Energetics in MRI driven turbulence

In these proceedings we present recent efforts to understand the energetics of magneto-hydrodynamic (MHD) turbulence driven by the magneto-rotational instability (MRI). These studies are carried out in the local (shearing box) approximation using the Athena simulation code. Athena is a higher order Godunov algorithm based on the piecewise parabolic method (PPM), the corner transport upwind (CTU) integration algorithm, and the constrained transport (CT) algorithm for evolving the magnetic field. This algorithm is particularly suited for these studies owing to the conservation properties of a Godunov scheme and the particular implementation of the shearing box source terms used here. We present a variety of calculations which may be compared directly to previously published results and discuss them in some detail. The only significant discrepancy found between the results presented here and in the published literature involves the turbulent heating rate. We observe the presence of recurrent channel solutions in calculations involving a mean vertical magnetic field and the associated time lag between the energy injection and thermalization rate. We also present the results of a shearing box calculation which includes an optically thin radiative term with a cooling rate selected to match the turbulent heating rate. Some properties which we find uniformly present in all of the calculations presented here are compressible fluctuations, spiral waves and weak shocks. It is found that these compressible modes dominate the temporal fluctuations in the probability distribution functions for most of the thermodynamic variables; only the specific entropy is relatively immune to their effects.