  (abridged) Magnetic fields are fundamental to the accretion dynamics of
protoplanetary disks and they likely affect planet formation. Typical methods
to study the magnetic field morphology observe the polarization of dust or
spectral lines. However, it has recently become clear that dust-polarization in
ALMA's spectral regime does not always faithfully trace the magnetic field
structure of protoplanetary disks, which leaves spectral line polarization as a
promising method for mapping the magnetic field morphologies of such sources.
We aim to model the emergent polarization of different molecular lines in the
ALMA wavelength regime that are excited in protoplanetary disks. We explore a
variety of disk models and molecules to identify those properties that are
conducive to the emergence of polarization in spectral lines and may therefore
be viably used for magnetic field measurements in protoplanetary disks. We used
PORTAL in conjunction with LIME. Together, they allowed us to treat the
polarized line radiative transfer of complex three-dimensional physical and
magnetic field structures. We present simulations of the emergence of spectral
line polarization of different molecules and molecular transitions in the ALMA
wavelength regime. We find that molecules that thermalize at high densities,
such as HCN, are also the most susceptible to polarization. We find that such
molecules are expected to be significantly polarized in protoplanetary disks,
while molecules that thermalize at low densities, such as CO, are only
significantly polarized in the outer disk regions. We present the simulated
polarization maps at a range of inclinations and magnetic field morphologies,
and we comment on the observational feasibility of ALMA linear polarization
observations of protoplanetary disks.
