Polarization diagrams computed for different heights above the solar surface (from 1 Mm in the leftmost frame to 9 Mm in the rightmost frame). In every diagram, the y-axis gives the degree of linear polarization , the x-axis the polarization angle α = 0.5   tan^-1(U/Q). p represents the maximum in the wavelength range covered by the He i multiplet. The plotted α has been sampled at the same wavelength. Red points correspond to “vertical” magnetic fields (θ_B < 54.7° or θ_B > 125.3°), while black points correspond to “horizontal” magnetic fields (54.7° < θ_B < 125.3°).