Brightness changes in and around a magnetic “flux tube” (schematic). Black: boundary of the magnetic structure. Red: τ = 1 surfaces for viewing angles μ = 1 (solid) and μ = 0.7 (dashed). Green arrows: direction of the specific intensity from the τ = 1 surfaces for these angles. At μ = 0.7 the “bright wall effect” is visible towards the solar limb (right side of the figure), in the region where the broken line lies below the solid line. At the disk center (left) side, the interior of the tube is obscured by the wall of the tube. Lateral influx of heat into the flux tube (heavy broken arrows) cool the surroundings, causing enhanced downflow (blue arrows) around the structure.