A low-mode dynamo with meridional circulation and dipolar symmetry of the magnetic field

A dynamic system for the Parker dynamo including meridional circulation that is applicable to astrophysical objects is constructed. The meridional circulation is able to control the regimes for the generation of magnetic fields. If the meridional flows are weak, regimes with steady oscillations, dynamo bursts, fluctuations, and chaotic components are possible. When the meridional circulation is strengthened, the range of dynamo numbers required for fluctuations and dynamo bursts is reduced, and gradually vanishes; at the same time, the range required for oscillations is increased and raised to higher dynamo numbers. The latitude-time distributions of the toroidal and poloidal magnetic fields for steady oscillations are presented.     

A dynamo in a torus as an explanation of magnetic fields in the outer rings of galaxies

It is currently generally believed that magnetic fields in the disks of spiral galaxies are generated by the dynamo mechanism, which is based on the joint action of differential rotation and the alpha effect, associated with turbulent motions in the interstellar gas. Together with their disks, outer rings are also encountered in galaxies, where magnetic fields may be present. In earlier studies, the generation of magnetic fields has been described in a planar approximation, whose essence is that the size of rings perpendicular to the plane of the galaxy is much smaller than their size in the radial direction. However, it is plausible that these sizesmay sometimes be comparable, so that it would be more logical to suppose that a ring has a toroidal form. A model for a dynamo in a toroidal ring is constructed in this study. This model describes the magnetic field using two functions, corresponding to the toroidal component of the field and the part of the vector potential characterizing its poloidal component. The possible generation of magnetic field in various cases is shown, with both quadrupolar symmetry (close to the fields obtained in the planar approximation) and dipolar symmetry (when two layers with oppositely directed magnetic fields form in the ring). The parameter values for which the generation of fields with one or the other type of symmetry is possible are estimated. The results can also be used to describe the evolution of the magnetic fields in other toroidal astrophysical objects.     

Phase space of a two-dimensional model of a Parker dynamo

The dependences of the magnetic-field strength, variations of the magnetic field, and the multipole level on the amplitudes of the α and ω effects are considered using a two-dimensional model for a Parker dynamo in a spherical layer. Calculations have been carried out for both traditional spatial distributions of α and ω and geostrophic regimes obtained from three-dimensional modeling of thermal convection. Two-dimensional distributions of the dynamo-wave velocities in the zone where magnetic field is generated are presented. Comparisons with the solar and planetary dynamos are considered.     

The effect of meridional circulation on dynamo waves approaching the solar poles

Dynamo waves approaching the solar poles are analyzed in the Parker approximation taking into account meridional circulation. Asymptotic solutions of the equations describing the generation of the magnetic field are constructed. It is shown in which cases the effect of meridional circulation results in traveling dynamo waves both incident on and reflected from the pole, or in a superposition of standing dynamo waves.     

Modeling the generation of the magnetic field in NGC 5775

A model for the generation of large-scale magnetic fields is constructed for the galaxy NGC 5775, in which the magnetic field has the form of a dipolar dynamo wave propagating along the galactic disk. The excitation of such a mode, which is unusual for galactic dynamos, can be explained by the strong variation of the galactic rotation with height above the plane of symmetry of the galactic disk.     

The structure of the global solar magnetic field excited by the turbulent dynamo mechanism

A mixing-length approximation is used to calculate Kλ for a Parker dynamo wave excited by the dynamo mechanism near the base of the solar convection zone (K is the wave number of the dynamo wave and λ the extent of the dynamo region). In a turbulent-dynamo model, this number characterizes the modes of the global magnetic field generated by a mechanism based on the joint action of the mean helical turbulence and solar differential rotation. Estimates are obtained for the helicity and radial angular-velocity gradient using the most recent helioseismological measurements at the growth phase of solar cycle 23. These estimates indicate that the dynamo mechanism most efficiently excites the fundamental antisymmetric (odd), dipole, mode of the poloidal field (Kλ≈?7) at low latitudes, while the conditions at latitudes above 50° are more favorable for the excitation of the lowest symmetric (even), quadrupole, mode (Kλ≈+8). The resulting north-south asymmetry of the poloidal field can explain the magnetic anomaly (“monopole” structure) of the polar fields observed near solar-cycle maxima. The effect of α quenching increases the calculated period of the dynamo-wave propagation from middle latitudes to the equator to about seven years, in rough agreement with the observed duration of the solar cycle.     

New perspectives on Mars’ crustal magnetic field

The planet Mars lacks, today, a planetary, dynamic magnetic field, but strong, intense, localized magnetic fields of lithospheric origin, one to two orders of magnitude larger than the terrestrial lithospheric field, are present. This lithospheric magnetic field is the result of magnetization processes in the presence of a magnetic dynamo and of demagnetization processes after the dynamo shutdown, such as impact or volcanoes. This crude scenario can be more accurately specified by interpreting global and local models of the current magnetic field of Mars. Some specific areas are studied, including the intensely magnetized Terra Sirenum, as well as the magnetic anomaly associated with Apollinaris Patera. Magnetic minerals could be of primary and/or secondary origin; this latter would imply an early hydration of a basaltic crust. A scenario, in which Mars experienced a major polar wander due to the Tharsis bulge, prior to the cessation of its dynamo, is proposed and discussed.     

Can Superflares Occur on the Sun? A View from Dynamo Theory

Recent data from the Kepler mission has revealed the occurrence of superflares in Sun-like stars which exceed by far any observed solar flares in released energy. Radionuclide data do not provide evidence for occurrence of superflares on the Sun over the past eleven millennia. Stellar data for a subgroup of superflaring Kepler stars are analysed in an attempt to find possible progenitors of their abnormal magnetic activity. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation can also be excited. For comparable conditions, magnetic field of second configuration is much stronger than that of the first one just because dynamo does not spend its energy for periodic magnetic field inversions but uses it for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. As suggested in a recent paper [1], we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.     

Magnetic-field structure in the accretion disks of semi-detached binary systems

The results of three-dimensional MHD numerical simulations are used to investigate the characteristic properties of the magnetic-field structures in the accretion disks of semi-detached binary systems. It is assumed that the intrinsic magnetic field of the accretor star is dipolar. Turbulent diffusion of the magnetic field in the disk is taken into account. The SS Cyg system is considered as an example. The results of the numerical simulations show the intense generation of a predominantly toroidal magnetic field in the accretion disk. Magnetic zones with well defined structures for the toroidal magnetic field form in the disk, which are separated by current sheets in which there ismagnetic reconnection and current dissipation. Possible observational manifestations of such structures are discussed. It is shown that the interaction of a spiral precessional wave with the accretor’s magnetosphere could lead to quasi-periodic oscillations of the accretion rate.     

Stability of toroidal magnetic fields in the radiation zone of a star

The stability of a toroidal magnetic field in the rotating radiation zone of a star is analyzed to estimate the maximum possible magnitude of relic fields. Equations for small perturbations are obtained taking into account the finite diffusivity and the stabilizing effect of the subadiabatic stratification. The numerical solution of the eigenvalue problem indicates that the threshold field strength for the onset of instability in the radiation zone of the Sun is about 600 G. This figure sets an upper bound for the strength of the relic field. The assumption that magnetic instabilities are present in the solar radiation zone disagrees with the observed abundance of lithium. Our analysis of joint stability of toroidal field and nonuniform rotation shows that two-dimensional MHD solutions for the solar tachocline are stable against three-dimensional perturbations.     

Small-Scale Dynamo in Accretion Discs

Astronomy Reports - Small-scale magnetic field is suggested as a mechanism for magnetic field self-excitation in accretion discs. Main features of small-scale dynamo action are compared with that...     

Model of a multiscaled MHD dynamo

A simple model for a multiscaled MHD dynamo is suggested. The uppermost tier of the model controls the evolution of the large-scale magnetic field, while the lower tiers are responsible for the evolution of the small-scale velocity and magnetic fields. This approach makes it possible to reproduce, e.g., the evolution of the Galactic magnetic field for realistic magnetic Reynolds numbers, which cannot be done using direct, detailed simulations.     

Moons' magnetism: from Io's and Ganymede's present magnetic signatures to the ancient lunar dynamo

Intrinsic magnetic fields, corresponding to virtual axial dipole moments of the order of 10²⁰ Am², have recently been evidenced for the Jovian satellites Io and Ganymede. By reviewing the rock magnetic and palaeomagnetic properties together with the history of Io, the hypothesis of a moment due to induced or remanent magnetization of crustal rocks acquired within the ambient Jovian field is clearly eliminated. This demonstration is all the more valid for Ganymede, which experiences a much lower Jovian field. The demonstration of a present dynamo action for these Jovian moons, possibly sustained by Jupiter through tidal heating and background magnetic field, may be an actualistic model for the early lunar history. The hypothesis of a lunar dynamo, active at 3–4 Ga, seems to be strongly supported by this analogy.     

Parker’s dynamo near the excitation threshold

A method of constructing asymptotic solutions for nonlinear mean-field dynamo equations near the excitation threshold is developed and applied to equations describing the solar dynamo in a Parker model. The form of solution obtained corresponds to the eigensolution for a kinematic dynamo, for the intensity of the generation sources at which self-excitation of the magnetic field begins (the so-called marginally stable eigenfunction). The wave amplitude is calculated.     

Current helicity and the small-scale dynamo

The small-scale dynamo inherent to mirror-asymmetric turbulence can generate a magnetic field characterized by substantial mirror asymmetry of the associated electric currents. In general, the corresponding helicity should be taken into account in calculations of the helicity balance, which is now used as a basis for models describing the suppression of the large-scale dynamo. However, the mirror asymmetry of the fluctuating magnetic fields is concentrated on scales much shorter than the magnetic-loop diameter. Therefore, the unaccounted-for contribution to the helicity balance is, in fact, not important.     

Active longitudes and north-south asymmetry of the activity the Sun as manifestations of its relic magnetic field

The distributions of dominant magnetic polarities in synoptic maps of photospheric magnetic fields and their extrapolations to the corona based on Stanford Observatory data are studied. Both dipolar and quadrupolar magnetic patterns are detected in the distributions of dominant polarities in the near-equatorial region of the photosphere for activity cycles 21, 22, and 23. The field in these patterns often has opposite signs on opposite sides of the equator, with this sign changing from cycle to cycle. A longitude-time analysis of variations of the mean solar magnetic field shows that the contribution of the large-scale magnetic patterns to the total field does not exceed 20 µT. The most stable magnetic structures at a quasi-source surface in the solar corona are separated by approximately 180° in heliographic longitude and are close to dipolar. The nature and behavior of these large-scale magnetic patterns are interpreted as a superposition of cyclic dynamo modes and the nonaxially symmetric relic field of the Sun. The contribution of the relic field to the mean solar magnetic field appears as a weak but stable rotational modulation whose amplitude does not exceed 8 µT.     

Variations of the dipole magnetic moment of the sun during the solar activity cycle

Observations of the large-scale solar magnetic field (synoptic maps) and measurements of the magnetic field of the Sun as a star (the total magnetic field) are used to determine the dipole magnetic moment and direction of the dipole field for three successive solar cycles. Both the magnetic moment and its vertical and horizontal components vary regularly during the cycle, but never disappear completely. A wavelet analysis of the total magnetic field shows that the amplitude of the 27-day variations of this field is very closely related to the magnetic moment of the horizontal dipole. The reversal of the global dipole field corresponds to a change in the inclination of its axis and occurs in a series of steps lasting one to two years rather than continuously. Before the onset of the reversal, the dipole axis precesses relative to the solar rotational axis, then shifts in a meridianal plane, reaching very low latitudes, where a substantial shift in longitude then begins. These results are discussed in connection with helioseismological data indicating the existence of oscillations with a period of about 1.3 yr and properties of dynamo processes for the case of an inclined rotator.     

Generation of large-scale magnetic fields in young solar-type stars

Differential-rotation and dynamo models are computed for a young, solar-mass star at the initial stage of the formation of its radiative core. It is argued that the global magnetic fields in the radiative zones of the contemporary Sun and similar stars are due to the action of a hydromagnetic dynamo at early evolutionary stages. Our computations suggest that this field should be nonaxisymmetric. Physical reasons for departures from axial symmetry are discussed in detail. It is suggested that nonaxisymmetric relic fields are responsible for the phenomenon of active longitudes.     

Impact of the magnetic field on the structure of accretion disks in semi-detached binaries

We discuss characteristic features of the magnetic gas-dynamical structure of the flows in a semi-detached binary system obtained from three-dimensional simulations, assuming that the intrinsic magnetic field of the accreting star is dipolar. The turbulent diffusion of the magnetic field is taken into account. The SS Cyg system is considered as an example. Including the magnetic field can alter the basic parameters of the accretion disk, such as the accretion rate and the characteristic density. The magnetic field in the disk is primarily toroidal.     

Magnetic fields in the radiative-transport zone and the solar cycle

It is shown that a hypothetical relict magnetic field in the solar radiative-transport zone that penetrates into the convective zone would affect the solar dynamo, resulting in radical changes in the butterfly diagrams. This would transform the traveling waves of activity into standing waves. A comparison of our results with the well-known butterfly diagrams for the Sun gives an upper limit of the order of some tens G for the value of relict magnetic field penetrating into the solar convective zone. At the same time, it is not ruled out that such relict magnetic fields in other solar-type stars are strong enough to make the activity waves become standing waves.