Magnetic fields in the infrared dark cloud G34.43+0.24
  We present the B-fields mapped in IRDC G34.43+0.24 using 850\,$\mu$m
polarized dust emission observed with the POL-2 instrument at JCMT. We examine
the magnetic field geometries and strengths in the northern, central, and
southern regions of the filament. The overall field geometry is ordered and
aligned closely perpendicular to the filament's main axis, particularly in
regions containing the central clumps MM1 and MM2, whereas MM3 in the north has
field orientations aligned with its major axis. The overall field orientations
are uniform at large (POL-2 at 14$\arcsec$ and SHARP at 10$\arcsec$) to small
scales (TADPOL at 2.5$\arcsec$ and SMA at 1.5$\arcsec$) in the MM1 and MM2
regions. SHARP/CSO observations in MM3 at 350\,$\mu$m from Tang et al. show a
similar trend as seen in our POL-2 observations. TADPOL observations
demonstrate a well-defined field geometry in MM1/MM2 consistent with MHD
simulations of accreting filaments. We obtained a plane-of-sky magnetic field
strength of 470$\pm$190\,$\mu$G, 100$\pm$40\,$\mu$G, and 60$\pm$34\,$\mu$G in
the central, northern and southern regions of G34, respectively, using the
updated Davis-Chandrasekhar-Fermi relation. The estimated value of field
strength, combined with column density and velocity dispersion values available
in the literature, suggests G34 to be marginally critical with criticality
parameter $\rm \lambda$ values 0.8$\pm$0.4, 1.1$\pm$0.8, and 0.9$\pm$0.5 in the
central, northern, and southern regions, respectively. The turbulent motions in
G34 are sub-Alfv\'{e}nic with Alfv\'{e}nic Mach numbers of 0.34$\pm$0.13,
0.53$\pm$0.30, and 0.49$\pm$0.26 in the three regions. The observed aligned
B-fields in G34.43+0.24 are consistent with theoretical models suggesting that
B-fields play an important role in guiding the contraction of the cloud driven
by gravity.
