Galactic magnetic X fields
  Our aim in this note is to compare a recent explanation of the galactic X
pattern in Faraday Rotation (XRM), to that produced by the advection part of
the classical dynamo.We find that the characteristic X magnetic field
polarization in the plane of the sky, found in edge-on spiral galaxies, can
develop magnetohydromagnetically from an initial disc magnetic field combined
with wind and rotation. The Rotation Measure develops a corresponding X
distribution in sign, but this distribution is not a `universal' behaviour
because it depends primarily on the velocity field. We use Cauchy evolution of
an initial magnetic field to find the field at some later time and place. A
`battery' mechanism that requires current to always flow out of a galaxy has
been recently suggested, which contrasts with the conclusions of this paper.
Either explanation has significant consequences for the structure of a spiral
galaxy. If the battery mechanism applies then we have a new method of producing
magnetic field independent of the traditional dynamo. If however the flow
mechanism applies; then constraints concerning the presence and nature of a
galactic wind, together with the signature of the mean radial magnetic field,
can be inferred.
