The JCMT BISTRO Survey: The Magnetic Field of the Barnard 1 Star-Forming
  Region
  We present the POL-2 850 $\mu$m linear polarization map of the Barnard 1
clump in the Perseus molecular cloud complex from the B-fields In STar-forming
Region Observations (BISTRO) survey at the James Clerk Maxwell Telescope. We
find a trend of decreasing polarization fraction as a function of total
intensity, which we link to depolarization effects towards higher density
regions of the cloud. We then use the polarization data at 850 $\mu$m to infer
the plane-of-sky orientation of the large-scale magnetic field in Barnard 1.
This magnetic field runs North-South across most of the cloud, with the
exception of B1-c where it turns more East-West. From the dispersion of
polarization angles, we calculate a turbulence correlation length of $5.0 \pm
2.5$ arcsec ($1500$ au), and a turbulent-to-total magnetic energy ratio of $0.5
\pm 0.3$ inside the cloud. We combine this turbulent-to-total magnetic energy
ratio with observations of NH$_3$ molecular lines from the Green Bank Ammonia
Survey (GAS) to estimate the strength of the plane-of-sky component of the
magnetic field through the Davis-Chandrasekhar-Fermi method. With a
plane-of-sky amplitude of $120 \pm 60$ $\mu$G and a criticality criterion
$\lambda_c = 3.0 \pm 1.5$, we find that Barnard 1 is a supercritical molecular
cloud with a magnetic field nearly dominated by its turbulent component.
