From Ulysses observations of magnetic field and plasma properties during its rapid pole-to-pole scan, we determine the causes of variations in the magnitude of the radial component of the heliospheric magnetic field (HMF). We find that the ambient radial HMF in the slow solar wind is a combination of the ambient radial HMF components of the two hemispheres with opposite polarities. It suggests that the source region of the slow solar wind has the same magnetic topology as the source region of the fast solar wind, i.e., both are open field regions. ©1999 American Institute of Physics.