The quiet Sun's magnetic flux estimated from Ca II H bright inter-granular G-band structures

Institut für Astrophysik, Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: ewiehr@astro.physik.uni-goettingen.de

Received: 4 March 2008
Accepted: 5 June 2008

Abstract

Aims. We determine the number density and area contribution of small-scale inter-granular Ca II bright G-band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations.

Methods. In a G-band image of the disk center at the activity minimum, 7593 small inter-granular structures were segmented with the “multiple-level tracking” pattern recognition algorithm. The scatterplot of the continuum versus the G-band brightness shows the known magnetic and non-magnetic branches. These branches are largely disentangled by applying an intrinsic Ca II H excess criterion. The thus obtained 2995 structures contain 1152 G-band bright points (BP) and 1843 G-band faint points (FP). They show a tendency toward increasing size with decreasing G-band excess, as expected from the “hot wall” picture. Their Ca II H and G-band brightness are slightly related, resembling the known relation of Ca II and magnetic field strength. The magnetic flux density of each individual BP and FP is estimated from their G-band brightness according to MHD model calculations.

Results. The entity of BP and FP covers the total FOV with a number density of 0.32 / Mm² and a total area contribution of 2.0%. Their individual calibrations yield a mean flux density of 20 Mx/cm² in the entire FOV and 13 Mx/cm² for inter-network regions.