Toward a hybrid dynamo model for the Milky Way

(Abridged) Based on the rapidly increasing all-sky data of Faraday rotation measures and polarised synchrotron radiation, the Milky Way's magnetic field is now modelled with an unprecedented level of detail and complexity. We aim to complement this heuristic approach with a physically motivated, quantitative Galactic dynamo model -- a model that moreover allows for the evolution of the system as a whole, instead of just solving the induction equation for a fixed static disc. Building on the framework of mean-field magnetohydrodynamics and extending it to the realm of a hybrid evolution, we perform three-dimensional global simulations of the Galactic disc. Closure coefficients embodying the mean-field dynamo are calibrated against resolved box simulations of supernova-driven interstellar turbulence. The emerging dynamo solutions comprise a mixture of the dominant axisymmetric S0 mode, with even parity, and a subdominant A0 mode, with odd parity. Notably, such a superposition of modes creates a strong localised vertical field on one side of the Galactic disc. We moreover find significant radial pitch angles, which decay with radius -- explained by flaring of the disc. In accordance with previous work, magnetic instabilities appear to be restricted to the less-stirred outer Galactic disc. Their main effect is to create strong fields at large radii such that the radial scale length of the magnetic field increases from 4 kpc (for the case of a mean-field dynamo alone) to about 10 kpc in the hybrid models. There remain aspects (e.g., spiral arms, X-shaped halo fields, fluctuating fields) that are not captured by the current model and that will require further development towards a fully dynamical evolution. Nevertheless, the work presented demonstrates that a hybrid modelling of the Galactic dynamo is feasible and can serve as a foundation for future efforts.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in A&

Do magnetic fields influence gas rotation in galaxies?

We aim to estimate the contribution of the radial component of the Lorentz force to the gas rotation in several types of galaxies. Using typical parameters for the exponential scale of synchrotron emission and the scale length of HI gas, under the assumption of equipartition between the energies of cosmic rays and total magnetic fields, we derive the Lorentz force and compare it to the gravitational force in the radial component of the momentum equation. We distinguish the different contributions between the large-scale and the small-scale turbulent fields by Reynolds averaging. We compare these findings with a dynamical dynamo model. We find a possible reduction of circular gas velocity in the very outer parts and an increase inside a radius of four times the synchrotron scale length. Sufficiently localized radial reversals of the magnetic field may cause characteristic modulations in the gas rotation curve with typical amplitudes of 10-20 km/s. It is unlikely that the magnetic field contributes to the flat rotation in the outer parts of galaxies. If anything, it will \emph{impede} the gravitationally supported rotation, demanding for an even higher halo mass to explain the observed rotation profile. We speculate that this may have consequences for ram pressure stripping and the truncation of the stellar disc

Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations

A mean-field theory of the electrodynamics of a turbulent fluid is formulated under the assumption that the molecular electric conductivity is correlated with the turbulent velocity fluctuation in the (radial) direction, $\mathbf{g}$. It is shown that for such homogeneous fluids a strong turbulence-induced field advection anti-parallel to $\mathbf{g}$ arises almost independently of rotation. For rotating fluids, an extra $\alpha$ effect appears with the known symmetries and with the expected maximum at the poles. Fast rotation, however, with Coriolis number exceeding unity suppresses this term. Numerical simulations of forced turbulence using the NIRVANA code demonstrate that the radial advection velocity, $\gamma$, always dominates the $\alpha$ term. We show finally with simplified models that $\alpha^2$ dynamos are strongly influenced by the radial pumping: for $\gamma<\alpha$ the solutions become oscillatory, while for $\gamma>\alpha$ they become highly exotic if they exist at all. In conclusion, dynamo models for slow and fast solid-body rotation on the basis of finite conductivity-velocity correlations are unlikely to work, at least for $\alpha^2\Omega$ dynamos without strong shear.Comment: 10 pages, 8 figures, to be published in A

Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations

A mean-field theory of the electrodynamics of a turbulent fluid is formulated under the assumption that the molecular electric conductivity is correlated with the turbulent velocity fluctuation in the (radial) direction, (Formula presented.). It is shown that for such homogeneous fluids a strong turbulence-induced field advection anti-parallel to (Formula presented.) arises almost independently of rotation. For rotating fluids, an extra (Formula presented.) effect appears with the known symmetries and with the expected maximum at the poles. Fast rotation, however, with Coriolis number exceeding unity suppresses this term. Numerical simulations of forced turbulence using the nirvana code demonstrate that the radial advection velocity, (Formula presented.), always dominates the (Formula presented.) term. We show finally with simplified models that (Formula presented.) dynamos are strongly influenced by the radial pumping: for (Formula presented.) the solutions become oscillatory, while for (Formula presented.) they become highly exotic if they exist at all. In conclusion, dynamo models for slow and fast solid-body rotation on the basis of finite conductivity–velocity correlations are unlikely to work, at least for (Formula presented.) dynamos without strong shear

Turbulent processes and mean-field dynamo

Mean-field dynamo theory has important applications in solar physics and galactic magnetism. We discuss some of the many turbulence effects relevant to the generation of large-scale magnetic fields in the solar convection zone. The mean-field description is then used to illustrate the physics of the $\alpha$ effect, turbulent pumping, turbulent magnetic diffusivity, and other effects on a modern solar dynamo model. We also discuss how turbulence transport coefficients are derived from local simulations of convection and then used in mean-field models.Comment: 64 pages, 20 figures, 2 tables, submitted to Space Science Reviews, special issue "Solar and stellar dynamos: a new era