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

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: jab@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
³ Image Processing Laboratory, Universidad de Valencia, 46980 Paterna, Valencia, Spain

Received: 6 October 2011
Accepted: 9 December 2011

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. We aim at measuring 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 the fraction of covered surface (FCS).

Methods. We count the area covered by BPs in G-band images obtained at various heliocentric angles with the 1-m Swedish Solar Telescope on La Palma. We restore the images to bring them close to the diffraction limit of the instrument (~0'.1).

Results. The FCS is largest at the disk center (≃1%), and then drops down to become  ≃0.2% at μ ≃ 0.3 (where μ is the cosine of the heliocentric angle). The relationship has a large scatter, which we evaluate by comparing different subfields within our FOVs. We develop a toy-model to describe the observed CLV, which considers the BPs as depressions in the mean solar photosphere characterized by a depth, a width, and a spread in the 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 the TSI variations, finding that 90% is contributed by BPs with μ > 0.5, and half of it is due to BPs with μ > 0.8.