Cuts through several source surfaces used in our PFSS models. Prolate source surfaces (red shapes) resemble zeppelin-shapes aligned with the solar rotation axis elongated along one axis, whereas oblate source surfaces (blue shapes) resemble thick disks elongated along two axes. The classical spherical source surface is drawn with a dotted black line. Various symbols introduced in the legend are used in Figs. 3–5, where they are shown at the corresponding positions.