Abstract
We determine the number density and area contribution of small-scale inter-granular calcium-II bright G-band structures in images of the quiet Sun as tracers of kilo-Gauss magnetic flux-concentrations. In a 149" x 117" G-band image of the disk center at the activity minimum, 7593 small inter-granular structures ['IGS']were segmented with the `multiple-level tracking' pattern recognition algorithm ['MLT_4']. The scatter-plot 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 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 of increasing size with decreasing G-band excess, as expected from the `hot wall' picture. Their Ca-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. The entity of BP and FP covers the total field-of-view ['FOV'] with a number density of 0.32/Mm^2 and a total area contribution of 2.0%. Their individual calibrations yield a mean flux density of 20 Mx/cm^2 in the entire FOV and 13 Mx/cm^2 for inter-network regions.
Highlights
The small-scale magnetic flux-concentrations on the non-active solar surface are an important ingredient for understanding solar magnetism
The segmentation and selection of small-scale inter-granular structures with a significant Ca II H excess allows lower limits to be determined for the number density and fractional area coverage of inter-granular kilo-Gauss magnetic flux-concentrations
Our method offers an alternate approach to polarimetry, it inevitably misses those magnetic structures that do not exhibit a Ca II H excess and “elusive magnetic structures” from fluctuations in thermodynamic properties
Summary
The small-scale magnetic flux-concentrations on the non-active solar surface are an important ingredient for understanding solar magnetism. The excess in the G-band brightness over the neighboring spectral continuum is a different measure than the excess over the mean photospheric G-band level, defining G-band bright points (BP; Muller & Roudier 1984) The latter cover two families of structures: (I) inter-granular BP, which are magnetic (Berger & Title 2001) and down-drafting (Langhans et al 2002), and (II) nonmagnetic BP, which move upwards with the respective granules (same two papers). The powerful pattern recognition algorithm, followed by a selective sampling of small structures, elimination of nonmagnetic ones by their G-band-to-continuum ratio, and classification of magnetic ones by the Ca II excess, qualifies our study as an alternate way to investigate small-scale solar kiloGauss magnetic flux-concentrations at the highest possible spatial resolution and independent of interpreting polarimetric measurements
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