Cosmic ray driven dynamo in barred and ringed galaxies

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 $\mu$G 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

Formation of gaseous arms in barred galaxies with dynamically important magnetic field : 3D MHD simulations

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. We performed 3D MHD numerical simulations of barred galaxies and polarization maps. 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.Comment: 4 pages, 4 figure

Simulations of galactic dynamos

We review our current understanding of galactic dynamo theory, paying particular attention to numerical simulations both of the mean-field equations and the original three-dimensional equations relevant to describing the magnetic field evolution for a turbulent flow. We emphasize the theoretical difficulties in explaining non-axisymmetric magnetic fields in galaxies and discuss the observational basis for such results in terms of rotation measure analysis. Next, we discuss nonlinear theory, the role of magnetic helicity conservation and magnetic helicity fluxes. This leads to the possibility that galactic magnetic fields may be bi-helical, with opposite signs of helicity and large and small length scales. We discuss their observational signatures and close by discussing the possibilities of explaining the origin of primordial magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria

Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection

Realistic astrophysical environments are turbulent due to the extremely high Reynolds numbers. Therefore, the theories of reconnection intended for describing astrophysical reconnection should not ignore the effects of turbulence on magnetic reconnection. Turbulence is known to change the nature of many physical processes dramatically and in this review we claim that magnetic reconnection is not an exception. We stress that not only astrophysical turbulence is ubiquitous, but also magnetic reconnection itself induces turbulence. Thus turbulence must be accounted for in any realistic astrophysical reconnection setup. We argue that due to the similarities of MHD turbulence in relativistic and non-relativistic cases the theory of magnetic reconnection developed for the non-relativistic case can be extended to the relativistic case and we provide numerical simulations that support this conjecture. We also provide quantitative comparisons of the theoretical predictions and results of numerical experiments, including the situations when turbulent reconnection is self-driven, i.e. the turbulence in the system is generated by the reconnection process itself. We show how turbulent reconnection entails the violation of magnetic flux freezing, the conclusion that has really far reaching consequences for many realistically turbulent astrophysical environments. In addition, we consider observational testing of turbulent reconnection as well as numerous implications of the theory. The former includes the Sun and solar wind reconnection, while the latter include the process of reconnection diffusion induced by turbulent reconnection, the acceleration of energetic particles, bursts of turbulent reconnection related to black hole sources as well as gamma ray bursts. Finally, we explain why turbulent reconnection cannot be explained by turbulent resistivity or derived through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection - Concepts and Applications", editors W. Gonzalez, E. N. Parke

Reconnection in weakly stochastic

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

Reconnection in weakly stochastic B-fields in 2D

The idea that there could be some way of producing fast magnetic reconnection even in highly conducting fluids is not new (Moffat 1978; Krause & Radler 1980), but early models of reconnection (see Parker 1957; Sweet 1958, e.g.) using realistic astrophysical temperatures and densities gave a very slow reconnection rate, ∼ VAS −1/2, where S ≡ η/(VAL) is the Lundquist number, η is the resistivity, and L is the size of the current sheet. It was Petschek (1964) who for the first time introduced a model for fast reconnection with a rate proportional to (log S) −1. Subsequent numerical simulations and theoretical analyses have shown that the Petschek reconnection rate is only attainable in very restricted circumstances. For instance, a modified version can stably persist in a collisionless plasma (see Drake et al. 2006a, e.g.). This means that the length of the current sheet should not exceed approximately 50 electron mean free paths (Uzdensky 2006; Yamada et al. 2006). This condition cannot be satisfied in many astrophysical environments, e.g. in the interstellar medium (Vishniac & Lazarian 1999). In a collisional plasma the X-point region required for Petschek reconnection will collapse to the Sweet-Parker geometry for large S (Biskamp 1996). The failure of the Petschek model has increased interest in the role of turbulence in reconnection. This interest has been further stimulated by the fact that the turbulence is ubiquitous in astrophysical environments where reconnection occurs, e.g. the ISM, stars, the Sun and accretion disks (Ruzmaikin et al. 1988). The idea that turbulence can affect reconnection has a long history, although usually studied in two-dimensions (2D) (Priest & Forbes 2000). Several researchers have approached this proble