Convergence history of the Trust Region Reflective and the Levenberg-Marquardt algorithms for the iterative solution of the nonlinear least-squares problem (9) for three different initial guesses. Left panel: sum of squared residuals, S, given by Eq. (8) as a function of the number of forward map, F, evaluations. The horizontal dashed line represents the tolerance of 10⁻⁶. Right panel: sequence of improving approximate solutions in the 3D space spanned by the (height-independent) magnetic field parameters [B, θ_B,χ_B].