Abstract

Context. It is unclear why there is a rather sharp boundary in sunspots between the umbra and the penumbra. Both regions exhibit magnetoconvection, which manifests 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 the size and brightness of convective features in the penumbra.

Highlights

  • Sunspots consist of a dark umbra and a bright penumbra

  • There is a smooth transition from umbral dots to penumbral filaments, with most physical parameters of the features being continuous functions of the length of the features

  • Apart from an offset in the continuum intensity, there are no obvious differences in the properties of the convective elements between the large and small spot that we analyzed

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Summary

Introduction

The penumbra is believed to exhibit magnetoconvection, which manifests in the penumbral filaments (Scharmer et al 2008; Zakharov et al 2008; Rempel et al 2009; Rempel 2011; Rempel & Cheung 2014). The direct manifestation of magnetoconvection in the umbra is the umbral dots (Danielson 1964; Parker 1979; Choudhuri 1986; Schüssler & Vögler 2006; Bharti et al 2010). Umbral dots are small bright features that appear in umbrae and pores. Depending on their location in the umbra, they are classified as central umbral dots (CUDs), located in the central parts of the umbra, or peripheral umbral dots (PUDs), located in the outer parts of the umbra (Grossmann-Doerth et al 1986)

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