(a) FR (orange circle) and PS (red structure) scheme, not to scale. The green frame (top) represents the simulated domain and the grey shaded area (bottom) contains the magnetic components that are outside of the simulation box. h₀ is the FR height and x_PS is the distance from the FR to the PS spine (red vertical line), whose height is parametrised by y_PS. The line dipole and the image current are located at depth d and h₀, respectively. (b) Internal structure of the FR: r is the radius, Δ is the thickness of the transition layer, R₋ is the radial coordinate from the FR centre, and ϕ the poloidal coordinate.