共查询到20条相似文献，搜索用时 31 毫秒
1.
Hirokazu Yoshimura 《Journal of Astrophysics and Astronomy》2000,21(3-4):365-371
We briefly describe historical development of the concept of solar dynamo mechanism that generates electric current and magnetic
field by plasma flows inside the solar convection zone. The dynamo is the driver of the cyclically polarity reversing solar
magnetic cycle. The reversal process can easily and visually be understood in terms of magnetic field line stretching and
twisting and folding in three-dimensional space by plasma flows of differential rotation and global convection under influence
of Coriolis force. This process gives rise to formation of a series of huge magnetic flux tubes that propagate along iso-rotation
surfaces inside the convection zone. Each of these flux tubes produces one solar cycle. We discuss general characteristics
of any plasma flows that can generate magnetic field and reverse the polarity of the magnetic field in a rotating body in
the Universe. We also mention a list of problems which are currently being disputed concerning the solar dynamo mechanism
together with observational evidences that are to be constraints as well as verifications of any solar cycle dynamo theories
of short and long term behaviors of the Sun, particularly time variations of its magnetic field, plasma flows, and luminosity. 相似文献
2.
In recent times evidence for bimodal distributions of stars in the H–R diagram has reached a striking evidence. These bimodal distributions seem to be correlated with a bimodal distribution of masses and angular velocities. The approach we propose to explain the observed bimodality suggests that this latter is due to a bimodal mass loss by magnetically controlled stellar winds during stellar evolution, owing to different magnetic field configurations. It is assumed a mechanism analogous to that which produces solar wind, with magnetic field generated by dynamo working in the convection zone. Different field geometries (dipole cr quadrupole), which depend on the mode the dynamo operates, can produce different but discrete mass losses during stellar evolution, thus producing bimodal distributions of masses and angular velocities. 相似文献
3.
Yu. V. Vandakurov 《Astronomy Letters》2002,28(8):560-567
The condition of minimum total dissipation is used to derive stationary rotation and azimuthal magnetic field distributions in the bulk of the solar convection zone with an upper boundary at which the relative radius is r/R=0.95. General equilibrium con figurations with symmetric and antisymmetric (about the equator) angular-velocity and field components are determined. The calculated rotation law matches the observed one in general parameters, but the decrease in angular velocity at high latitudes in theory is larger than that in observations. Besides, there are additional sharp variations in the rotation and field distributions in the theoretical curves near the generation zone of solar torsional waves. The possible cause of the latter discrepancy is discussed. The change in equilibrium distributions due to the presence of an inverse molecular-weight gradient at the base of the convection zone is also studied. This gradient is known to be produced by accelerated gravitational helium settling in the convection zone. 相似文献
4.
An asymptotic solution of generation equations for the solar mean magnetic field is given and studied. The variation of rotational angular velocity with depth is taken from helioseismological data. Average helicity is prescribed according to the mixing length theory. It is shown that three dynamo waves of the magnetic field are excited. The first wave is generated at the surface layer and concentrates at latitudes of about 60°. Its activity becomes apparent in the poleward migration of the zone of polar faculae formation. The second more powerful wave of the field is excited in the center of the convection zone and its activity shows up in a sunspot cycle. The third wave which is similar to the first wave, is generated at the bottom of the convection zone and attenuates towards the surface. Its activity may appear as a three-fold reversal of the polar magnetic field. 相似文献
5.
It is thought that the large-scale solar-cycle magnetic field is generated in a thin region at the interface of the radiative core (RC) and solar convection zone (SCZ). We show that the bulk of the SCZ virogoursly generates a small-scale turbulent magnetic field. Rotation, while not essential, increases the generation rate of this field.Thus, fully convective stars should have significant turbulent magnetic fields generated in their lower convection zones. In these stars the absence of a radiative core, i.e., the absence of a region of weak buoyancy, precludes the generation of a large-scale magnetic field, and as a consequence the angular momentum loss is reduced. This is, in our opinion, the explanation for the rapid rotation of the M-dwarfs in the Hyades cluster.Adopting the Utrecht's group terminology, we argue that the residual chromospheric emission should have three distinctive components: the basal emission, the emission due to the large-scale field, and the emission due to the turbulent field, with the last component being particularly strong for low mass stars.In the conventional dynamo equations, the dynamo frequencies and the propagation of the dynamo wave towards the equator are based on the highly questionable assumption of a constant . Furthermore, meridional motions, a necessary consequence of the interaction of rotation with convection, are ignored. In this context we discuss Stenflo's results about the global wave pattern decomposition of the solar magnetic field and conclude that it cannot be interpreted in the framework of the conventional dynamo equations.We discuss solar dynamo theories and argue that the surface layers could be essential for the generation of the poloidal field. If this is the case an -effect would not be needed at the RC-SCZ interface (where the toroidal field is generated). The two central problems facing solar dynamo theories may the transport of the surface poloidal field to the RC-SCZ interface and the uncertainty about the contributions to the global magnetic field by the small-scale magnetic features.Visitor, National Solar Observatory, National Optical Astronomy Observatories.The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 相似文献
6.
A kinematic -dynamo model of magnetic field generation in a thin convection shell with nonuniform helicity for large dynamo numbers is considered in the framework of Parker's migratory dynamo. The asymptotic solution obtained of equations governing the magnetic field has the form of an anharmonic travelling dynamo wave. This wave propagates over most latitudes of the solar hemisphere from high latitudes to the equator, and the amplitude of the magnetic field first increases and then decreases with propagation. Over the subpolar latitudes, the dynamo wave reverses; there the dynamo wave propagates polewards and decays with latitude. The half-width of the maximum of the magnetic field localisation and the phase velocity of the dynamo wave are calculated. Butterfly diagrams are plotted and analysed and these show that even a simple model may reveal some properties of the solar magnetic fields. 相似文献
7.
Electric current helicity in the solar atmosphere 总被引：2，自引：0，他引：2
N. Seehafer 《Solar physics》1990,125(2):219-232
In the theories of solar magnetism, kinetic and magnetic helicities, which arise as a consequence of the rotation of the Sun, play a key role. The dynamo for the main field is assumed to operate in the convection zone. The solar rotation also may be the ultimate cause for the generation of dc electric currents in the atmosphere, needed as the energy source for flares. Then in the atmosphere the electric current helicity, H
C
= B · × B, which is a pseudo-scalar quantity, should be antisymmetric about the equatorial plane. An inspection of 16 active regions, for which H
C has been estimated by using extrapolation of measured photospheric magnetic fields, leads to the result that the electric current helicity is predominantly negative in the northern and positive in the southern hemisphere. The helicity of the large-scale currents generated according to standard dynamo theory by the alpha effect in the convection zone is just opposite in sign. Current generation due to rotational motions of sunspots and other magnetic elements in accordance with the global differential rotation, i.e., counter-clockwise in the northern and clockwise in the southern hemisphere, however, can explain the rule found. Also in some alternative dynamo models for the global field, in which the dynamo operates at the base of the convection zone, the large-scale current helicity generated by the alpha effect has the sign needed. 相似文献
8.
A model for the solar dynamo, consistent in global flow and numerical method employed with the differential rotation model, is developed. The magnetic turbulent diffusivity is expressed in terms of the entropy gradient, which is controlled by the model equations. The magnetic Prandtl number and latitudinal profile of the alpha-effect are specified by fitting the computed period of the activity cycle and the equatorial symmetry of magnetic fields to observations. Then, the instants of polar field reversals and time-latitude diagrams of the fields also come into agreement with observations. The poloidal field has a maximum amplitude of about 10 Gs in the polar regions. The toroidal field of several thousand Gauss concentrates near the base of the convection zone and is transported towards the equator by the meridional flow. The model predicts a value of about 1037 erg for the total magnetic energy of large-scale fields in the solar convection zone. 相似文献
9.
We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer α Ω dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
We investigated the generation of dynamo waves in the solar convection zone through a numerical simulation. We integrated the axisymmetric α–Ω kinematic dynamo equations in a spherical geometry, where the α- and Ω-profiles depend on the spatial coordinates. The model results show that the fundamental parameter that determines the behavior of the system is the product between the characteristic intensities of the α and Ω contributions. In particular, we found three different regimes in which the system exhibits different behaviors: a regime without a dynamo effect, one with an exponential amplification of the magnetic field, and one with dynamo waves. 相似文献
11.
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
12.
Magnetic buoyancy is thought to play an important role in the dynamical behavior of the Sun's magnetic field in the convection zone. Magnetic buoyancy is commonly thought to cause inescapable rapid loss of toroidal flux from much of the convection zone, thereby suppressing effective operation of a solar dynamo. This paper re-examines the detailed character of magnetic buoyancy, especially as it is influenced by the magnetic field's effect on heat transport and temperature gradients in the convection zone. It is suggested that suppression of convective heat transport across strong magnetic flux tubes can alter the temperature within the tubes and can subdue, or even reverse, the effect of magnetic buoyancy. 相似文献
13.
Guided by the recent observational result that the meridional circulation of the Sun becomes weaker at the time of the sunspot
maximum, we have included a parametric quenching of the meridional circulation in solar dynamo models such that the meridional
circulation becomes weaker when the magnetic field at the base of the convection zone is stronger. We find that a flux transport
solar dynamo tends to become unstable on including this quenching of meridional circulation if the diffusivity in the convection
zone is less than about 2×1011 cm2 s−1. The quenching of α, however, has a stabilizing effect and it is possible to stabilize a dynamo with low diffusivity with sufficiently strong
α-quenching. For dynamo models with high diffusivity, the quenching of meridional circulation does not produce a large effect
and the dynamo remains stable. We present a solar-like solution from a dynamo model with diffusivity 2.8×1012 cm2 s−1 in which the quenching of meridional circulation makes the meridional circulation vary periodically with solar cycle as observed
and does not have any other significant effect on the dynamo. 相似文献
14.
E. E. Benevolenskaya 《Solar physics》1995,161(1):1-8
The source of the poloidal magnetic field was fixed using a uniform series of surface low-resolution magnetic field observations begun at Wilcox Solar Observatory at Stanford. The results obtained confirm the idea that low-frequency dynamo waves with a period approximately equal to 22 years and a high-frequency wave of a quasi-two-year period can coexist. It seems that an interaction between these components in the convection zone takes place on the Sun. Surface large-scale solar magnetic fields are analyzed using a two-dimensional Fourier method technique to study the poloidal field distribution. The first harmonic approximately equals the period of the magnetic cycle, appears at all latitudes, and reaches its the maximum value in the polar regions. Moreover, spectral analyses of axisymmetric magnetic field derivative in time found that the second important harmonic of a period approximately equal to two years appears at all latitudes. This second high-frequency harmonic dominates the polar latitude regions at the same time as the low-frequency one. 相似文献
15.
We assume the large-scale diffuse magnetic field of the Sun to originate from the poloidal component of a dynamo operating at the base of the convection zone, whereas the sunspots are due to the toroidal component. The evolution of the poloidal component is studied to model the poleward migration of the diffuse field seen on the solar surface and the polar reversal at the time of sunspots maxima (Dikpati and Choudhuri 1994, 1995). 相似文献
16.
Arnab Rai Choudhuri 《Solar physics》2003,215(1):31-55
Mean field dynamo theory deals with various mean quantities and does not directly throw any light on the question of existence of flux tubes. We can, however, draw important conclusions about flux tubes in the interior of the Sun by combining additional arguments with the insights gained from solar dynamo solutions. The polar magnetic field of the Sun is of order 10 G, whereas the toroidal magnetic field at the bottom of the convection zone has been estimated to be 100000 G. Simple order-of-magnitude estimates show that the shear in the tachocline is not sufficient to stretch a 10 G mean radial field into a 100000 G mean toroidal field. We argue that the polar field of the Sun must get concentrated into intermittent flux tubes before it is advected to the tachocline. We estimate the strengths and filling factors of these flux tubes. Stretching by shear in the tachocline is then expected to produce a highly intermittent magnetic configuration at the bottom of the convection zone. The meridional flow at the bottom of the convection zone should be able to carry this intermittent magnetic field equatorward, as suggested recently by Nandy and Choudhuri (2002). When a flux tube from the bottom of the convection zone rises to a region of pre-existing poloidal field at the surface, we point out that it picks up a twist in accordance with the observations of current helicities at the solar surface. 相似文献
17.
Allan Sacha Brun 《Solar physics》2004,220(2):333-345
We have performed 3-D numerical simulations of compressible convection under the influence of rotation and magnetic fields
in spherical shells. They aim at understanding the subtle coupling between convection, rotation and magnetic fields in the
solar convection zone. We show that as the magnetic Reynolds number is increased in the simulations, the magnetic energy saturates
via nonlinear dynamo action, to a value smaller but comparable to the kinetic energy contained in the shell, leading to increasingly
strong Maxwell stresses that tend to weaken the differential rotation driven by the convection. These simulations also indicate
that the mean toroidal and poloidal magnetic fields are small compared to their fluctuating counterparts, most of the magnetic
energy being contained in the non-axisymmetric fields. The intermittent nature of the magnetic fields generated by such a
turbulent convective dynamo confirms that in the Sun the large-scale ordered dynamo responsible for the 22-year cycle of activity
can hardly be located in the solar convective envelope. 相似文献
18.
We present a straightforward comparison of model calculations for the α-effect, helicities, and magnetic field line twist
in the solar convection zone with magnetic field observations at atmospheric levels. The model calculations are carried out
in a mixing-length approximation for the turbulence with a profile of the solar internal rotation rate obtained from helioseismic
inversions. The magnetic field data consist of photospheric vector magnetograms of 422 active regions for which spatially-averaged
values of the force-free twist parameter and of the current helicity density are calculated, which are then used to determine
latitudinal profiles of these quantities. The comparison of the model calculations with the observations suggests that the
observed twist and helicity are generated in the bulk of the convection zone, rather than in a layer close to the bottom.
This supports two-layer dynamo models where the large-scale toroidal field is generated by differential rotation in a thin
layer at the bottom while the α-effect is operating in the bulk of the convection zone. Our previous observational finding
was that the moduli of the twist factor and of the current helicity density increase rather steeply from zero at the equator
towards higher latitudes and attain a certain saturation at about 12 – 15∘. In our dynamo model with algebraic nonlinearity, the increase continues, however, to higher latitudes and is more gradual.
This could be due to the neglect of the coupling between small-scale and large-scale current and magnetic helicities and of
the latitudinal drift of the activity belts in the model. 相似文献
19.
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
20.
The first results of the solar dynamo model that allows for the diamagnetic effect of inhomogeneous turbulence and the nonlocal
alpha-effect due to the rise ofmagnetic loops are discussed. The nonlocal alpha-effect is not subject to the catastrophic
quenching related to the conservation of magnetic helicity. Given the diamagnetic pumping, the magnetic fields are concentrated
near the base of the convection zone, although the distributed-typemodel covers the entire thickness of the convection zone.
Themagnetic cycle period, the equatorial symmetry of the field, its meridional drift, and the polar-to-toroidal field ratio
obtained in the model are in agreement with observations. There is also some disagreement with observations pointing the ways
of improving the model. 相似文献