A Far-Infrared Observational Test of the Directional Dependence in
  Radiative Grain Alignment
  The alignment of interstellar dust grains with magnetic fields provides a key
method for measuring the strength and morphology of the fields. In turn, this
provides a means to study the role of magnetic fields from diffuse gas to dense
star-forming regions. The physical mechanism for aligning the grains has been a
long-term subject of study and debate. The theory of radiative torques, in
which an anisotropic radiation field imparts sufficient torques to align the
grains while simultaneously spinning them to high rotational velocities, has
passed a number of observational tests. Here we use archival polarization data
in dense regions of the Orion molecular cloud (OMC-1) at 100, 350, and
$850\,\mu$m to test the prediction that the alignment efficiency is dependent
upon the relative orientations of the magnetic field and radiation anisotropy.
We find that the expected polarization signal, with a 180-degree period, exists
at all wavelengths out to radii of 1.5 arcminutes centered on the BNKL object
in OMC-1. The probabilities that these signals would occur due to random noise
are low ($\lesssim$1\%), and are lowest towards BNKL compared to the rest of
the cloud. Additionally, the relative magnetic field to radiation anisotropy
directions accord with theoretical predictions in that they agree to better
than 15 degrees at $100\,\mu$m and 4 degrees at $350\,\mu$m.
