Interaction of the magnetorotational instability with hydrodynamic turbulence in accretion disks

Abstract

Accretion disks in which angular momentum transport is dominated by the magnetorotational instability (MRI) can also possess additional, purely hydrodynamic, drivers of turbulence. Even when the hydrodynamic processes, on their own, generate negligible levels of transport, they may still affect the evolution of the disk via their influence on the MRI. Here, we study the interaction between the MRI and hydrodynamic turbulence using local MRI simulations that include hydrodynamic forcing. As expected, we find that hydrodynamic forcing is generally negligible if it yields a saturated kinetic energy density that is small compared to the value generated by the MRI. For stronger hydrodynamic forcing levels, we find that hydrodynamic turbulence modifies transport, with the effect varying depending upon the spatial scale of hydrodynamic driving. Large scale forcing boosts transport by an amount that is approximately linear in the forcing strength, and leaves the character of the MRI (for example the ratio between Maxwell and Reynolds stresses) unchanged, up to the point at which the forced turbulence is an order of magnitude stronger than that generated by the MRI. Low amplitude small scale forcing may modestly suppress the MRI. We conclude that the impact of hydrodynamic turbulence on the MRI is generically ignorable in cases, such as convection, where the additional turbulence arises due to the accretion energy liberated by the MRI itself. Hydrodynamic turbulence may affect (and either enhance or suppress) the MRI if it is both strong, and driven by independent mechanisms such as self-gravity, supernovae, or solid-gas interactions in multiphase protoplanetary disks.