(a) FR and CH scheme not to scale. The turquoise frame indicates the simulated region. h₀ is the FR (orange circle) height and D is the FR distance to the CH (purple-shaded area) whose width is parametrised by W. The line dipole and the image current are located at depth d and h₀, respectively. (b) Current wire layers: 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.