Recent studies have shown that the heliospheric magnetic field can be much more radial than the standard Parker spiral in the rarefactions of co‐rotation interaction regions (CIRs). This effect can be explained by the motion of open magnetic field foot points at the Sun across the coronal hole boundary. Such motion causes magnetic field lines to be connected and stretched across the CIR's fast‐slow wind transition, creating a less transverse, “sub‐Parker” magnetic field spiral. Here we include foot point motion at the Sun in a steady‐state magneto‐hydrodynamic (MHD) model to quantify the formation and evolution of such sub‐Parker spirals out to 5 AU. We compare these results with Ulysses CIR observations during its first polar orbit, and find that sub‐Parker spirals consistent with our model are almost always present in CIR rarefactions. Thus, we explain the existence and frequent occurrence of the sub‐Parker spiral structure of heliospheric magnetic field.