Comparison of the spectrum (in antenna temperature units) computed when the magnetic-sublevel populations are degenerate (solid black line) under LTE conditions, with the spectra polarized perpendicular and parallel to the magnetic field (dash-dotted red and dotted blue lines, respectively), computed when the magnetic sublevels are explicitly considered in the detailed balance equations. All spectra are from the central cell (R = Z = 0). The green points show the polarization fraction (right y axis). With the dashed black line we mark a lenient detection limit of 0.1% polarization. As was expected, under LTE conditions, I_⊥ = I_∥ = I_total/2 and the polarization fraction is zero to numerical accuracy (note that the right y axis is in logarithmic scale spanning twenty-one orders of magnitude). The underlying physical model used for these radiative-transfer simulations is shown in Fig. 3.