Time–distance plot of vertical slice V1 at y = 2.6 Mm (left panel, case NS) and y = 6.1 Mm (right panel, case S). See Figure 4 for slice references. MSFs of 3.5 and 5 mHz are displayed for parallel velocity, including harmonics. The dashed lines over-plotted represent the trajectory of an object moving at the Alfvén speed. For this, the average local Alfvén speed is calculated for the specified times for z = 1.5–2.5 Mm, and for z = 2.5–6.5 Mm.