  The influence of magnetic fields (B-fields) in the formation and evolution of
bipolar bubbles, due to the expanding ionization fronts (I-fronts) driven by
the Hii regions that are formed and embedded in filamentary molecular clouds,
has not been well-studied yet. In addition to the anisotropic expansion of
I-fronts into a filament, B-fields are expected to introduce an additional
anisotropic pressure which might favor expansion and propagation of I-fronts to
form a bipolar bubble. We present results based on near-infrared polarimetric
observations towards the central $\sim$8'$\times$8' area of the star-forming
region RCW57A which hosts an Hii region, a filament, and a bipolar bubble.
Polarization measurements of 178 reddened background stars, out of the 919
detected sources in the JHKs-bands, reveal B-fields that thread perpendicular
to the filament long axis. The B-fields exhibit an hour-glass morphology that
closely follows the structure of the bipolar bubble. The mean B-field strength,
estimated using the Chandrasekhar-Fermi method, is 91$\pm$8 {\mu}G. B-field
pressure dominates over turbulent and thermal pressures. Thermal pressure might
act in the same orientation as those of B-fields to accelerate the expansion of
those I-fronts. The observed morphological correspondence among the B-fields,
filament, and bipolar bubble demonstrate that the B-fields are important to the
cloud contraction that formed the filament, gravitational collapse and star
formation in it, and in feedback processes. The latter include the formation
and evolution of mid-infrared bubbles by means of B-field supported propagation
and expansion of I-fronts. These may shed light on preexisting conditions
favoring the formation of the massive stellar cluster in RCW57A.
