Mean value of the χ²-merit function between the observed 𝕀^obs(x, y, λ) and synthetic 𝕀^syn(x, y, λ) Stokes profiles (Eq. (3)) over the entire field-of-view as a function of the inversion iteration (i-even) performed with the Firtez inversion code (Sect. 3.1). Results for NOAA AR 10944 (left-hand panel in Fig. 1) are indicated in orange, while results for NOAA AR 10944 (right-hand panel in Fig. 1) are shown in blue.