  The linear polarization of thermal dust emission provides a powerful tool to
probe interstellar and circumstellar magnetic fields, because aspherical grains
tend to align themselves with magnetic field lines. While the Radiative
Alignment Torque (RAT) mechanism provides a theoretical framework to this
phenomenon, some aspects of this alignment mechanism still need to be
quantitatively tested. One such aspect is the possibility that the reference
alignment direction changes from the magnetic field ("B-RAT") to the radiation
field k-vector ("k-RAT") in areas of strong radiation fields. We investigate
this transition toward the Orion Bar PDR, using multi-wavelength SOFIA HAWC+
dust polarization observations. The polarization angle maps show that the
radiation field direction is on average not the preferred grain alignment axis.
We constrain the grain sizes for which the transition from B-RAT to k-RAT occur
in the Orion Bar (grains > 0.1 {\mu}m toward the most irradiated locations),
and explore the radiatively driven rotational disruption that may take place in
the high-radiation environment of the Bar for large grains. While the grains
susceptible to rotational disruption should be in supra-thermal rotation and
aligned with the magnetic field, k-RAT aligned grains would rotate at thermal
velocities. We find that the grain size at which the alignment shifts from
B-RAT to k-RAT corresponds to grains too large to survive the rotational
disruption. Therefore, we expect a large fraction of grains to be aligned at
supra-thermal rotation with the magnetic field, and potentially be subject to
rotational disruption depending on their tensile strength.
