  We examine the range of rest-mass densities, temperatures and magnetic fields
involved in simulations of binary neutron star mergers (BNSM) and identify the
conditions under which the ideal magneto-hydrodynamics (MHD) breaks down using
recently computed conductivities of warm, magnetized plasma created in such
systems. While previous dissipative MHD studies of BNSMs assumed that
dissipation sets in due to low conduction at low rest-mass densities, we show
that this paradigm must be shifted: the ideal MHD is applicable up to the
regime where the hydrodynamic description of matter breaks down. We also 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-density plane where the hydrodynamic
description breaks down.
