Cosmic ray driven dynamo in galactic disks: effects of resistivity, SN rate and spiral arms

We present a further development of the first numerical model of the magnetohydrodynamical dynamo, driven by cosmic rays in the interstellar medium (Hanasz et al. 2004). The cosmic rays are produced in randomly occurring supernova remnants. Our model is described by equations of magnetohydrodynamics supplemented with the diffusion-advection equation governing the propagation of cosmic rays. The other essential elements of the model are: vertical gravity of the disk, differential rotation and resistivity leading to reconnection of magnetic field lines. We obtain amplification of the large-scale magnetic field on a timescale of galactic rotation. The model represents a kind of fast, cosmic ray driven galactic dynamo proposed by Parker (1992). The amplification of the large-scale magnetic field results from the buoyancy of the cosmic ray component in the interstellar medium. We describe five models characterized by different resistivity magnitudes, supply rates of cosmic rays and discuss in more detail a model with periodic SN activity, which mimics the presence of spiral arms in the disk. We find that both the resistivity and spiral arms enhance the efficiency of the cosmic-ray dynamo. The timescale of magnetic field amplification in this model is as short as 140 Myr. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.