Methods. We derived ensemble averages of the physical properties of different types of convective features based on observations of two sunspots with Hinode. Results. There are strong similarities between the convective features in the outer parts of the umbra and the ones in the penumbra, with most physical parameters being smooth and continuous functions of the length of the features. Conclusions. Our results indicate that the transition in brightness from the umbra to the penumbra is solely caused by an increased effectiveness of magnetoconvection within individual convective cells. There is no significant difference in the number density of convective elements between the outer umbra and the inner penumbra. Penumbral filaments exhibit a larger area and a higher brightness compared to umbral dots. It is still unclear, how exactly the underlying magnetic field causes the increase in size and brightness of convective features in the penumbra.