  We seek the conditions for a {\it steady} mean field galactic dynamo. The
parameter set is reduced to those appearing in the $\alpha^2$ and
$\alpha/\omega$ dynamo, namely velocity amplitudes, and the ratio of sub-scale
helicity to diffusivity. The parameters can be allowed to vary on conical
spirals. We analyze the mean field dynamo equations in terms of scale invariant
logarithmic spiral modes and special exact solutions. Compatible scale
invariant gravitational spiral arms are introduced and illustrated in an
appendix, but the detailed dynamical interaction with the magnetic field is
left for another work. As a result of planar magnetic spirals `lifting' into
the halo, multiple sign changes in average rotation measures forming a regular
pattern on each side of the galactic minor axis, are predicted. Such changes
have recently been detected in the CHANG-ES survey.
