The right hand side of the previous equation can be considered in two parts: j × × B = j z ( z ^ ^ × × B ⊥ ⊥ ) + j ⊥ ⊥ × × z ^ ^ B z , where the ⊥ ⊥ subscript denotes the component in the plane perpendicular to the z -axis. The z component of the current in the above equation can be written in terms of the one-dimensional vector potential as j z = − − 1 μ μ 0 ∇ ∇ 2 A .