We present three-dimensional global numerical simulations of the cosmic-ray (CR) driven dynamo in barred galaxies. We study the evolution of the interstellar medium of the barred galaxy in the presence of non-axisymmetric component of the potential, i.e. the bar. The magnetohydrodynamical dynamo is driven by CRs, which are continuously supplied to the disk by supernova (SN) remnants. No magnetic field is present at the beginning of simulations but one-tenth of SN explosions is a source of a small-scale randomly oriented dipolar magnetic field. In all models we assume that 10% of 10 51 erg SN kinetic energy output is converted into CR energy.To compare our results directly with the observed properties of galaxies we construct realistic maps of polarized radio emission. The main result is that the CR-driven dynamo can amplify weak magnetic fields up to a few µG within a few Gyr in barred galaxies. The obtained e-folding time is equal to 300 Myr and the magnetic field reaches equipartition at time t ∼ 4.0 Gyr. Initially, completely random magnetic field evolves into large-scale structures. An even (quadrupoletype) configuration of the magnetic field with respect to the galactic plane can be observed. Additionally, the modeled magnetic field configuration resembles maps of the polarized intensity observed in barred galaxies. Polarization vectors are distributed along the bar and between spiral arms. Moreover, the drift of magnetic arms with respect to the spiral pattern in the gas density distribution is observed during the whole simulation time.

Aims. We present results of three-dimensional nonlinear MHD simulations of a large-scale magnetic field and its evolution inside a barred galaxy with the back reaction of the magnetic field on the gas. The model does not consider the dynamo process. To compare our modeling results with observations, we construct maps of the high-frequency (Faraday-rotation-free) polarized radio emission on the basis of simulated magnetic fields. The model accounts for the effects of projection and the limited resolution of real observations. Methods. We performed 3D MHD numerical simulations of barred galaxies and polarization maps. Results. The main result is that the modeled magnetic field configurations resemble maps of the polarized intensity observed in barred galaxies. They exhibit polarization vectors along the bar and arms forming coherent structures similar to the observed ones. In the paper, we also explain the previously unsolved issue of discrepancy between the velocity and magnetic field configurations in this type of galaxies. The dynamical influence of the bar causes gas to form spiral waves that travel outwards. Each gaseous spiral arm is accompanied by a magnetic counterpart, which separates and survives in the inter-arm region. Because of a strong compression, shear of non-axisymmetric bar flows and differential rotation, the total energy of modeled magnetic field grows constantly, while the azimuthal flux grows slightly until 0.05 Gyr and then saturates.

We study two-dimensional turbulent magnetic reconnection in a compressible fluid in the gas pressure dominated limit. We use open boundary conditions and start from a Harris current sheet configuration with a uniform total pressure. A small perturbation of the vector potential initiates laminar reconnection at the Sweet-Parker rate, which is allowed to evolve for several dynamical times. Subsequently sub-Alfvenic turbulence is produced through random forcing at small wave numbers. The magnetic field topology near the current sheet is strongly affected by the turbulence. However, we find that the resulting reconnection speed depends on the resistivity. In contrast to previous results in three dimensions, we find no evidence for fast reconnection. The reconnection speed exhibits strong variations, but the time averages increase smoothly with the strength of the turbulence.

Aims. We study numerically the large-scale gas and magnetic field evolution of barred galaxies in the gravitational potential of a disk, bulge, halo, and bar. We solve non-linear MHD equations including the back-reaction of the magnetic field to the gas. We do not take into account any dynamo process. Methods. We apply the numerical MHD code to calculate the model of the galaxy in three dimensions. We construct realistic maps of high-frequency (Faraday rotation free) polarized radio emission on the basis of the simulated magnetic fields. The polarization model includes the effects of projection and limited resolution. Results. The main result is that our modeled polarization maps resemble the radio polarization structures observed in barred galaxies. The modeled polarization B-vectors distribution along the bar and between spiral arms resembles the observed topology of the magnetic field in barred galaxies. Our calculations for several different rotational velocities and sound speeds give the same result we got in our previous earlier published model. The reason of this behaviour is the dynamical evolution of the bar that causes gas to form spiral waves going radially outward. A gaseous spiral arms in turn generates magnetic ones, which live much longer in the inter-arm disk space than the gaseous pattern.

Context. For the first time, our magnetohydrodynamical numerical calculations provide results for a three-dimensional model of barred galaxies involving a cosmic-ray driven dynamo process that depends on star formation rates. Furthermore, we argue that the cosmic-ray driven dynamo can account for a number of magnetic features in barred galaxies, such as magnetic arms observed along the gaseous arms, magnetic arms in the inter-arm regions, polarized emission that is at the strongest in the central part of the galaxy, where the bar is situated, polarized emission that forms ridges coinciding with the dust lanes along the leading edges of the bar, as well as their very strong total radio intensity. Aims. Our numerical model probes what kind of physical processes could be responsible for the magnetic field topology observed in barred galaxies (modes, etc.). We compare our modelled results directly with observations, constructing models of high-frequency (Faraday rotation-free) polarized radio emission maps out of the simulated magnetic field and cosmic ray pattern in our modeled galaxy. We also take the effects of projection into account as well as the limited resolution. Methods. We applied global 3D numerical calculations of a cosmic-ray driven dynamo in barred galaxies with different physical input parameters such as the supernova (SN) rate. Results. Our simulation results lead to the modelled magnetic field structure similar to the one observed on the radio maps of barred galaxies. Moreover, they cast new light on a number of properties in barred and spiral galaxies, such as fast exponential growth of the total magnetic energy to the present values. The quadrupole modes of magnetic field are often identified in barred galaxies, but the dipole modes (e.g., in NGC 4631) are found very seldom. In our simulations the quadrupole configuration dominates and the dipole configuration only appears once in the case of model S100, apparently as a consequence of the choice of the random number seed. Synthetic radio maps of our models display X-type structure similar to what is observed in real galaxies. Conclusions. We conclude that a cosmic-ray driven dynamo process in barred galaxies can amplify magnetic fields efficiently. The fastest rate of magnetic field increase is 195 yr for a SN frequency of 1/50 yr −1 .The obtained strength of magnetic field corresponds to the observational values (a few μG in spiral arms). The polarization and rotation measure maps also agree with observations. We found the effect of shifting magnetic arms in 4 models (out of the sample of 5).

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Abstract. We study the global evolution of the magnetic field and interstellar medium (ISM) of the barred and ringed galaxies in the presence of non-axisymmetric components of the potential, i.e. the bar and/or the oval perturbations. The magnetohydrodynamical dynamo is driven by cosmic rays (CR), which are continuously supplied to the disk by supernova (SN) remnants. Additionally, weak, dipolar and randomly oriented magnetic field is injected to the galactic disk during SN explosions. To compare our results directly with the observed properties of galaxies we construct realistic maps of high-frequency polarized radio emission. The main result is that CR driven dynamo can amplify weak magnetic fields up to few µG within few Gyr in barred and ringed galaxies. What is more, the modelled magnetic field configuration resembles maps of the polarized intensity observed in barred and ringed galaxies.

Abstract. We present the results of the three-dimensional, fully non-linear MHD simulations of the large-scale magnetic field evolution in a barred galaxy with the back reaction of magnetic field to gas. We also include the process of the cosmic-ray driven dynamo. In addition, we check what physical processes are responsible for the magnetic field evolution in the tidally influenced spiral galaxies. We solve the MHD equations for the gas and magnetic field in a spiral galaxy with gravitationally prescribed bulge, disk and halo which travels along common orbit with the second body. In order to compare our modeling results with the observations we also construct the maps of high-frequency (Faraday rotation-free) polarized radio emission from the simulated magnetic fields. The model accounts for the effects of projection and limited resolution.We found that the obtained magnetic field configurations are highly similar to the observed maps of the polarized intensity of barred galaxies, because the modeled vectors form coherent structures along the bar and spiral arms. We also found a physical explanation of the problem of inconsistency between the velocity and magnetic fields character present in this type of galaxies. Due to the dynamical influence of the bar, the gas forms spiral waves which go radially outward. Each spiral arm forms the magnetic arm which stays much longer in the disk than the gaseous spiral structure. The modeled total energy of magnetic field and magnetic flux grows exponentially due to the action of the cosmic-ray driven dynamo. We also obtained the polarization maps of tidally influenced spiral galaxies which are similar to observations.