Left panel: diagram representing the initial conditions. The domain is periodic in the x-direction, with rigid boundaries in the y-direction after the perturbation is applied. The guide field B_z (out of the plane) is shown in blue, overlaid with field lines showing the x-component of the magnetic field B_x. The red dotted line shows the form of the perturbation applied at the boundary: a sinusoidal inflow and outflow pattern with red arrows indicating direction, equal and opposite at the upper and lower y boundaries for a short period of time (see Eq. (9)). Right panel: the initial force free magnetic field components.