  Local shearing box simulations of stratified magneto rotational turbulence
invariably exhibit cyclic field patterns which propagate away from the disc
midplane. A common explanation for this is magnetic buoyancy. The recent
analysis by Shi et al. however shows that the flow is buoyantly stable below
one disc scale height H, necessitating an alternative explanation in this
region. We here conduct and analyse direct numerical simulations to explain the
observed behaviour by means of a mean-field description. Apart from the mean
radial and azimuthal field, we monitor the small-scale current helicity, which
we propose as a key indicator for saturation. Reconstructing the horizontally
averaged field, we demonstrate that the problem can be reduced to a
one-dimensional induction equation. By means of the so-called test field
method, we then determine the underlying closure parameters. Our analysis shows
that, apart from a possible direct MRI dynamo, two distinct indirect dynamo
mechanisms operate in the disc. This resolves the issue of the "wrong" sign of
the MRI dynamo effect. Finally, we use the obtained closure parameters to run a
dynamically quenched dynamo model. This model approximately recovers the
observed field patterns in the mean fields. Moreover, the model reproduces the
prevailing parity and the distinct phase pattern in the small-scale current
helicity. The latter property might open a potential route to understand the
saturation of MRI induced turbulence.
