Inclination angle of the magnetic field at log₁₀(τ) = − 1.0 (photosphere; panel a). Expansion factor of the magnetic field between photosphere and chromosphere (i.e., at log₁₀(τ) = − 4.6) (panel b). Density ratio between chromosphere and photosphere (panel c). Total magnetic field gradient (panel d). The white contour in four panels represents the umbra-penumbra boundary as seen in the continuum intensity. The hatched area indicates the central region of the umbra where saturation effects and low photon flux are detected in the photosphere (Stangalini et al. 2018). The magnetic field, the inclination angles, and the density ratios were derived from the NICOLE inversions.