Panel a: evolution of the y-axis components of the gas pressure gradient and Lorentz force averaged over the rectangular region shown in Fig. 2 for the simulations with clump length L = 3 Mm and infinite. Panel b: evolution of the x- and y-axis components of the velocity averaged over the rectangular region shown in Fig. 2 for the simulations with clump length L = 3 Mm and infinite. Panel c: map of the number density n at t = 190 s for the simulation with infinite clump length. Green lines are magnetic field lines drawn from the left-hand boundary.