Milky Way: circular velocity for a distribution of the regular magnetic field with an exponential profile (solid), compared to the profile given by Eq. (7). The exponential scale length is 10 kpc, and r_m = 7 kpc. One cannot distinguish the two profile shapes from a measurement of the (normalised) polarised intensity in the inner 10 kpc. The density distribution of Kalberla & Dedes (2008) is marked with crosses.