  It is generally assumed that magnetic fields play an important role in the
formation and evolution of protoplanetary disks. One way of observationally
constraining magnetic fields is to measure polarized emission and absorption
produced by magnetically aligned elongated dust grains. The fact that radiation
also becomes linearly polarized by light scattering at optical to millimeter
wavelengths complicates magnetic field studies. We characterize the linear
polarization of mid-infrared radiation due to scattering of the stellar
radiation and dust thermal re-emission radiation (self-scattering). We find
that the thermal re-emission radiation is stronger than the scattered stellar
radiation for disks with inner holes smaller than 10 au within the considered
parameter range. The mid-infrared polarization due to scattering shows several
clear trends: For scattered stellar radiation only, the linear polarization
degree decreases slightly with increasing radial distance, while it increases
with radial distance for thermal re-emission radiation only and for a
combination of scattered stellar radiation and thermal re-emission radiation.
The linear polarization degree decreases with increasing disk flaring and
luminosity of the central star. An increasing inner radius shifts the increase
of the linear polarization degree further outside, while a larger scale height
increases the linear polarization degree for small radial distances and
decreases this degree further outside. For longer wavelengths, the linear
polarization degree converges more slowly.
