  We examine the range of rest-mass densities, temperatures and magnetic fields
involved in simulations of binary neutron-star mergers and identify the
conditions under which the ideal-magnetohydrodynamics approximation breaks down
and hence the magnetic-field decay should be accounted for. We use recent
calculations of the conductivities of warm correlated plasma in envelopes of
compact stars and find that the magnetic-field decay timescales are much larger
than the characteristic timescales of the merger process for lengthscales down
to a meter. Because these are smaller than the currently available resolution
in numerical simulations, the ideal-magnetohydrodynamics approximation is
effectively valid for all realistic simulations. At the same time, we find that
the Hall effect can be important at low densities and low temperatures, where
it can induce a non-dissipative rearrangement of the magnetic field. Finally,
we mark the region in temperature and density where the hydrodynamic
description breaks down.
