  The origin of polarized emission from protoplanetary disks is uncertain.
Three mechanisms have been proposed for such polarized emission: grain
alignment with magnetic fields, grain alignment with radiation gradients, and
self-scattering of thermal dust emission. Aiming to observationally identify
the polarization mechanisms, we present ALMA polarization observations of the
0.87 mm dust continuum emission toward the circumstellar disk around HD 142527
with high spatial resolution. We confirm that the polarization vectors in the
northern region are consistent with self-scattering. Furthermore, we show that
the polarization vectors in the southern region are consistent with grain
alignment by magnetic fields, although self-scattering cannot be ruled out. To
understand the differences between the polarization mechanisms, we propose a
simple grain size segregation model: small dust grains ($\lesssim$ 100 microns)
are dominant and aligned with magnetic fields in the southern region, and
middle-sized ($\sim100$ microns) grains in the upper layer emit self-scattered
polarized emission in the northern region. The grain size near the middle plane
in the northern region cannot be measured because the emission at 0.87 mm is
optically thick. However, it can be speculated that larger dust grains
($\gtrsim$ cm) may accumulate near this plane. These results are consistent
with those of a previous analysis of the disk, in which large grain
accumulation and optically thick emission from the northern region were found.
This model is also consistent with theories where smaller dust grains are
aligned with magnetic fields. The magnetic fields are toroidal, at least in the
southern region.
