A New Mechanism of Nonlinearity in the Disc Dynamo

We propose and discuss a new mechanism for the saturation of magnetic field growth for mean-field dynamos in galactic and accretion discs. A peculiar property of these dynamo systems is that the disc thickness grows with the magnetic field. The efficiency of magnetic field regeneration can be shown to have a maximum at a moderate value of the disc half-thickness h (for the Solar vicinity of the Milky Way, at about 600 pc). Therefore, at small h the dynamo action gets stronger with h and the magnetic field may grow super-exponentially. The disc thickness eventually becomes so large that the dynamo action is suppressed and a steady state is established. By comparing this nonlinearity with α-quenching we show that the latter is more efficient in establishing the steady-state strength of the mean magnetic field in real spiral galaxies. The disc inflation caused by the growing magnetic field may be important for the history of magnetic field evolution in galaxies; this effect can strongly accelerate the generation of the regular magnetic field in young galaxies. 1