  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.
