Diagram representations of coronal field extrapolation models. (a) The PFSS model assumes a “source surface” at which all magnetic field lines are open and radial. Moving the PFSS source surface to a lower height increases the open flux because it forces more higher-order structures to become open. (b) The HCCSSS model includes a “cusp surface,” which we explain in the text. Moving this cusp surface in or out also affects the open flux calculated by this model.