Do we need non-ideal magnetohydrodynamics to model protostellar discs?
  We investigate and discuss protostellar discs in terms of where the various
non-ideal magnetohydrodynamics (MHD) processes are important. We find that the
traditional picture of a magnetised disc (where Ohmic resistivity is dominant
near the mid-plane, surrounded by a region dominated by the Hall effect, with
the remainder of the disc dominated by ambipolar diffusion) is a great
oversimplification. In simple parameterised discs, we find that the Hall effect
is typically the dominant term throughout the majority of the disc. More
importantly, we find that in much of our parameterised discs, at least two
non-ideal processes have coefficients within a factor of 10 of one another,
indicating that both are important and that naming a dominant term underplays
the importance of the other terms. Discs that were self-consistently formed in
our previous studies are also dominated by the Hall effect, and the ratio of
ambipolar diffusion and Hall coefficients is typically less than 10, suggesting
that both terms are equally important and listing a dominant term is
misleading. These conclusions become more robust once the magnetic field
geometry is taken into account. In agreement with the literature we review, we
conclude that non-ideal MHD processes are important for the formation and
evolution of protostellar discs. Ignoring any of the non-ideal processes,
especially ambipolar diffusion and the Hall effect, yields an incorrect
description of disc evolution.
