Vertical component of the magnetic field (top panel) and vertical component of the current density (bottom panel) calculated at the bottom boundary of the simulation box, at t = 608.6 s, for the case when plasma is rotated over the negative polarity N1 and at the same time moved southward over the positive polarity P11–P12. The contour lines at the bottom panel show the position of the negative (white line) and positive (black line) polarities.