Collisional polarization of molecular ions: a signpost of ambipolar
  diffusion
  Magnetic fields play a role in the dynamics of many astrophysical processes,
but they are hard to detect. In a partially ionized plasma, a magnetic field
works directly on the ionized medium but not on the neutral medium, which gives
rise to a velocity drift between them: ambipolar diffusion. This process is
suggested to be important in the process of star formation, but has never been
directly observed. We introduce a method that could be used to detect ambipolar
diffusion and the magnetic field that gives rise to it, where we exploit the
velocity drift between the charged and neutral medium. By using a
representative classical model of the collision dynamics, we show that
molecular ions partially align themselves when a velocity drift is present
between the molecular ion and its main collision partner H2. We demonstrate
that ambipolar diffusion potently aligns molecular ions in regions denser than
their critical density, which subsequently leads to partially polarized
emission from these species. We include a model for HCO+ and show that
collisional polarization could be detectable for the ambipolar drifts predicted
by numerical simulations of the inner protostellar disk regions. The
polarization vectors are aligned perpendicular to the magnetic field direction
projected on the plane of the sky.
