Center-to-limb variation of the area covered by magnetic bright points in the quiet Sun

TL;DR

This study measures how the surface coverage of magnetic bright points varies from the center to the limb of the Sun, revealing their potential impact on solar irradiance and modeling their physical properties.

Contribution

It provides the first detailed measurement of the center-to-limb variation of quiet Sun magnetic bright points and introduces a simple model to describe their observed distribution.

Findings

FCS is about 1% at disk center, decreasing to 0.2% near the limb.

Most TSI contribution (>90%) comes from BPs with mu > 0.5.

BPs are shallow, wide structures with diverse inclinations.

Abstract

CONTEXT: The quiet Sun magnetic fields produce ubiquitous bright points (BPs) that cover a significant fraction of the solar surface. Their contribution to the total solar irradiance (TSI) is so-far unknown. AIMS: To measure the center-to-limb variation (CLV) of the fraction of solar surface covered by quiet Sun magnetic bright points. The fraction is referred to as 'fraction of covered surface', or FCS. METHODS: Counting of the area covered by BPs in G-band images obtained at various heliocentric angles with the 1-m Swedish Solar Telescope on La Palma. Through restoration, the images are close to the diffraction limit of the instrument (~0.1 arcsec). RESULTS: The FCS is largest at disk center (~1 %), and then drops down to become 0.2 % at 'mu'= 0.3 (with 'mu' the cosine of the heliocentric angle. The relationship has large scatter, which we evaluate comparing different subfields within our FOVs. We work out a toy-model to describe the observed CLV, which considers the BPs to be depressions in the mean solar photosphere characterized by a depth, a width, and a spread of inclinations. Although the model is poorly constrained by observations, it shows the BPs to be shallow structures (depth < width) with a large range of inclinations. We also estimate how different parts of the solar disk may contribute to TSI variations, finding that 90 % is contributed by BPs having 'mu' > 0.5, and half of it is due to BPs with 'mu' > 0.8.