Evolution of | v_B | at the proper null, which shows a longitudinal daughter disturbance, in the xz-plane with y = 0. It is nonlinear and everywhere cospatial to the Alfvén wave. It arises due to the action of the Alfvén wave’s ponderomotive force, which carries the disturbance along its path as it propagates. The daughter disturbance does not impact upon the medium as it passes through.