The flux-surface functions
are directly related to the lowest-order perpendicular
velocities of the various plasma species in the island rest frame. In fact, as is clear from Sections 4.2, 5.2, and 8.2,
refer to the components of the perpendicular electron, MHD, and ion
fluid velocities, respectively. Note that , , and are all zero inside the magnetic separatrix of the island chain.