Dmitri Sokoloff

Active longitudes, nonaxisymmetric dynamos and phase mixing

We discuss the problem of solar active longitudes from the viewpoint of dynamo theory. We start from a recent observational analysis of the problem undertaken by Berdyugina & Usoskin (2003, AA however we can not exclude in principle such an explanation. We relate the phenomenon of solar active longitudes to the information available concerning stellar active longitudes, and also consider evidence from other tracers of solar activity.

Current Helicity of Active Regions as a Tracer of Large-scale Solar Magnetic Helicity

We demonstrate that the current helicity observed in solar active regions traces the magnetic helicity of the large-scale dynamo generated field. We use an advanced two-dimensional mean-field dynamo model with dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic two-dimensional mean-field dynamo model with simple algebraic alpha-quenching only. Using these numerical models we obtained butterfly diagrams both for the small-scale current helicity and also for the large-scale magnetic helicity, and compared them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by −A · B evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation. Here B and A are, respectively, the dynamo generated mean magnetic field and its vector potential. A theoretical interpretation of these results is given.

Multiscale magnetic fields in spiral galaxies: evolution and reversals

Context. Magnetic fields in nearby, star-forming galaxies reveal large-scale patterns, as predicted by dynamo models, but also a variety of small-scale structures. In particular, a large-scale field reversal may exist in the Milky Way while no such reversals have been observed so far in external galaxies. Aims. The effects of star-forming regions in galaxies need to be included when modelling the evolution of their magnetic fields, which can then be compared to future radio polarization observations. The conditions leading to large-scale field reversals also need clarification. Methods. Our simplified model of field evolution in isolated disc galaxies includes a standard mean-field dynamo and continuous injection of turbulent fields (the effect of supernova explosions) in discrete star forming regions by implicit small-scale dynamo action. Synthetic maps of radio synchrotron emission and Faraday rotation measures are computed for galaxies at different evolutionary stages. Results. A large-scale dynamo is essential to obtain regular large-scale spiral magnetic fields, as observed in many galaxies. These appear, on kpc scales in near energy equilibrium with the turbulence, after 1–2 Gyr (corresponding to redshift about 4−3). The injection of turbulent fields generates small-scale field structures. Strong injected small-scale fields and a large dynamo number (e.g. rapid rotation) of a galaxy favour the generation of field reversals. Depending on the model parameters, large-scale field reversals may persist over many Gyrs and can survive until the present epoch. Significant polarized radio synchrotron emission from young galaxies is expected at redshift ≤4. Faraday rotation measures (RM) are crucial to detect field reversals. Large-scale RM patterns of rotation measures can be observed at redshift ≤3. Conclusions. Our model can explain the general form of axisymmetric spiral fields with many local distortions, as observed in nearby galaxies. For a slightly different choice of parameters, large-scale field reversals can persist over the lifetime of a galaxy. Comparing our synthetic radio maps with future observations of distant galaxies with the planned Square Kilometre Array (SKA) and its precursors will allow testing and refinement of models of magnetic field evolution.

The formation of regular interarm magnetic fields in spiral galaxies

Observations of regular magnetic fields in several nearby galaxies reveal magnetic arms situated between the material arms. The nature of these magnetic arms is a topic of active debate. Previously we found a hint that taking into account the effects of injections of small-scale magnetic fields generated, e.g., by turbulent dynamo action, into the large-scale galactic dynamo can result in magnetic arm formation. We now investigate the joint roles of an arm/interarm turbulent diffusivity contrast and injections of small-scale magnetic field on the formation of large-scale magnetic field (magnetic arms) in the interarm region. We use the relatively simple no-

Reversals of the solar dipole

z

Magnetic and gaseous spiral arms in M83

model for the galactic dynamo. This involves projection on to the galactic equatorial plane of the azimuthal and radial magnetic field components; the field component orthogonal to the galactic plane is estimated from the solenoidality condition. We find that addition of diffusivity gradients to the effect of magnetic field injections makes the magnetic arms much more pronounced. In particular, the regular magnetic field component becomes larger in the interarm space compared to that within the material arms.The joint action of the turbulent diffusivity contrast and small-scale magnetic field injections (with the possible participation of other effects previously suggested) appears to be a plausible explanation for the phenomenon of magnetic arms.

Enhancement of magnetic fields arising from galactic encounters

Context. During a solar magnetic field reversal the magnetic dipole moment does not vanish, but migrates between poles, in contradiction to the predictions of mean-field dynamo theory. Aims. We try to explain this as a consequence of magnetic fluctuations. Methods. We used the statistics of fluctuations to estimate observable signatures. Results. Simple statistical estimates, taken with results from mean-field dynamo theory, suggest that a non-zero dipole moment may persist through a global field reversal. Conclusions. Fluctuations in the solar magnetic field may play a key role in explaining reversals of the solar dipole.

Resonances for activity waves in spherical mean field dynamos

Isotropic and anisotropic wavelet transforms are used to decompose the images of the spiral galaxy M83 in various tracers to quantify structures in a range of scales from 0.2 to 10 kpc. We used radio polarization observations at {lambda}6 cm and 13 cm obtained with the VLA, Effelsberg and ATCA telescopes and APEX sub-mm observations at 870 {mu}m, which are first published here, together with maps of the emission of warm dust, ionized gas, molecular gas, and atomic gas. The spatial power spectra are similar for the tracers of dust, gas, and total magnetic field, while the spectra of the ordered magnetic field are significantly different. The wavelet cross-correlation between all material tracers and total magnetic field is high, while the structures of the ordered magnetic field are poorly correlated with those of other tracers. -- The magnetic field configuration in M83 contains pronounced magnetic arms. Some of them are displaced from the corresponding material arms, while others overlap with the material arms. The magnetic field vectors at {lambda}6 cm are aligned with the outer material arms, while significant deviations occur in the inner arms and in the bar region, possibly due to non-axisymmetric gas flows. Outside the bar region, the typical pitch angles of the material and magnetic spiral arms are very close to each other at about 10{deg}. The typical pitch angle of the magnetic field vectors is about 20{deg} larger than that of the material spiral arms. One of the main magnetic arms in M83 is displaced from the gaseous arms, while the other main arm overlaps a gaseous arm. We propose that a regular spiral magnetic field generated by a mean-field dynamo is compressed in material arms and partly aligned with them. The interaction of galactic dynamo action with a transient spiral pattern is a promising mechanism for producing such complicated spiral patterns as in M83.

Galactic winds and the origin of large-scale magnetic fields

Context. Galactic encounters are usually marked by a substantial increase in synchrotron emission of the interacting galaxies when compared with the typical emission from similar non-interacting galaxies. This increase is believed to be associated with an increase in the star formation rate and the turbulent magnetic fields resulting from the encounter, while the regular magnetic field is usually believed to decrease as a result of the encounter. Aims. We attempt to verify these expectations. Methods. 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 resulting from star forming regions. We represent an encounter by the

Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

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 mean-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.