Panel (a): 2D sketch representing the “topological” path (orange solid line). Magnetic closed-line apexes are denoted by the dark-edged rhomboids, and the starting point is denoted by the red cross. Panel (b): sketch of the 3D scenario. Only magnetic field lines crossing the orange cylinders are considered. The apex of each magnetic closed line is denoted by the rhomboids as in panel (a), and edged rhomboids represent the minima of the apexes inside the cylinder. Panel (c): example of the “topological” path (the orange line connecting the orange dots) calculated in a 2D example from the initial position e₀. The gray lines indicate closed field lines in a pseudostreamer topology (and helmet streamer to the left) and the blue lines are open field lines.