Density scatter plots (2D histograms) of the brightness temperature T_b versus gas temperature T_gas taken at the effective formation heights for 0.4 mm, 1 mm, 3 mm, and 4.5 mm. Darker color indicates more pixels in the bin. Solid lines denote T_b = T_gas. Blue contour (dark gray in the printed version) encloses 95% of the spatial locations with weak magnetic field (| B | < 30 G at the effective formation height), red contour (light gray in the printed version) includes 95% of the locations of strong magnetic field (| B | ≥ 30 G at the effective formation height).