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Magnetic fields play essential roles in protoplanetary disks. Magnetic fieldsin the disk atmosphere are of particular interest, as they are connected to thewind-launching mechanism. In this work, we study the polarization of the lightscattered off of magnetically aligned grains in the disk atmosphere, focusingon the deviation of the polarization orientation from the canonical azimuthaldirection, which may be detectable in near-IR polarimetry with instruments suchas VLT/SPHERE. We show with a simple disk model that the polarization can evenbe oriented along the radial (rather than azimuthal) direction, especially inhighly inclined disks with toroidally dominated magnetic fields. Thispolarization reversal is caused by the anisotropy in the polarizibility ofaligned grains and is thus a telltale sign of such grains. We show that thenear-IR light is scattered mostly by $\mu$m-sized grains or smaller at the$\tau=1$ surface and such grains can be magnetically aligned if they containsuperparamagnetic inclusions. For comparison with observations, we generatesynthetic maps of the ratios of $U_\phi/I$ and $Q_\phi/I$, which can be used toinfer the existence of (magnetically) aligned grains through a negative$Q_\phi$ (polarization reversal) and/or a significant level of $U_\phi/I$. Weshow that two features observed in the existing data, an asymmetricdistribution of $U_\phi$ with respect to the disk minor axis and a spatialdistribution of $U_\phi$ that is predominantly positive or negative, areincompatible with scattering by spherical grains in an axisymmetric disk. Theyprovide indirect evidences for scattering by aligned non-spherical grains.

Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized Optical/IR Light Scattered by Aligned Grains