Physical interpretation of field line helicity in the DeVore gauge. The red circles show the direction of A₀ (contours of ψ) arising from a strong magnetic source B_r> 0. The shaded surfaces are radial projections of the field lines L₁ and L₂. Both field lines have a contribution to from any flux linking through these surfaces (owing to the second term of Eq. (4)), but only L₁ has a contribution from A₀, since the projection of L₂ is perpendicular to A₀.