Cosmic-ray-driven dynamo in galactic disks

A parameter study

¹ Toruń Centre for Astronomy, Nicolaus Copernicus University, 87-148 Toruń/Piwnice, Poland e-mail: mhanasz@astri.uni.torun.pl
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
³ Department of Physics and Astronomy, McMaster University, 1280 Main St. W., Hamilton, ON L8S 4M1, Canada
⁴ Astronomical Observatory, Munich University, Scheinerstr. 1, 81679 Munich, Germany

Received: 28 May 2008
Accepted: 19 December 2008

Abstract

Aims. We present a parameter study of the magnetohydrodynamical-dynamo driven by cosmic rays in the interstellar medium (ISM), focusing on the efficiency of magnetic-field amplification and the issue of energy equipartition between magnetic, kinetic, and cosmic-ray (CR) energies.

Methods. We perform numerical CR-MHD simulations of the ISM using an extended version of ZEUS-3D code in the shearing-box approximation and taking into account the presence of Ohmic resistivity, tidal forces, and vertical disk gravity. CRs are supplied in randomly-distributed supernova (SN) remnants and are described by the diffusion-advection equation, which incorporates an anisotropic diffusion tensor.

Results. The azimuthal magnetic flux and total magnetic energy are amplified in the majority of models depending on a particular choice of model parameters. We find that the most favorable conditions for magnetic-field amplification correspond to magnetic diffusivity of the order of 310²⁵ cm² s^-1, SN rates close to those observed in the Milky Way, periodic SN activity corresponding to spiral arms, and highly anisotropic and field-aligned CR diffusion. The rate of magnetic-field amplification is relatively insensitive to the magnitude of SN rates spanning a range of 10% to 100% of realistic values. The timescale of magnetic-field amplification in the most favorable conditions is 150 Myr, at a galactocentric radius equal to 5 kpc, which is close to the timescale of galactic rotation. The final magnetic-field energies reached in the efficient amplification cases fluctuate near equipartition with the gas kinetic energy. In all models CR energy exceeds the equipartition values by a least an order of magnitude, in contrast to the commonly expected equipartition. We suggest that the excess of cosmic rays in numerical models can be attributed to the fact that the shearing box does not permit cosmic rays to leave the system along the horizontal magnetic field, as may be the case for true galaxies.