Observations of dwarf galaxies suggest the presence of large-scale magnetic fields. However the size and slow rotation of these galaxies appear insufficient to support a mean-field dynamo action to excite such fields. Here we suggest a new mechanism
to explain large-scale magnetic fields in galaxies that are too small to support mean-field dynamo action. The key idea is that we do not identify large-scale and mean magnetic fields. In our scenario the the magnetic structures originate from a small-scale dynamo which produces small-scale magnetic field in the galactic disc and a galactic wind that transports this field into the galactic halo where the large turbulent diffusion increases the scale and order of the field. As a result, the magnetic field becomes large-scale; however its mean value remains vanishing in a strict sense. We verify the idea by numerical modelling of two distinct simplified configurations, a thin disc model using the no-$z$ approximation, and an axisymmetric model using cylindrical $r,z$ coordinates. Each of these allows reduction of the problem to two spatial dimensions. Taken together, the models support the proposition that the general trends will persist in a fully 3D model. We demonstrate that a pronounced large-scale pattern can develop in the galactic halo for a wide choice of the dynamo governing parameters. We believe that our mechanism can be relevant to explaining the presence of the fields observed in the halos of dwarf galaxies. We emphasize that detailed modelling of the proposed scenario needs 3D simulations, and adjustment to the specific dynamo governing parameters of dwarf galaxies.
Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. We cons
ider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with $R_alpha<0$ have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. We have explored and extended the solutions known for thin accretion discs.
Galactic encounters are usually marked by a substantial increase of synchrotron emission of the interacting galaxies compared to the typical emission from similar isolated galaxies. This is believed to be associated with an increase of the star forma
tion rate and the associated turbulent magnetic fields. The regular magnetic field is usually believed to decrease. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase of the star formation rate caused by the encounter. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction, leading to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. The encounter causes enhanced total and polarized emission without increase of the regular magnetic field strength. The increase of synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase of the star formation rate, depending on the choice of parameters.The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr).
We study activity waves of the kind that determine cyclic magnetic activity of various stars, including the Sun, as a more general physical rather than a purely astronomical problem. We try to identify resonances which are expected to occur when a me
an-field dynamo excites waves of quasi-stationary magnetic field in two distinct spherical layers. We isolate some features that can be associated with resonances in the profiles of energy or frequency plotted versus a dynamo governing parameter. Rather unexpectedly however the resonances in spherical dynamos take a much less spectacular form than resonances in many more familiar branches of physics. In particular, we find that the magnitudes of resonant phenomena are much smaller than seem detectable by astronomical observations, and plausibly any related effects in laboratory dynamo experiments (which of course are not in gravitating spheres!) are also small. We discuss specific features relevant to resonant phenomena in spherical dynamos, and find parametric resonance to be the most pronounced type of resonance phenomena. Resonance conditions for these dynamo wave resonances are rather different from those for more conventional branches of physics. We suggest that the relative insignificance of the phenomenon in this case is because the phenomena of excitation and propagation of the activity waves are not well-separated from each other and this, together with the nonlinear nature of more-or-less realistic dynamos, suppress the resonances and makes them much less pronounced than resonant effects, for example in optics.
We demonstrate that a quasi-uniform cosmological seed field is a much less suitable seed for a galactic dynamo than has often been believed. The age of the Universe is insufficient for a conventional galactic dynamo to generate a contemporary galacti
c magnetic field starting from such a seed, accepting conventional estimates for physical quantities. We discuss modifications to the scenario for the evolution of galactic magnetic fields implied by this result. We also consider briefly the implications of a dynamo number that is significantly larger than that given by conventional estimates.
