Top panel: energy of the linear force-free field with outer boundary surface at r_L and the same relative helicity and boundary conditions on the stellar surface as the non-linear Wolfson field with n = 0.1. The solid line is a spline interpolation through the computed values as indicated by the open diamonds. The energy of the non-linear field is indicated by the horizontal dashed line. Bottom panel: the value of the force-free parameter α₀ of the linear field satisfying the above conditions vs. the radius of the outer boundary surface. The solid line is a spline interpolation through the computed values (open diamonds).