In-plane magnetic field divergence diagnostics. (a) Spatial distributions of the in-plane ∇ · B (in nT ﹩{R}_{s}^{-1}﹩) for the MHD true values (left) and the model prediction (right) at the first rolling step for a representative test sample; (b) the median (solid line) and 90th percentile (dashed line) of the normalized in-plane divergence ﹩\frac{| {\rm{\nabla }}\cdot {\boldsymbol{B}}| \,h}{| {\boldsymbol{B}}| }﹩ as functions of the number of rolling steps, for the model prediction (red squares) and the MHD true values (blue circles) calculated using the same finite-difference operator.