Illustration of the formation and evolution of a simple bipolar AR from the emergence of an isolated magnetic flux tube. The local photospheric surface is shown as a gray transparent plane. Panel (a) summarizes graphically the evolution of magnetic flux (green color) over the lifetime of the bipolar AR. Initially (after the onset of emergence, t_start; panel (c)), the two opposite polarities (N and P) form a PIL and self-separate rapidly (separation distance and speed are overplotted as black and white curves, respectively). The self-separation of opposite flux progressively slows down until the end of the flux emergence phase, where the bipole has assumed its final (typically maximum) self-separation at the surface (t_end; panel (d)). The decay phase begins (t_decay; panel (e)) when surface effects spread and transport the flux from the initially compact polarity concentrations and lead them to cancel out and submerge at the PIL, leading to a reduction of flux observed at the surface.