J. A. Bonet, I. Cabello, J. Sanchez Almeida
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.
View original:
http://arxiv.org/abs/1112.2513
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