Imposed velocity drivers. Upper left: schematic of the drivers imposed on each of the foot points of magnetic field lines. Upper right: imposed velocity field on the lower boundary of the medium amplitude AC simulation. Centre: temporal evolution of the magnitude of the imposed driver on the lower boundary of the medium amplitude AC simulation on the line y = 0 Mm. Lower: temporal evolution of the magnitude of the imposed driver on the lower boundary of the medium amplitude DC simulation on the line y = 0 Mm.