Magnetic field vector maps of nearby spiral galaxies
  We present a method for determining directions of magnetic field vectors in a
spiral galaxy using two synchrotron polarization maps, an optical image, and a
velocity field. The orientation of the transverse magnetic field is determined
with a synchrotron polarization map of higher frequency band and the
$180^\circ$-ambiguity is solved by using sign of the Rotation Measure (RM)
after determining geometrical orientation of a disk based on a assumption of
trailing spiral arms. The advantage of this method is that direction of
magnetic vector for each line of sight through the galaxy can be inexpensively
determined with easily available data and with simple assumptions. We applied
this method to three nearby spiral galaxies using archival data obtained with
the Very Large Array (VLA) to demonstrate how it works. The three galaxies have
both clockwise and counter-clockwise magnetic fields, which implies that all
three galaxies are not classified in simple Axis-Symmetric type but types of
higher modes and that magnetic reversals commonly exist.
