Magnetic field lines originating from the bottom left footpoint (dark grey) and from the upper right footpoint (light grey) are shown at t = 60t_A for L_uni (top) and L_mix (bottom). At the onset of instability, two plots are shown: one with isosurfaces of current (left) and the other with isosurfaces of |    σ^∗    |, a proxy for viscous heating (right).