We consider the physics of magnetic flares in the energetic radiation field of an accretion disk corona (ADC). The X-ray emission from these flares is thought to be responsable for the observed hard powerlaw component in the X-ray spectra of galactic black hole candidates in their ‘high’ spectral state. During the flare event (inverse Compton) scattering of soft photons from the underlying disk into hard photons occurs on accelerated electrons in current sheets. The electrons are decelerated by the radiation drag force that results from the up-scattering. This friction-like effect of the intense background radiation field on the motion of the electrons in the sheet can be considered as a form of resistivity in the magnetohydrodynamical picture of the current sheet: Compton resistivity. A spectrum is derived for the up-scattered radiation from current sheets in the ADC and it is found that this spectrum mimics a powerlaw above a critical photon energy.

Low-order models of nonlinear dynamos can be used to investigate generic properties of more realistic mean field dynamos. Reducing the partial differential equations to a set of ordinary differential equations makes it possible to explore the bifurcation structure in considerable detail and to compute unstable solutions as well as ones that are stable. Complicated time-dependent behaviour is typically associated with a homoclinic or heteroclinic bifurcation. Destruction of periodic orbits at saddles or saddle-foci gives rise to Lorenz-like or Shil'nikov-like chaotic oscillations, while destruction of a quasiperiodic orbit leads to aperiodically modulated cycles. Changes in spatial symmetry can also be investigated. The interaction between solutions (steady or periodic) with dipole and quadrupole symmetry gives rise to a complicated bifurcation structure, with several recognizably different mixed-mode solutions; similar behaviour has also been found in spherical dynamo models. These results have implications for the expected behaviour of stellar dynamos.

We study the magnetic field structures of fast kinematic dynamos for a class of steady chaotic flows with stagnation points. We find that the dynamos are generated by a stretch-fold-shear mechanism, which is effective only when the chaotic flow region is large enough and overlaps significantly with the rotating flux tubes.

The large-scale magnetic field generation by the turbulent motion energy, known as turbulent dynamo [1], is perspective candidate to explain the observed stationary magnetic fields of cosmic objects.

1. The reflective non-symmetric (gyrotropic) stochastic magnetic hydrodynamics (MHD) turbulence is investigated by a quantum-field (QF) theory method including a renormalization group (RG), that has been successfully applied to the fully developed HD turbulence [1,2] and to the ordinary (reflective symmetric) MHD turbulence [3].

The exact numerical solution of the simplest non-linear equation from the hierarchy of non-linear equations for the averaged Green function shows that such solution allows to calculate the diffusivity and α-effect coefficient with a good accuracy for an arbitrary spectra of turbulence for all values of the characteristic parameter. It is derived also the improved equation describing the evolution of admixture fluctuations in a turbulent medium which takes into account the non-linear equation for the averaged Green function.

We discuss a dynamo mechanism driven by weak plasma turbulence and show that turbulent plasma waves may generate and maintain cosmic magnetic field. A dynamo equation is derived from the magnetic induction equation based on mean field electrodynamics. In the usual α–ω dynamo theory, the source term in the dynamo equation arises from α–effect associated with the convective motion of the fluid. In contrast, in our theory the source term is determined by “P–effcct” associated with weakly turbulent waves (e.g. Alfvén waves) in the plasma. We suggest that “P–ω” dynamo may be operative either in the presence or absence of convection. The sole requirement for its operation is the existence of weak plasma turbulence in the source region of the cosmic magnetic field.

Our starting point is a formal linear stochastic differential equation of first order (higher order equations can be transformed to systems of these) where I, a, W are stochastic functions with and analogously for a and W. I, a, and W are allowed to depend on the element ω of a set Ω in which a probability measure is defined in the usual way (see e.g. Doob, 1953; de Witt-Morette, 1981). To get a solution of eq.(1) for the mean intensity we treat the problem according the Reynolds averaging technique in the usual manner : The stochastic equation is changed into an infinte hierarchical system of equations for the correlations.

A method to calculate the electrical conductivity of plasmas with electromagnetic waves with small wave number is discussed.

The nonlinear (in terms of the large-scale magnetic field) effect of the modification of the magnetic force by an advanced small-scale magnetohydrodynamic (MHD) turbulence is considered. The phenomenon is due to the generation of magnetic fluctuations at the expense of hydrodynamic pulsations. It results in a decrease of the elasticity of the large-scale magnetic field.

The renormalization group (RNG) method was employed for the investigation of the MHD turbulence at the large magnetic Reynolds number. It was found that the level of the magnetic fluctuations can exceed that obtained from the equipartition assumption due to the inverse energy cascade in advanced MHD turbulence.

This effect can excite an instability of the large-scale magnetic field due to the energy transfer from the small-scale turbulent pulsations. This instability is an example of the inverse energy cascade in advanced MHD turbulence. It may act as a mechanism for the large-scale magnetic ropes formation in the solar convective zone and spiral galaxies.

The problem of preferential excitation of non-axially symmetric magnetic fields in most of the observed spiral galaxies and in planets as Uranus and Neptune is studied. It is suggested that a dynamo acting in a thin conductive shell is possible in these objects.

It is shown that in this thin shell axisymmetric and non-axisymmetric magnetic field modes can be excited with equal efficiency. The preferential excitation of non-axially symmetric magnetic fields is possible, when the gradient of the angular velocity of rotation is comparatively small and has an essential component perpendicular to the shell.

A striking formal analogy between sunspots and the type II superconductor magnetic vortex has been pointed out by Saniga (1990). On the basis of this similarity, it was shown that, by postulating the existence of a complex-valued scalar field on the Sun (a “Higgs-like field”), many features of sunspots can be reproduced. Here this formal analogy is exploited further to show that the Abelian-Higgs sunspot acquires a non-zero electric charge in the background of a curved space-time endowed with a constant, completely antisymmetric torsion tensor. The space-time torsion is also shown to relate the spot's electric and magnetic fileds.

Optical techniques were first used to detect magnetic fields in cosmic objects. Hale used the Zeeman effect to detect solar magnetic fields in 1908. Babcock extended the Zeeman technique to show the existence of magnetic fields in Ap stars in 1946. Optical polarization observations were made as early as 1920 by W.F. Meyer who observed the Hubble variable nebula NGC 2261. Polarization observations of the Andromeda nebula were made by Öhman in 1942. However at first the interpretation of these optical polarization observations was in terms of scattered light only. The theoretical work of Davis and Greenstein suggested that optical polarization could also be due to dust grains aligned in magnetic fields. Observations of Hiltner and Hall supported this interpretation. Extensive surveys of starlight polarization were made by many observers giving information about nearby magnetic fields in the Galaxy. Optical polarization observations of galaxies gave some information on the Magellanic Clouds and other nearby objects but due to lack of sensitivity progress was slow.

The pitch angles of the local magnetic field in our Galaxy, previously derived from Rotation Measures (RMs) of pulsars by many authors, are not consistent with each other and with the pitch angles of the local spiral arms. That may be due to the fact that the used pulsar samples are located in different arms in which the directions of the magnetic fields are different. In this paper 2-D and 3-D models for the local magnetic field based on spiral arms are proposed for fitting the RMs of 129 nearby pulsars. In our models the amplitude of the uniform field changes sinusoidally to avoid abrupt reversals, and the directions are parallel or anti-parallel. The best-fitting 2-D model shows that in the Orion arm the strength of the regular field component is 2.4 ± 0.3μG, with its direction towards l = 73° ± 3°. There is a direction reversal in Sagittarius-Carina arm beginning at D rev = 0.3Kpc. The half “wavelength” of the sinusoidal variation is about 1.7 ± 0.4Kpc. The best fitting 3-D model shows that the scale height is only about 0.16 Kpc, which means that the local uniform field is confined in the galactic plane. The strongest regular field in this 3-D model is about 2.8±0.3μG. The results from both, the 2-D model as well as the 3-D model, show that the orientation of the field is in excellent agreement with the spiral arms.

Results of linear αΩ-dynamo models are confronted with radio polarization observations of spiral galaxies. The general distribution of polarized emission and the magnetic field pitch angle can be described with sufficient accuracy. The occurrance of systematic large-scale variations in Faraday rotation (RM) is the strongest argument in favour of dynamo theory. However, the predominance of axisymmetric SO modes could not be confirmed by observations; S1 modes are about equally frequent. The azimuthal variations of field pitch angles and, in two cases, the phases of the RM variations are inconsistent with a classical αΩ-dynamo. Locally deviating RM values indicate field lines bending out of the plane. There is increasing evidence that galactic fields cannot be described by simple dynamo modes. This calls for more realistic dynamo models, taking into account non-axisymmetric velocity fields and galactic winds.

Interpretation of radio observations is difficult because Faraday depolarization can seriously affect the data. Observations of small-scale field structures are summarized which show the path for future research. Instrumental needs for such investigations are discussed.

The interacting galaxy NGC4631 is unique among edge-on galaxies in that it shows a prominent synchrotron radio halo. We derive the intrinsic magnetic field structure in this halo. The field lines are mainly radially outgoing from the central region. The possible mechanisms generating such magnetic fields in the halo, i.e. outflows and dynamo processes, are discussed.

The face-on spiral galaxy IC 342 is known to possess an axisymmetric spiral magnetic field (ASS) with magnetic field lines being orientated inwards everywhere at least in the analyzed radial range between 5 and 13 kpc1 (Krause et al., 1989a; Sokoloff et al., 1992). The ASS field configuration seems to be even with respect to the galactic plane.

High frequency polarization observations reveal the existence of a large-scale ordered magnetic field in the disk of the spiral galaxy NGC 6946. At lower frequencies the disk is no longer transparent to polarized radio waves due to Faraday depolarization. The spiral pattern of the uniform magnetic field and the distribution of polarized intensities are fairly well simulated by a dynamo model. The model parameters indicate that the dynamo does not only operate in the disk, but also in the halo.

We present observations of the polarized emission and the derived magnetic field structure of the spiral galaxy M 51 at different wavelengths: 2.8 cm, 6.2 cm, 18 and 20.5 cm.

This southern galaxy with its fairly regular pattern is well suited to study the influence of a bar onto the magnetic field in a galaxy. With its low inclination of 24° it provides a geometry similar to M 51. Also the magnetic field strength of 11±2 μG is in the same range.