Magnetic arms generated by multiple interfering galactic spiral patterns
  Interfering two- and three-arm spiral patterns have previously been inferred
to exist in many galaxies and also in numerical simulations, and invoked to
explain important dynamical properties, such as lack of symmetry, kinks in
spiral arms, and star formation in armlets. The non-axisymmetric galactic
mean-field dynamo model of Chamandy et al. is generalized to allow for such
multiple co-existing spiral patterns in the kinetic alpha_k effect, leading to
the existence of magnetic spiral arms in the large-scale magnetic field with
several new properties. The large-scale magnetic field produced by an evolving
superposition of two- and three-arm (or two- and four-arm) patterns evolves
with time along with the superposition. Magnetic arms can be stronger and more
extended in radius and in azimuth when produced by two interfering patterns
rather than by one pattern acting alone. Transient morphological features arise
in the magnetic arms, including bifurcations, disconnected armlets, and
temporal and spatial variation in arm strength and pitch angles. Pitch angles
of the large-scale magnetic field and magnetic arm structures (ridges) are
smaller than those typically inferred from observations of spiral galaxies for
model parameters of Chamandy et al., but can become comparable to typically
inferred values for certain (still realistic) parameters. The magnetic field is
sometimes strongest in between the alpha_k-arms, unlike in standard models with
a single pattern, where it is strongest within the alpha_k-arms. Moreover, for
models with a two- and three-arm pattern, some amount of m=1 azimuthal symmetry
is found to be present in the magnetic field, which is generally not the case
for forcing by single two- or three-arm patterns. (abridged)
