The opacity of magnetic flux concentrations is reduced, because they are evacuated, so photons escape from deeper in the atmosphere, that is, optical depth surfaces are depressed into the interior (Wilson depression, Maltby, 2000). Radiatively, they are holes in the surface. The temperature structure in these concentrations is nearly in radiative equilibrium with radiative heating from fluid flowing down along their sides and cooling from emission in vertical rays (Bercik, 2002, Figure 21). Where the flux concentration is narrow, heating from the side walls raises the internal temperature at optical depth unity and the concentration appears bright (Spruit, 1976). Small magnetic flux concentrations may appear especially bright in the continuum (Bercik, 2002; Keller et al., 2004; Carlsson et al., 2004; Steiner, 2010). This enhanced brightness extends for all the sight lines that pass through the low density, optically thinner, magnetic concentration where photons escape from deeper, hotter layers (Figures 22 and 23). Where the concentrations are wide, the side wall heating is not significant and the flux concentrations appear darker than the surroundings as pores or sunspots.
In the G-band there is an additional, smaller, effect – the CH molecule becomes dissociated in the low density magnetic concentrations (Steiner et al., 2001; Carlsson et al., 2004; Shelyag et al., 2004; Steiner, 2005). The bottom panel of Figure 22 shows the temperature as function of . The contrast in temperature between magnetic concentrations and non-magnetic areas increases with decreasing optical depth giving larger intensity contrast with increasing opacity (e.g., Ca H,K). The G-band has its mean formation height (black line in bottom panel) at corresponding to a mean formation height 54 km above where , therefore giving a larger contrast than in the continuum. The contrast enhancement by the destruction of CH is seen as a dip in the curve showing the mean formation optical depth in the bottom panel. Note also that the G-band intensity has its peak contribution at similar heights as the continuum (that is why the granulation pattern looks similar). Bright points in the G-band have been used as a proxy for magnetic field concentrations. While G-band bright points are a good proxy for strong magnetic fields, there are many more regions of strong field that appear dark in the G-band, typically because they cover a larger area (Figure 24). In simulations, all the bright points correspond to locations of large field magnitude, but not all large field locations correspond to bright points (Vögler et al., 2005; Stein and Nordlund, 2006). Further, the field has a longer lifetime than the bright points. The contrast in the G-band has also been studied by Rutten et al. (2001), Sánchez Almeida et al. (2001), and Steiner et al. (2001).
Living Rev. Solar Phys. 9, (2012), 4
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