Contrast Analysis of Solar Faculae and Magnetic Bright Points

Abstract

The morphology and contrast of small-scale solar elements at four disk positions is analyzed. The data were obtained at the Swedish 1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets have disk positions near disk center (average μ = cos θ > 0.8) and show numerous magnetic bright points (MBPs), and two are sufficiently limbward to show prominent (average μ ≤ 0.6). The filtergrams are obtained in the 430.5 nm G band and 436.4 nm continuum bandpasses; the magnetograms are Fe I 630.25 nm Stokes V images taken with the Solar Optical Universal Polarimeter (SOUP) tunable filter. In all images we achieve nearly diffraction-limited resolution (~100 km in the G band). Analysis shows that MBPs and faculae are distinct radiative signatures of the field: MBPs have a constant or slightly decreasing contrast with increasing magnetogram signal, while facular contrast increases linearly with magnetogram signal. Faculae are much larger than MBPs, with an average radial width of 400 km. The observations support recent modeling showing that faculae are granules seen through the opacity reduction provided by elements (or groups thereof), while MBPs are caused by lateral radiation leakage scattering from deeper layers of the element.