Abstract
The magnetorotational instability (MRI) is believed to be responsible for most of the angular momentum transport in accretion discs. However, molecular dissipation processes may drastically change the efficiency of MRI turbulence in realistic astrophysical situations. The physical origin of this dependency is still poorly understood as linear and quasi linear theories fail to explain it. In this paper, we look for the link between molecular dissipation processes and MRI transport of angular momentum in non stratified shearing box simulations including a mean vertical field. We show that magnetic helicity is unimportant in the model we consider. We perform a spectral analysis on the simulations tracking energy exchanges in spectral space when turbulence is fully developed. We find that the energy exchanges are essentially direct (from large to small scale) whereas some non linear interactions appear to be non local in spectral space. We speculate that these non local interactions are responsible for the correlation between turbulent transport and molecular dissipation. We argue that this correlation should then disappear when a significant scale separation is achieved and we discuss several methods by which one can test this hypothesis.
Highlights
The transport of angular momentum in astrophysical discs is a central problem in accretion theory
We find that the energy exchanges are essentially direct whereas some non-linear interactions appear to be non-local in spectral space
magnetorotational instability (MRI) generated turbulence is generally efficient at transporting angular momentum (Hawley et al 1995), recent results have shown a strong sensitivity for MRI turbulence on small-scale dissipation processes (Lesur & Longaretti 2007; Fromang et al 2007), and in particular on the magnetic Prandtl number Pm
Summary
The transport of angular momentum in astrophysical discs is a central problem in accretion theory. MRI generated turbulence is generally efficient at transporting angular momentum (Hawley et al 1995), recent results have shown a strong sensitivity for MRI turbulence on small-scale dissipation processes (Lesur & Longaretti 2007; Fromang et al 2007), and in particular on the magnetic Prandtl number Pm (ratio of microscopic viscosity to resistivity). This effect, called the α − Pm correlation, casts doubts on the actual efficiency of the MRI in realistic situations since Pm can vary by several orders of magnitude in discs (Balbus & Henri 2008). This kind of process would allow for direct communication between the injection scales (transport scales) and the largest dissipation scale (either resistive or viscous)
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