Interacting loops: images show specific interpolated magnetic field lines (traced from the base of the simulation domain, at in non-dimensional units), colour-coded by location in , at different stages of the experiment. Blue field lines are traced from regions where , red field lines are traced from only, while green field lines are traced from . Thus (prior to any reconnection) blue field lines are initially associated with the left-hand flux tube and green field lines with the right-hand tube, separated by red field lines. Purple isosurfaces (where present) indicate regions of current above the critical value.