CAN THE TURBULENT GALACTIC DYNAMO GENERATE LARGE-SCALE MAGNETIC-FIELDS

Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo, However, the same turbulence also leads to a smal l-scale dynamo which generates magnetic noise at a more rapid rate. Th e efficiency of the large-scale dynamo depends on how this noise satur ates. We examine this issue, taking into account ambipolar drift, whic h obtains in a galaxy with significant neutral gas. Pie argue as follo ws, (i) The small-scale dynamo generated field does not fill the volum e, but is concentrated into intermittent rope-like structures, The flu x ropes ape curved on the turbulent eddy scales, Their thickness is se t by the diffusive scale determined by the effective ambipolar diffusi on. (ii) For a largely neutral galactic gas, the small-scale dynamo sa turates, as a result of inefficient random stretching, when the peak f ield in a flux rope has grown to a few times the equipartition value, (iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume, (iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. Th e diffusive effects needed fur the large-scale dynamo operation are th en preserved until the large-scale field itself grows to near equipart ition levels.