共查询到20条相似文献，搜索用时 31 毫秒
1.
M. P. White 《Astronomische Nachrichten》1978,299(4):209-216
Axisymmetric αω-dynamo models are investigated numerically for the galactic dynamo. Contrasting to the eigenvalue formulation of the problem by STIX (1976), an initial-value formulation is developed in a manner which is a generalization of the approach to the solar dynamo by JEPPS (1975). It is found, for STIX's model, that the critical dynamo numbers, Pc, obtained by this approach do not agree with those obtained by STIX . In order to resolve this disagreement SOWARD (1977) has evaluated an asymptotic formula for Pc which confirm the results presented here. Having established this approach, the dependence of the models upon boundary conditions and the relevant astrophysical parameters is investigated, and an attempt is made to simulate nonlinear effects. Finally a comparison is made between predictions of the dynamo models and the observed radiation of certain external galaxies which provides insight into the nature of the intergalactic medium. 相似文献
2.
P.J. Kpyl 《Astronomische Nachrichten》2011,332(1):43-50
Global dynamo simulations solving the equations of magnetohydrodynamics (MHD) have been a tool of astrophysicists who try to understand the magnetism of the Sun for several decades now. During recent years many fundamental issues in dynamo theory have been studied in detail by means of local numerical simulations that simplify the problem and allow the study of physical effects in isolation. Global simulations, however, continue to suffer from the age‐old problem of too low spatial resolution, leading to much lower Reynolds numbers and scale separation than in the Sun. Reproducing the internal rotation of the Sun, which plays a crucial role in the dynamo process, has also turned out to be a very difficult problem. In the present paper the current status of global dynamo simulations of the Sun is reviewed. Emphasis is put on efforts to understand how the large‐scale magnetic fields, i.e. whose length scale is greater than the scale of turbulence, are generated in the Sun. Some lessons from mean‐field theory and local simulations are reviewed and their possible implications to the global models are discussed. Possible remedies to some current issues of solar simulations are put forward (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
3.
C. G. Campbell J. C. B. Papaloizou & V. Agapitou 《Monthly notices of the Royal Astronomical Society》1998,300(1):315-320
We consider the problem of poloidal magnetic field advection and bending of an initially vertical field owing to radial inflow in thin accretion discs. For a ratio of kinematic viscosity to magnetic diffusivity of order unity, significant bending of an externally applied vertical field cannot occur in a disc with no internal dynamo. However, we show that if poloidal field is generated by a dynamo operating near its critical state, then significant field bending may be possible. Our results are of particular relevance to wind launching from accretion discs. 相似文献
4.
M. R. E. Proctor 《Astronomische Nachrichten》1977,298(1):19-25
The kinematic α-effect dynamo problem is investigated in the case of an exterior perfect conductor. It is shown that certain approximate symmetries discovered in the numerical analysis of ROBERTS (1972) are exact for this case. As an illustration, an exact solution is given in a cylindrical geometry, where the equations can be written in terms of one variable. The implications for the earth's dynamo are discussed. 相似文献
5.
The generation of magnetic field in a homogeneous, electrically conducting fluid – as required for the dynamo generation of the fields of many astrophysical bodies – is normally a threshold process; the dynamo mechanism, applicable to such bodies in unmagnetised environments, requires motions of sufficient strength to overcome the innate magnetic diffusion. In the presence of an ambient field, however, the critical nature of the field generation process is relaxed. Motions can distort and amplify the ambient field for all amplitudes of flow. For motions with appropriate geometries, an internal ‘dynamo‐like’ field of appreciable strength can be generated, even for relatively weak flows. At least a minority of planets, moons and other bodies exist within significant external astrophysical fields. For these bodies, the ambient field problem is more relevant than the classical dynamo problem, yet it remains relatively little studied. In this paper we consider the effect of an axial ambient field on a spherical mean‐field α 2ω dynamo model, through nonlinear calculations with α ‐quenching feedback. Ambient fields of varying strengths, and both stationary and oscillatory in time, are imposed. Particular focus is placed on the effects of these fields on the equatorial symmetry and the time dependence of the preferred solutions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
6.
M. Reyes-Ruiz 《Monthly notices of the Royal Astronomical Society》2000,319(4):1039-1046
In the light of recent results from numerical simulations of accretion disc MHD turbulence, we revisit the problem of the configuration of large-scale magnetic fields resulting from an α Ω dynamo operating in a thin accretion disc. In particular, we analyse the consequences of the peculiar sign of the α -effect suggested by numerical simulations . We determine the symmetry of the fastest-growing modes in the kinematic dynamo approximation and, in the framework of an ' α -quenched' dynamo model, study the evolution of the magnetic field. We find that the resulting field for this negative polarity of the α -effect generally has dipole symmetry with respect to the disc midplane, although the existence of an equilibrium configuration depends on the properties of the turbulence. The role of magnetic field dragging is discussed and, finally, the presence of an external uniform magnetic field is included to address the issue of magneto centrifugal wind launching from accretion discs. 相似文献
7.
For a simple spherically symmetric mean‐field dynamo model we investigate the possibility of determining the radial dependence of the coefficient α. Growth rates for different magnetic field modes are assumed to be known by measurement. An evolutionary strategy (ES) is used for the solution of the inverse problem. Numerically, we find quite different α‐profiles giving nearly the same eigenvalues. The ES is also applied to find functions α(r) yielding zero growth rates for the lowest four magnetic field modes. Additionally, a slight modification of the ES is utilized for an “energetic” optimization of α2‐dynamos. The consequences of our findings for inverse dynamo theory and for the design of future dynamo experiments are discussed. 相似文献
8.
The problem of whether the solar dynamo is quasi-periodic or chaotic is addressed by examining 1500 years of sunspot, geomagnetic and auroral activity cycles. We find sub-harmonics of the fundamental solar cycle period during the years preceding the Maunder minimum and loss of phase of the subharmonic on emergence from it. These phenomena are indicative of chaos. They indicate that the solar dynamo is chaotic and is operating in a region close to the transition between period doubling and chaos. Since Maunder type minima reoccur irregularly for millennia, it appears that the Sun remains close to this transition to and from chaos. We postulate this as a universal characteristic of solar type stars caused by feedback in the dynamo number. 相似文献
9.
A. F. Lanza M. Rodonò R. Rosner 《Monthly notices of the Royal Astronomical Society》2000,314(2):398-402
The conjecture is presented that the gap in the distribution of the orbital periods of cataclysmic variables is related to a particular kind of hydromagnetic dynamo, called an interface dynamo, operating near the base of the convective envelope of their secondary components. Such a dynamo is characterized by the spatial separation of the regions where differential rotation and the α effect operate. Unlike conventional dynamos, the linear growth rate of an interface dynamo becomes negative for highly supercritical dynamo numbers, leading to the disappearance of the dynamo action. If such a result, from linear theory, is confirmed by non-linear calculations, it may provide a physical basis for the so-called disrupted magnetic braking hypothesis, invoked to explain the existence of the period gap by several evolutionary models of cataclysmic variables. 相似文献
10.
Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non‐rotating wall‐bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non‐axisymmetric instabilities of shearinduced axisymmetric toroidal velocity or magnetic fields, such as Kelvin‐Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high‐Reynolds number shear flows could act as a large‐scale driving mechanism of turbulent flows that would in turn generate an independent small‐scale dynamo. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
11.
A two-layer spherical alpha2 dynamo model consisting of an inner electrically conducting core (magnetic diffusivity lambdai and radius ri) with alpha=0 surrounded by an electrically conducting spherical shell (magnetic diffusivity lambdao and radius ro) with a constant alpha is shown to exhibit oscillatory behavior for values of beta=lambdai&solm0;lambdao and ri&solm0;ro relevant to the solar dynamo. Time-dependent dynamo solutions require ri&solm0;ro>/=0.55 and beta相似文献
12.
The problem of magnetic field generation under screw motion in a toroidal channel is studied numerically. The screw dynamo in the cylinder with periodical boundary conditions was found to be a suitable approximation for generation of the magnetic field by a screw flow in a thin torus. For the thick torus, a principally new solution of the screw dynamo problem was obtained. In this case the growing global magnetic field mode has the scale of a maximal geometrical size of the torus and does not vanish on the axis of the torus (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
S.-I. Akasofu 《Planetary and Space Science》1983,31(1):25-48
This paper emphasizes an effort to link processes which relate solar activity and magnetospheric disturbances in terms of energy transfer through a chain of four elements. In this view, each element is explicitly thought to be powered by a dynamo, namely the solar wind generation dynamo, the solar flare dynamo, the solar wind-magnetosphere dynamo and the auroral dynamo, respectively. Each dynamo powers a plasma acceleration process by the Lorentz force and the plasma flows thus generated are the solar wind, the flare-generated solar wind disturbance, the magnetospheric plasma convection and the ionospheric convection, respectively. Each plasma flow conveys the energy from one element to the next in the chain. Some of the kinetic energy of the photospheric plasma is eventually deposited in the polar ionosphere as heat energy. 相似文献
14.
Rapidly rotating young (T Tauri, pre-Main-Sequence, and Main-Sequence) stars as well as subgiants seem to show starspots not only at low and middle latitudes, as the Sun, but also at high latitudes and even around the poles. We consider a simple nonlinear Parker migratory dynamo model working in a thin shell in order to investigate how high latitude and polar spots may be produced for different values of the dynamo layer radius and thickness and for various rotation rates. Simple assumptions on the angular velocity gradient and helicity distribution are made according to symmetry properties and recent solar and stellar observations. A recently proposed asymptotic WKB-type approach is used to solve the dynamo problem and its drawbacks and advantages in the solar and stellar contexts are discussed. As a general result, we find that a sizable toroidal field can be produced over a much more extended latitude range than in the Sun, thus explaining in a natural way the occurrence of activity from the poles to the equator in such stars. Our approach complements that proposed by Schüssler et al. (1996) which is focused on the instability and emergence of the azimuthal flux tubes, as well as the analyses based on a dynamo working over an extended part of the stellar convective envelope (Moss, Tuominen, and Brandenburg, 1991; Moss et al., 1995). 相似文献
15.
In the context of astrophysical dynamos we illustrate that the no-cosines flow, with zero mean helicity, can drive fast dynamo
action and we study the dynamo’s mode of operation during both the linear and non-linear saturation regimes. It turns out
that in addition to a high growth rate in the linear regime, the dynamo saturates at a level significantly higher than normal
turbulent dynamos, namely at exact equipartition when the magnetic Prandtl number Prm∼ 1. Visualization of the magnetic and velocity fields at saturation will help us to understand some of the aspects of the
non-linear dynamo problem. 相似文献
16.
Numerical studies with a spherical dynamo model have shown two remarkable phenomena. The model consists of a spherical body of an electrically conducting incompressible uid surrounded by free space. In addition to a rotation of the body an inner motion due to a given forcing is considered which satisfies a no–slip condition at the boundary. The full interaction of magnetic field and motion is taken into account. Starting from a fluid motion capable of dynamo action and a very weak magnetic field it was observed that the growing magnetic field destroys the dynamo property of the motion and then decays, and that the system ends up in a state with another motion incapable of dynamo action and zero magnetic field. In another case with a motion unable to prevent small magnetic fields from decay it proved to be possible that stronger magnetic fields deform it so that a dynamo starts to work which enables the system to approach a steady state with a finite magnetic field. 相似文献
17.
Shao-Lan Bi Tan-Da Li Kang Liu Jie Jiang Ya-Guang Li Jing-Hua Zhang Xian-Fei Zhang Ya-Qian Wu 《天文和天体物理学研究(英文版)》2021,(4):157-164
The change of sound speed has been found at the base of the convection during the solar cycles,which can be used to constrain the solar internal magnetic field.We aim to check whether the magnetic field generated by the solar dynamo can lead to the cyclic variation of the sound speed detected through helioseismology.The basic configuration of magnetic field in the solar interior was obtained by using a Babcock-Leighton(BL) type flux transport dynamo.We reconstructed one-dimensional solar models by assimilating magnetic field generated by an established dynamo and examined their influences on the structural variables.The results show that magnetic field generated by the dynamo is able to cause noticeable change of the sound speed profile at the base of the convective zone during a solar cycle.Detailed features of this theoretical prediction are also similar to those of the helioseismic results in solar cycle 23 by adjusting the free parameters of the dynamo model. 相似文献
18.
Eric G. Blackman George B. Field 《Monthly notices of the Royal Astronomical Society》2000,318(3):724-732
We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between energy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in large magnetic Reynolds number rotators, there must then be an escape of relative magnetic helicity associated with the mean field, accompanied by an equal and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity including the dissipation of magnetic energy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo simulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert 1 AGN coronae. 相似文献
19.
Colin B. Roald 《Monthly notices of the Royal Astronomical Society》1998,300(2):397-410
In an attempt to produce a simple representation of an interface dynamo, I examine a dynamo model composed of two one-dimensional (radially averaged) pseudo-spherical layers, one in the convection zone and possessing an α-effect, and the other in the tachocline and possessing an ω-effect. The two layers communicate by means of an analogue of Newton's law of cooling, and a dynamical back-reaction of the magnetic field on ω is provided. Extensive bifurcation diagrams are calculated for three separate values of η, the ratio of magnetic diffusivities of the two layers. I find recognizable similarities to, but also dramatic differences from, the comparable one-layer model examined by Roald &38; Thomas. In particular, the solar-like dynamo mode found previously is no longer stable in the two-layer version; in its place there is a sequence of periodic, quasi-periodic and chaotic modes probably created in a homoclinic bifurcation. These differences are important enough to provide support for the view that the solar dynamo cannot be meaningfully modelled in one dimension. 相似文献
20.
M. Yu. Reshetnyak 《Astronomy Letters》2016,42(7):482-487
The inverse problem in a spherical shell to find the two-dimensional spatial distributions of the α-effect and differential rotation in a mean-field dynamo model has been solved. The derived distributions lead to the generation of a magnetic field concentrated inside the convection zone. The magnetic field is shown to have no time to rise from the region of maximum generation located in the lower layers to the surface in the polarity reversal time due to magnetic diffusion. The ratio of the maximum magnetic energy in the convection zone to its value at the outer boundary reaches two orders of magnitude or more. This result is important in interpreting the observed stellar and planetary magnetic fields. The proposed method of solving the inverse nonlinear dynamo problem is easily adapted for a wide class of mathematical-physics problems. 相似文献