Projections of the magnetic field lines onto the meridional plane for the non-linear force-free model of Wolfson (1995) with n = 0.5 as described in Sect. 2.2.1 (solid lines). The linear model with the same photospheric boundary conditions and magnetic helicity, confined within the radius r_L = 7.0 R, is shown for comparison (dotted lines). The magnetic bounding surface at r = r_L is also shown (dashed line).