ν_∥ (left) and u_∥/c₀ (right) shown in the top panel for the strong field and radial driving case at t = 58.5 s. In the middle panel, both components are overplotted by black lines of ν_∥ = ±0.5. In the bottom panel, both plots are overplotted by black lines of u_∥/c₀ = ±0.01. Red, black (non-contours) and white lines are as in Figure 2. Note: we have only kept the magnetic field lines anchored at the piston edges (black lines) and the β = 1 line for the two bottom plots for simplicity. We also note that the colour range is different for the two variables.