Panel a: evolution of the y-axis components of the gas pressure gradient (−∇P_y, red solid curve), Lorentz force (LF_y, blue solid curve), magnetic tension (T_y, blue dashed curve), and magnetic pressure gradient (−∇Pm_y, blue dash-dotted curve) averaged over the rectangular region shown in Fig. 2. Panel b: evolution of the x- and y-axis components of the velocity averaged over the rectangular region shown in Fig. 2.