Variation of the merit function with the inclination θ_B and azimuth χ_B of the field vector. The minima are located at eight different positions. Four of them correspond to a field vector pointing to the observer, i.e., positive Stokes V signals. These four solutions are connected through the 90° ambiguity and the 180° ambiguity of the Hanle effect (see arrows). The vertical dotted lines correspond to the Van Vleck angles 54.74° and 125.26°. The merit function here has been calculated using the parameters derived from the inversion of profile (a) in Fig. 4: B = 17 G, Δτ = 1.13, θ = 71.4°, h = 61.̋8, Δλ_D = 6 km s^-1, and a = 0.33. The contours delimit connected regions by the same value of Stokes Q/I (dashed) and U/I (solid).