  Dynamos in astrophysical disks are usually explained in terms of the standard
alpha-omega mean field dynamo model where the local helicity generates a radial
field component from an azimuthal field. The subsequent shearing of the radial
field gives rise to exponentially growing dynamo modes. There are several
problems with this model. The exponentiation time for the galactic dynamo is
hard to calculate, but is probably uncomfortably long. Moreover, numerical
simulations of magnetic fields in shearing flows indicate that the presence of
a dynamo does not depend on a non-zero average helicity. However, these
difficulties can be overcome by including a fluctuating helicity driven by
hydrodynamic or magnetic instabilities. Unlike traditional disk dynamo models,
this `incoherent' dynamo does not depend on the presence of systematic fluid
helicity or any kind of vertical symmetry breaking. It will depend on geometry,
in the sense that the dynamo growth rate becomes smaller for very thin disks,
in agreement with constraints taken from the study of X-ray novae. In this
picture the galactic dynamo will operate efficiently, but the resulting field
will have a radial coherence length which is a fraction of the galactic radius.
