  We propose a simple ansatz for estimating the value of the numerical
resistivity and the numerical viscosity of any Eulerian MHD code. We test this
ansatz with the help of simulations of the propagation of (magneto)sonic waves,
Alfven waves, and the tearing mode instability using the MHD code Aenus. By
comparing the simu- lation results with analytical solutions of the
resistive-viscous MHD equations and an empirical ansatz for the growth rate of
tearing modes we measure the numerical viscosity and resistivity of Aenus. The
comparison shows that the fast-magnetosonic speed and wavelength are the
characteristic velocity and length, respectively, of the aforementioned
(relatively simple) systems. We also determine the dependance of the numerical
viscosity and resistivity on the time integration method, the spatial
reconstruction scheme and (to a lesser extent) the Riemann solver employed in
the simulations. From the measured results we infer the numerical resolution
(as a function of the spatial reconstruction method) required to properly
resolve the growth and saturation level of the magnetic field amplified by the
magnetorotational instability in the post-collapsed core of massive stars. Our
results show that it is to the best advantage to resort to ultra-high order
methods (e.g., 9th-order Monotonicity Preserving method) to tackle this problem
properly, in particular in three dimensional simulations.
