We examine effects of density waves on the local galactic αω-dynamo. Oscillations of the magnetic field and the dynamo parameters due to the density wave perturbation irreversibly couple with the dynamo action to enhance the growth of the magnetic fields.

The generation and maintenance of galactic magnetic fields are considered when the dynamo-acted disk is perturbed by spiral density waves. The periodic change of gaseous density and shear is coupled with the field oscillation as a dynamo wave. We find the conditions for parametric resonance which increases the efficiency of dynamos. Special attention is paid to two-armed spiral galaxies, in which the BSS field structure is possibly the most effectively excited configuration owing to the present mechanism.

We report here some preliminary results of 3–D numerical simulations of an α–ω dynamo in galaxies with differential rotation, small–scale turbulence, and a shock wave induced by a stellar density wave. We obtain the magnetic field from the standard dynamo equation, but include the spiral shock velocity field from a hydrodynamic simulation of the gas flow in a gravitational field with a spiral perturbation (Johns and Nelson, 1986).

A steady density wave in the stellar background of a disk–like galaxy is supposed to force a spiral shock wave in the interstellar gas. The jump in vorticity across the shock leads to a locally enhanced helicity, and so to an α–effect which is steady but azimuth–dependent in the frame rotating with the angular velocity ω of the density wave. This is simulated by the adoption of the form for the local dynamo growth rate arising when the standard kinematic dynamo equation is treated by the thin–disk approximation (Ruzmaikin et al 1988). The global magnetic field is proportional to the function Q satisfying where η is the turbulent resistivity (for simplicity assumed uniform) and is the laminar angular velocity of the gas in the inertial frame. We look for solutions of the form where is a global eigen-value, and the non-vanishing of couples all odd or all the even m-values. Anticipating that the strong differential rotation will ensure that in the modes with the largest growth-rate the higher-m parts are weak, the equations are truncated, leaving just a pair in q1, q-1, to describe a basically bisymmetric (m = 1) mode. Approximate treatment by the WKBJ technique suggests that a corotating growing mode (with Γ real and positive) will differ significantly from zero over the range between the points where Numerical solutions have been found for a set of illustrative parameters with corotation occurring at 6.67 kpc, and the turbulence parameters close to those in the M51 mode studied by Ruzmaikin et al which extends over = 1 kpc. Three growing corotating modes were found, the fastest extending for ~ 3 kpc, the other two for over 4 kpc. The first two grow 2-3 times faster, the third somewhat slower, than the M51 mode.

Magnetic vortex tubes (MVTs) on a hierarchy of scales occur universally. On the largest scale they channel bipolar outflows of gas. A pinched region of MVT provides an acceleration mechanism capable of yielding the maximum cosmic ray energies.

Matter in accretion disks spirals inward because of angular-momentum losses due to intrinsic shear torques. For galactic disks, the accretion rates can be as high as 1M⊙/yr, corresponding to a value larger than unity of the dimension-less viscosity parameter α. The required torques exceed turbulent ones by at least an order of magnitude; they can, however, be the result of magnetic tensions.

In fast-revolving, clumpy disks, magnetic pressures can exceed static gas pressures and approach ram pressures. Suggestive candidates for such magnetic dominance are the inner, near-rigidly rotating parts of galactic disks.

The advent of 3-dimensional, electromagnetic, and fully relativistic particle simulations allows a detailed study of a magnetized plasma galaxy model. When two such models are simulated, an interaction yielding results resembling observational data from double radio sources, including the emission of synchrotron radiation, are obtained. Simulation derived morphologies, radiation intensities, frequency spectra and isophote patterns are produced by the model which can be directly compared to observational data. Long time simulation runs (~109 years) show the evolution of barred spiral galaxies with large scale bisymmetric magnetic field distributions having 100μG field strengths.

The amplification of a seed magnetic field by the hydrodynamic behavior of a disk galaxy is an important and challenging problem. We present our initial work on a numerical simulation of the amplification of such a field to large scales and its subsequent evolution. The computations will be carried out using a 3-D magnetohydrodynamic code.

Various mechanisms of creation of the extended spectrum of MHD turbulence in the interstellar medium are reviewed. Within the scales 1014 cm ≲ L ≲ 1019 cm the turbulence mostly consists of the ensemble of weak shock waves. At 1012 cm ≲ L ≲ 1014 cm the principle mechanism is generation of MHD waves by cosmic rays.

We study the exact phase space dynamics of relativistic test particles propagating in static one-dimensional Alfvén waves, modelling cosmic ray propagation in the interplanetary medium and in the interstellar medium. The result shows that the conventional approach should not be considered adequate to explain important features of particle propagation in Alfvén waves.

A numerical model of magnetic field structure in the presence of turbulent motions of the interstellar gas has been made. We solved the kinematic equation of magnetic field transport in a limited volume of ISM. Diffusion effects smoothing out small-scale structure are allowed as well. A permanent helical configuration of field lines has been found. This justifies, at least partly, searching solutions in the dynamo theory in the form of field modes. The presence of diffusion appears essential for the time evolution of magnetic field and magnetic energy density.

We show that large–scale motions of the interstellar gas, such as those associated with galactic density waves, easily develop, over a wide range of scales, shocks and discontinuities which are expected to generate turbulence. The latter is supposed to evoke diffusion of magnetic fields and cosmic rays on scales down to a few parsecs. We suggest that these processes may be of major importance in discussions of interconnections between the observed radio emission of the disks of spiral galaxies and the gas density distribution within them. In particular, we predict that the density of cosmic rays and magnetic field energy must be much less contrasted (on scales of ~1 pc and up to the scales of galactic shocks) than the gas density, hence the synchrotron radio emission is not as contrasted as is predicted under the hypothesis of a fully frozen-in magnetic field.

The magnetic field structure in the swept-up HII shell and the stellar wind region around an early type star is described. It is argued that the field does not become dynamically so important as to prevent self-similar flow. A condition under which reconnexion removes magnetic energy from the bubble is derived. It follows that the process of turbulent mixing between the HII region and the bubble works efficiently enough to cause severe departures from uniform expansion, even in the presence of magnetic fields.

Violent events such as supernova explosions or gradual processes such as mass loss from hot supergiant stars produce diamagnetic cavities: the interstellar magnetic field, frozen into ambient plasma, is excluded from a region around the source of expanding injected plasma. Particle-in-cell simulations illustrate the formation of such diamagnetic cavities in both the steady and impulsive cases. Plasma loss along the field means that the cavity always tends toward a cylindrical or barrel shape, even for very weak magnetic fields. Ion acceleration is expected out the ends of the barrel for all parameters of injection and initial field.

The magnetic field in the gaseous disk of the galaxy is dynamically unstable to undulations with wavelengths of the order of 1 kpc. The elevated portions of the field are subject to rapid inflation (~ 50 km/sec) by the cosmic rays produced within the gaseous disk. The result is a magnetic halo of 1–3 × 10−6 gauss, composed of close packed bipolar lobes of field extending outward from the surface of the disk to distances of several kpc. The inflation is presumably irregular, producing tangential discontinuities (current sheets) throughout the extended bipolar fields. A major portion of the magnetic energy is dissipated by rapid reconnection at these current sheets, heating the tenuous halo gas to temperatures of 106 − 107 °K and producing the X-ray emission observed from the halos of many spiral galaxies.

The plasma density fluctuations responsible for interstellar scintillations occur on the same scales as interstellar magnetohydrodynamic waves (Alfvén waves), which are responsible for many important processes such as the acceleration of the cosmic rays. This suggests that these density fluctuations represent a compressive component of MHD waves, and raises the exciting possibility that radioastronomical observations can provide more or less direct measurements of interstellar microphysical processes. Extraction of MHD wave properties from the radio scattering measurements requires a sound theoretical understanding of the relationship between the magnetic field in an MHD wave and the corresponding plasma density perturbation. We present a plasma kinetic theory treatment of the density compression associated with an MHD wave field. The density perturbation may be expressed as the sum of three terms. These terms are proportional to the wave amplitude, wave intensity, and sine transform of the wave intensity, respectively. The coefficients of these three terms are functions of the plasma β, the electron-to-ion temperature ratio, and the angle of wave propagation with respect to the large scale magnetic field. This relation can serve as the basis for inferring the MHD wave field given a radio scattering measurement of the density fluctuation statistics. In an attempt to apply these ideas to the interstellar plasma turbulence, we have made VLBI angular broadening measurements of sources whose lines of sight pass close to supernova remnants. The intensity of MHD waves is expected to be high in the vicinity of the shock waves associated with supernova remnants. We do not yet have unambiguous evidence of enhanced radio wave scattering due to shock-associated MHD waves. However, we have found anomalously high scattering for the source CL4, whose line of sight passes through the Cygnus Loop.

Concentrating on our own Galaxy we discuss the dynamics of the outer halo, its magnetic structure and the occurrence of a supersonic mass loss in the form of a Galactic wind. The cosmic rays, as the nonthermal high energy component, de facto not influenced by gravity, play an essential role in the wind dynamics.

The symbiotic nature of the relationship between galactic winds and cosmic rays (CR's) is attracting increasing interest since, on the one hand, galactic wind termination shocks have been invoked to explain the origin of the highest energy CR's (Jokipii and Morfill, 1985) while, on the other hand, it is thought that CR's may be the principal agent responsible for driving winds (Ipavich, 1975). A simple, spherically-symmetric galactic wind model which includes the dynamical effect of spatially diffusing CR's with an outward pressure gradient is presented. A hydrodynamic form of the cosmic ray transport equation is used, and the spatial diffusion coefficient κ is modelled as κ ∞ ρα (ρ the gas density) for some parameter α. The choice of α < 0 yields flow solutions which are subsonic at the source and supersonic at infinity. Such a choice of α corresponds to assuming that the level of Alfvén wave turbulence decreases with increasing distance from the galaxy. Near the galaxy κ is small and the cosmic rays are strongly coupled to the thermal gas, thus supplying additional momentum in the subsonic wind regime. Further from the galaxy, as κ increases, the cosmic rays decouple from the thermal gas and no longer contribute to the dynamics of the flow. Hence far from the galaxy, the Euler equations should be a suitable model. Furthermore, unlike classical wind models, a multiplicity of saddle points, and therefore flow solutions, exists. Finally, we show that α < 0 is most likely to lead to stable, steady-state flows.

Basing on a simple 1-dim model, the process of diffusive particle acceleration at a galactic bow-shock is discussed.

The effect that cosmic rays and the Alfvén waves generated by them have on the structure of the plasma distribution perpendicular to the galactic disc in a hydrostatic model is examined. It is shown that the plasma distribution exhibits two length scales, essentially because the cosmic rays and Alfvén waves lift the gas up and stretch it out beyond the galactic plane. The predicted gas density far from the galactic plane indicates that models involving hydrostatic equilibrium should be replaced by those allowing for a galactic wind.