New radio continuum surveys allowed the determination of the magnetic field in the Large Magellanic Cloud. The magnetic field is filamentary, extending to the south of the 30 Doradus nebula. Some possible explanations for this unusual morphology are discussed.

The hitherto known essentials of the present-day dynamo theory of the galactic magnetic fields are reported. The vertical stratification of the interstellar turbulence is exclusively considered as the source of the α-effect. New expressions for this tensorial effect and its quenching by the field are applied. Although its anisotropy and the influence of the halo easily yield the excitation of non-axisymmetric magnetic configurations of diverse equatorial symmetry, the galactic differential rotation finally leads to axisymmetric and quadrupolar solutions. The magnetic strength of the dynamo fields slightly exceeds the equipartition value while its angular momentum transport (into the intergalactic space) is very small.

For increasingly steep vertical gradients of the turbulence intensity a saturation of the field strength is found since the eddy diffusivity grows with the α-effect. Only beyond the maximum of the field the equatorial symmetry changes to an equatorial antisymmetry, i.e. to dipolar solutions.

In order to produce the observed values of the pitch angles one has only to choose correlation times of the turbulence slightly exceeding the usually accepted 107 yrs.

We discuss some aspects of the interrelationship between the dynamo problem for galaxies and their dynamics. First, we consider the generation of magnetic fields in the presence of fountain flows and galactic winds. Next, we discuss the distortion of a steady magnetic field by tidal effects and other transient spiral features. Finally, we give an expression for the amplitude of density waves generated by large-scale non-axisymmetric fields.

We present calculations showing how both transient effects associated with nonaxisymmetric seed fields, and an interaction with a companion, can produce BSS type magnetic structure in a spiral galaxy.

We describe here the results of 3-D numerical simulations of an αω-dynamo in galaxies with differential rotation, small scale turbulence, and a shock wave induced by a stellar density wave. A non-linear quenching mechanism for the dynamo instability is used, and with the model parameters employed the field achieves a steady state which closely resembles observed fields in galaxies. The magnetic field vectors are parallel to the plane in the disc, with the magnetic intensity decreasing away from the plane. The vectors are also nearly parallel to the spiral arms in the disc, and the field direction is axisymmetric about the galactic centre, but with significant increase of intensity in the arms. The magnetic intensity rises steeply towards the centre of the galaxy, where the field becomes dominated by the vertical component. Nowhere in the parameter range covered is the bi-symmetric field mode dominant.

A nonlinear thin-disk galactic dynamo model based on α-quenching is proposed. Assuming that the mean helicity depends on the magnetic field strength averaged across the disk, we derive a universal form of nonlinearity in the radial dynamo equation. We discuss the evolution of the regular magnetic field in the Milky Way and the Andromeda Nebula. It is argued that the reversals of the regular magnetic field in the Galaxy are a relic inherited from the structure of the seed field. We also briefly discuss the role of the turbulent diamagnetism and the effects of galactic evolution on the dynamo.

The solution of the boundary layer problem in a nonlinear galactic dynamo is described.

A new mechanism is presented for the local amplification and possible global dynamo maintenance of non-axisymmetric large-scale magnetic fields in disk galaxies. Shear in a galactic wind or large-scale flow of ionised gas with components axial and radial to the disk plane may regenerate large-scale magnetic fields.

Numerical results are presented from kinematic mathematical models based on a local (thin disk) approximation and an exact three-dimensional formulation. The one-dimensional thin-disk model illustrates the possibility of exponential amplification and the resulting local axial spatial structure of large-scale galactic magnetic fields. Three-dimensional results support the possibility of global wind dynamo action.

The origin of a regular magnetic field in astrophysical jets is discussed. It is shown that jet plasma flow can generate a magnetic field provided the streamlines are helical. The dynamo of this type, known as the screw dynamo, generates magnetic fields with the dominant azimuthal wave number m = 1 whose field lines also have a helical shape. The field concentrates into a relatively thin cylindrical shell and its configuration is favorable for the collimation and confinement of the jet plasma.

The evolution of galactic magnetic fields under the influence of spiral arms is investigated numerically by 2-dimensional simulation. We compute several models, especially when the swing-excitation mechanism works. We consider also the modification of the mean velocity fields driven by the Lorentz force of the growing magnetic fields. The characteristic structure of magnetic fields around the arm in each model is compared with the observations.

We describe the basic process of coupling between dynamo and density waves in galaxies. The growth rate of magnetic field as a result of coupling is derived, applying the method of multiple time scales to the marginal state of disk dynamo. It is shown that the 1st-order resonance in a perturbation of density and thus the linear swing excitation, is possible. Moreover, the growth rate of magnetic field is always positive and does not depend on the initial phase difference between the magnetic and density waves. Both the numerical and analytical calculations show that ω = 2ω0 (ω: density-wave freq., ω0: dynamo freq.) is still the best condition for resonance due to the linear effect of swing excitation.

The absence of parametric resonance by generation of the large-scale bisymmetric magnetic field in the framework of the simplest thin disk galactic dynamo model is shown.

We provide a simple and realistic expression for the alpha-tensor in the Galactic disk, when turbulence is driven by supernova explosions and superbubbles.

The turbulence in galaxies is assumed to be driven by a sample of stochastically occurring supernova explosions. For simplicity, we replace the complicated real flow by the velocity field of the explosions themselves. The resulting turbulence model can be used to determine the turbulent electromotive force, < u′ × B′ >, a basic quantity in the dynamo theory.

If the distribution function of the SN explosion rate is known, it is sufficient to compute the α-effect for a single explosion in a rotating, density stratified galaxy. We use an axisymmetric simulation code to determine the helical velocity field of the explosion, and consider only explosions located on the rotation axis. The (homogeneous) magnetic field is assumed to be purely vertical. Therefore, only the αzz -component of the α-tensor can be obtained with this method.

The induced magnetic field fluctuations must be correlated with the velocity field in order to get the turbulent electromagnetic force. The resulting α-effect for a sample of explosions starting in the galactic midplane possesses a z-profile with an amplitude of about 60m/s. It is always negative in the northern hemisphere and positive in the southern hermisphere. The scale of the α-profile is about 60pc. The influence of the density distribution proved to be small. We consider the extreme case of uniform explosion rate and find α-values of only few meters per second for a density scale height of 100pc.

Stimulated by recent high frequency radio polarization measurements of M83 and M51, we consider the influence of non-axisymmetric features (bars, spiral arms, etc…) on galactic magnetic fields. The time scale for the field amplification due to the non-axisymmetric velocity field is related to the time scale of angular momentum transport in the disk by the non-axisymmetric features. Due to its dissipational character (cooling and angular momentum transport) the gas plays a major role for the excitation of non-axisymmetric instabilities. Since it is the gaseous component of the interstellar gas in which magnetic field amplification takes place we consider the interplay of gasdynamical processes triggered by gravitational instabilities and magnetic fields. A comparison with the time scale for dynamo action in a disk from numerical models for disk dynamos gives the result that field amplification by non-axisymmetric features is faster in galaxies like M83 (strong bar) and M51 (compagnion and very distinct spiral structure), than amplification by an axisymmetric dynamo. Furthermore, we propose that axisymmetric gravitational instabilities may provide the turbulent magnetic diffusivity ηT. Based on standard galaxy models we obtain a radially dependent diffusivity whose numerical value rises from 1025cm 2s −1 to 1027cm 2s −1, declining for large radii.

We imagine the interstellar medium as a system of molecular clouds and isolated magnetic flux tubes. We assume that: the flux tubes appear as a result of disruption of molecular clouds containing magnetic field, their length is comparable to the mean intercloud distance of about 100 pc and diameter comparable to the diameter of the clouds ∼ 10 pc.

A fully three-dimensional kinematic model is applied to simulate the evolution of the magnetic field in a small (120 pc in size) volume of the turbulent interstellar medium (ISM) in the presence of the field diffusion. The turbulent motions arc approximated by a sequence of non-overlapping in time vortices, which have the form of the rotating (with a non-zero hclicity) columns much longer than the parcel size. They moved vertically and at the inclination of ±30° to the galactic plane. The resulting magnetic field structure closely resembles a fragment of a classical twisted flux tube, well-ordered over the whole parcel of the ISM.

The model of the polarized emission from a spiral galaxy similar to M31, containing helically twisted magnetic field has been constructed. Helical magnetic field was found to cause a specific asymmetry of the distribution of the polarized brightness of the galactic disk.

We reexamine the minimum energy method to determine the magnetic field strength in spiral galaxies from the cosmic ray standpoint of view. It is shown that for example in M51 the estimated field strength is about a factor of 2 lower than obtained with the standard method. As a by-product the corresponding γ-ray flux from the galaxies can be calculated, which will allow further improvement of the method provided reliable γ-ray spectra are at hand.

It is argued that the description of the magnetic field in halos of galaxies should take into account its dynamical coupling to the other major components of the interstellar medium, namely thermal plasma and cosmic rays (CR's). It is then inevitable to have some loss of gas and CR's (galactic wind) provided that there exist some “open” magnetic field lines, facilitating their escape, and a sufficient level of self-generated waves which couple the particles to the gas. We discuss qualitatively the topology of the magnetic field in the halo and show how galactic rotation and magnetic forces can be included in such an outflow picture.