  The density profiles of dark halos are usually inferred from the rotation
curves of disk galaxies based on the assumption that the gas is a good tracer
of the gravitational potential of the galaxies. Some authors have suggested
that magnetic pinching forces could alter significantly the rotation curves of
spiral galaxies. In contrast to other studies which have concentrated in the
vertical structure of the disk, here we focus on the problem of magnetic
confinement in the radial direction to bound the magnetic effects on the HI
rotation curves. It is shown that azimuthal magnetic fields can hardly speed up
the HI disk of galaxies as a whole. In fact, based on virial constraints we
show that the contribution of galactic magnetic fields to the rotation curves
cannot be larger than ~10 km/s at the outermost point of HI detection, if the
galaxies did not contain dark matter at all, and up to 20 km/s in the
conventional dark halo scenario. The procedure to estimate the maximum effect
of magnetic fields is general and applicable to any particular galaxy disk. The
inclusion of the surface terms, namely the intergalactic (thermal, magnetic or
ram) pressure, does not change our conclusions. Other problems related with the
magnetic alternative to dark halos are highlighted. The relevance of magnetic
fields in the cuspy problem of dark halos is also discussed.
