CHANG-ES XV: Large-scale magnetic field reversals in the radio halo of
  NGC 4631
  NGC 4631 is an interacting galaxy which exhibits one of the largest gaseous
halos observed among edge-on galaxies. We aim to examine the synchrotron and
polarization properties of its disk and halo emission with new radio continuum
data. Radio continuum observations of NGC 4631 were performed with the Karl G.
Jansky Very Large Array at C-band (5.99 GHz) in the C & D array configurations,
and at L-band (1.57 GHz) in the B, C, & D array configurations. The Rotation
Measure Synthesis algorithm was utilized to derive the polarization properties.
  We detected linearly polarized emission at C-band and L-band. The magnetic
field in the halo is characterized by strong vertical components above and
below the central region of the galaxy. The magnetic field in the disk is only
clearly seen in the eastern side of NGC 4631, where it is parallel to the plane
of the major axis of the galaxy. We detected for the first time a large-scale,
smooth Faraday depth pattern in a halo of an external spiral galaxy, which
implies the existence of a regular (coherent) magnetic field.
  A quasi-periodic pattern in Faraday depth with field reversals was found in
the northern halo of the galaxy. The field reversals in the northern halo of
NGC 4631, together with the observed polarization angles, indicate giant
magnetic ropes (GMRs) with alternating directions. To our knowledge, this is
the first time such reversals are observed in an external galaxy.
