Longitudinal magnetic field and velocity gradients in the photosphere inferred from THEMIS multiline observations

We present multiline observations taken with the French-Italian telescope THEMIS operated by CNRS and CNR on the island of Tenerife. Several spectral lines are observed simultaneously to study magnetic phenomena at different altitudes of line formation, comprising the Fe i 5250.65 Å, Cr i 5247.57 Å, Fe i 5250.22 Å, Fe i 6301.51 Å, Fe i 6302.50 Å, and Fe i 6151.62 Å lines. The observations were analyzed using Milne-Eddington inversion algorithms modified to allow for non-normal Zeeman triplet lines, and to take additionally into account the vertical velocity gradient. We show the self-consistency of the solutions found from the inversion and those obtained from the quantitative bisector method. Results from the different lines observed simultaneously yield the height dependence of the magnetic field strength. From modeling the line formation heights applied on selected points with a longitudinal magnetic field between 200 and 1000 G, we determine the gradient of the vertical component (absolute value) of the magnetic field to be − 1.18 G/km.