  The extra stiffness that the magnetic field adds to the ISM changes the way
it reacts to the presence of a spiral perturbation. At intermediate to high z,
the gas shoots up before the arm, flows over, and falls behind it, as it
approaches the next arm. This generates a multicell circulation pattern, within
each of which the net radial mass flux is positive near the midplane and
negative at higher z. The flow distorts the magnetic field lines. In the arm
region, the gas flows nearly parallel to the arm, and therefore, the magnetic
field adopts a similar pitch angle. Between the arms, the gas flows out in
radius, generating a negative pitch angle in the magnetic field. The intensity
and direction of the field yield synthetic synchrotron maps that reproduce some
features of the synchrotron maps of external galaxies, like the islands of
emission and the displacement between the gaseous and synchrotron arms. When
comparing the magnitude of the field with the local gas density, two
distinctive relations appear, depending on whether the magnetic pressure is
dominant. Above the plane, the density structure develops a shape resembling a
breaking wave. This structure collapses and rises again with a period of about
60 Myr, similar to that of a vertical oscillation mode. The falling gas plays
an important part in the overall hydrostatics, since its deceleration
compresses the low z gas, raising the average midplane pressure in the interarm
region above that provided by the weight of the material above.
