Magnetic field structure of the Galactic plane from differential
  analysis of interstellar polarization
  A new method for measuring the global magnetic field structure of the
Galactic plane is presented. We have determined the near-infrared polarization
of field stars around 52 Cepheids found in recent surveys toward the Galactic
plane. The Cepheids are located at the galactic longitudes $-10^{\circ}\leq \,
l\, \leq +10.5^{\circ}$ and latitudes $-0.22^{\circ}\leq \, l\, \leq
+0.45^{\circ}$, and their distances are mainly in the range of 10 to 15 kpc
from the Sun. Simple classification of the sightlines is made with the
polarization behavior vs. $H-K_{\mathrm S}$ color of field stars, and typical
examples of three types are presented. Then, division of the field stars in
each line of sight into (a) foreground, (b) bulge, and (c) background is made
with the $Gaia$ DR2 catalog, the peak of the $H-K_{\mathrm S}$ color histogram,
and $H-K_{\mathrm S}$ colors consistent with the distance of the Cepheid in the
center, respectively. Differential analysis between them enables us to examine
the magnetic field structure more definitely than just relying on the
$H-K_{\mathrm S}$ color difference. In one line of sight, the magnetic field is
nearly parallel to the Galactic plane and well aligned all the way from the Sun
to the Cepheid position on the other side of the Galactic center. Contrary to
our preconceived ideas, however, sightlines having such well-aligned magnetic
fields in the Galactic plane are rather small in number. At least 36 Cepheid
fields indicate random magnetic field components are significant. Two Cepheid
fields indicate that the magnetic field orientation changes more than 45 in the
line of sight. The polarization increase per color change $P$/ ($H-K_{\mathrm
S}$) varies from region to region, reflecting the change in the ratio of the
magnetic field strength and the turbulence strength.
