We quantify the effects of magnetic fields, cosmic rays and gas pressure on the rotational velocity of HI gas in the Milky Way, at galactic distances between Rsun and 2Rsun. The magnetic field is modelled by two components; a mainly azimuthal magnetic component and a small-scale tangled field. We construct a range of plausible axisymmetric models consistent with the strength of the total magnetic field as inferred from radio synchrotron data. In a realistic Galactic disk, the pressure by turbulent motions, cosmic rays and the tangled turbulent field provide radial support to the disk. Large-scale (ordered) magnetic fields may or may not provide support to the disk, depending on the local radial gradient of the azimuthal field. We show that for observationally constrained models, magnetic forces cannot appreciably alter the tangential velocity of HI gas within a galactic distance of 2Rsun.

11 pages, 8 figures, accepted for publication in MNRAS