Ulysses' observations of the heliospheric magnetic field have shown that, above a narrow low-latitude region, the intensity of the radial component of the field is independent of latitude. Here we present a theoretical explanation of this behaviour based on an MHD model of the solar wind applicable at solar minimum, which corresponds to the Ulysses' pole to pole pass. A discussion of other models' predictions is also presented. ¿ American Geophysical Union 1996