Structure inversions of the four target models using RLS. The left column shows the inferred buoyancy frequency difference (orange full line) and the difference between the models (blue dashed line) for target models #1 to #4 in order. The observational uncertainty is indicated by the light blue regions around the inferred profile. For most of these inferred profiles, the regions are approximately the size of the line width. The right column shows the period-spacing pattern of the reference model (blue circles), the target models (orange plus symbols) and the fitted modes (gray crosses; defined in Eq. (4)). The inferred profile is discretized in 70 grid points, of which 50 are located in regions of the largest difference in buoyancy frequency, indicated by the red lines. The regularization parameter varies between 10^−4.8 and 10^−4.2 between the different target models. Except for the closest target model, the inferred profiles show significant deviation from the actual difference between the models. The period-spacing patterns show that the overall quality of the fit is limited, especially at higher radial orders.