Issue |
A&A
Volume 498, Number 2, May I 2009
|
|
---|---|---|
Page(s) | 335 - 346 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/200810279 | |
Published online | 05 March 2009 |
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
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.
Key words: galaxies: ISM / galaxies: magnetic fields / magnetohydrodynamics (MHD) / ISM: cosmic rays / ISM: kinematics and dynamics / ISM: magnetic fields
© ESO, 2009
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