  We study the global consequences in the halos of spiral galaxies of the
steady, axially symmetric, mean field dynamo. We use the classical theory but
add the possibility of using the velocity field components as parameters in
addition to the helicity and diffusivity. The analysis is based on the simplest
version of the theory and uses scale-invariant solutions. The velocity field
(subject to restrictions) is a scale invariant field in a `pattern' frame, in
place of a full dynamical theory. The `pattern frame' of reference may either
be the systemic frame or some rigidly rotating spiral pattern frame. One type
of solution for the magnetic field yields off-axis, spirally wound, magnetic
field lines. These predict sign changes in the Faraday screen rotation measure
in every quadrant of the halo of an edge-on galaxy. Such rotation measure
oscillations have been observed in the CHANG-ES survey.
