Polarisation of oscillations for different field lines within the domain at t = 1100 s. Sixteen field lines are selected on a grid at positions indicated by the labels at the top and right-hand side of the figure. Three hundred points are selected along each field line and depicted with a colour that shows their position along the field line as a percentage of the total field line length. In order to show the direction of polarisation, the horizontal velocity (v_y against v_x) is shown for each point. Notice the large resonance for row 3 column 3.