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1.
In the traditional axisymmetric models of the 11-year solar cycle, oscillations of the magnetic fields appear in the background of nonoscillating (over time scale considered) turbulent velocity fields and differential rotation. In this paper, an alternative approach is developed: The excitation of magnetic oscillations with the 22-year period is the consequence of hydrodynamic oscillations with the 11-year period. In the excitation of hydrodynamic oscillations, two processes taking place in high latitudes near the interface between the convective and radiative zones play a key role. One is forcing of the westerly zonal flow, the conditions for which are due to deformation of the interfacial surface. The other process is the excitation of a shear instability of zonal flow as a consequence of a strong radial gradient of angular velocity. The development of a shear instability at some stage brings about the disruption of the forcing of differential rotation. In the first (hydrodynamic) part of the paper, the dynamics of axisymmetric flows near the bottom of the convection zone is numerically simulated. Forcing of differential rotation having velocity shear in latitude and the existence of solutions in the form of torsional waves with the 11-year oscillation period are shown. In the second part the dynamics of the magnetic field is studied. The most pronounced peculiarities of the solutions are the existence of forced oscillations with the 22-year period and the drift of the toroidal magnetic field component from the mid latitudes to the equator. In high and low latitudes after cycle maximum, the toroidal component is of opposite sign in accordance with observations. In the third part, the transport of momentum from the bottom of the convection zone to the outer surface by virtue of diffusivity is considered. The existence of some sources of differential rotation in the convection zone is not implied. A qualitative correspondence of the differential rotation profile in the bulk of the convection zone and on its outer surface to experimental data is shown. The time correspondence between torsional and magnetic oscillations is also in accordance with observations. 相似文献
2.
Some consequences of a nonlinear coupling between magnetic field and rotation are studied within a solar type 2D dynamo model for a spherical convective shell. The magnetic feedback on the rotation law produces two main effects. First, the torsional oscillations are excited. Second, a long-term amplitude modulation of the dynamo cycles is produced. The latter may be identified with the grand cycle of solar activity. The dynamo model seems to be in accord with the phase relations between the torsional and magnetic activity oscillations observed in the 11-year cycle as well as in the 55-year grand cycle. It, however, fails to reproduce the observationally suggested global decreasing trend in the equatorial rotation rate. 相似文献
3.
H. M. Antia Sarbani Basu & S. M. Chitre 《Monthly notices of the Royal Astronomical Society》1998,298(2):543-556
A new set of accurately measured frequencies of solar oscillations is used to infer the rotation rate inside the Sun, as a function of radial distance as well as latitude. We have adopted a regularized least-squares technique with iterative refinement for both 1.5D inversion, using the splitting coefficients, and 2D inversion using individual m splittings. The inferred rotation rate agrees well with earlier estimates showing a shear layer just below the surface and another one around the base of the convection zone. The tachocline or the transition layer where the rotation rate changes from differential rotation in the convection zone to an almost latitudinally independent rotation rate in the radiative interior is studied in detail. No compelling evidence for any latitudinal variation in the position and width of the tachocline is found, although it appears that the tachocline probably shifts to a slightly larger radial distance at higher latitudes and possibly also becomes thicker. However, these variations are within the estimated errors and more accurate data would be needed to make a definitive statement about latitudinal variations. 相似文献
4.
A. A. Loginov O. K. Cheremnykh V. N. Krivodubskij N. N. Salnikov 《Bulletin of the Crimean Astrophysical Observatory》2012,108(1):58-63
Numerical simulation results of the global solar flows are presented. The conclusion on the common hydrodynamic nature of the torsional oscillations and spatial-temporal variations of the poloidal flow was made. Both processes were shown to be toroidal and poloidal components of a single hydrodynamic oscillatory flow that is asymmetric about the solar equator. The basis for these processes is the physical mechanism of the loss of stability of the solar differential rotation. 相似文献
5.
Observers have long measured solar rotation with different techniques and obtained different results. This paper compares differential rotation measurements from four techniques: Doppler shift, Doppler feature tracking, magnetic feature tracking, and p-mode splittings. The different rotation rates measured by the first three techniques are interpreted as rotation rates of solar phenomena which depend on the properties and depth of that which is measured. This interpretation is supported by comparison with rotation measurements obtained from p-mode splittings except for Doppler features. The rotation rate of the plasma corresponds to the surface rate obtained by inversions; the rates of magnetic features correspond to the rotation rate at various depths within the convection zone. Supergranulation rotates at a rate greater than the maximum rotation rate within the convection zone, suggesting that supergranules are not simple convection cells anchored at a particular depth. 相似文献
6.
The problem of the interaction between magnetic fields and differential rotation in the radiative zone of the Sun is investigated. It is demonstrated that effects of magnetic buoyancy can be neglected in the analysis of this interaction. It is shown that hydromagnetic torsional waves propagating from the solar core cannot be responsible for the 22-year solar cycle. A possible geometry of the magnetic field that conforms with stationary differential rotation is considered. A verifying method for hypotheses on the structure of the magnetic field and torsional oscillations in the radiative zone of the Sun is proposed based on helioseismic data. 相似文献
7.
R. A. Gulyaev 《Solar System Research》2006,40(4):337-340
We define the principal plane of the solar corona as the equatorial plane of the dipole component of the solar magnetic field at the source surface. The position of this plane defines the orientation of the heliospheric current sheet and the outer solar corona in 3D space, in particular, their inclination with respect to the solar equatorial plane. Their tilt varies from almost zero at solar minimum to almost 90° at solar maximum. But this change is not monotonic; more or less regular oscillations are superimposed onto the average curve. Data on the inclination of the coronal principal plane during 1994–2000 were analyzed. Oscillations with a quasiperiod of 1.3 years were clearly revealed. These oscillations were found to be correlated with variations of the solar rotation rate at the base of the solar convection zone. The latter variations have the same quasi-period of 1.3 years, but they are 1 month out of phase. 相似文献
8.
William J. Merryfield 《Solar physics》1990,128(1):305-320
The continual emergence of magnetic flux in solar active regions suggests that a substantial reservoir of flux is present somewhere beneath the photosphere. It has been proposed that this flux could be stored in an azimuthal field of order 3000 G residing in the lower portion of the convection zone. Such a field may be large enough to substantially influence the dynamics of the convection: linear stability analyses indicate that donut-like convective rolls having azimuthal symmetry might then be preferred to banana cells aligned with the rotation axis. Observational detections of such azimuthal rolls have been claimed.The problem of pattern selection by convection in the presence of rotation and a horizontal magnetic field is examined here in a model system consisting of a planar Boussinesq fluid layer. Nonlinear solutions are obtained numerically. It is found that solutions consisting solely of donut cells can exist even at parameter values at which linear theory suggests that banana cells should be preferred instead. However, when the horizontal field decays below a critical value, banana cells may then grow. This leads to the destruction of the horizontal field and a permanent transition to banana cells. 相似文献
9.
Hirokazu Yoshimura 《Solar physics》1972,22(1):20-33
The relation between the systematic time variations of the solar differential rotation at middle latitudes and the asymmetry of global distribution of the solar activity is discussed in connection with the study of the maintenance of the solar differential rotation. The systematic variations at middle latitudes are inferred from a peculiar correlation in the time variations of the solar differential rotation which is shown in this paper to be implied in the data of Howard and Harvey (1970) of spectroscopic measurements of rotational velocities. If we adopt the working hypothesis of the solar equatorial acceleration maintained by the angular momentum transport due to the very large scale convection, the two phenomena are related through the concurrent presence of the neighboring modes with the presumed dominant mode of the very large scale convection. 相似文献
10.
We model stellar differential rotation based on the mean-field theory of fluid dynamics. DR is mainly driven by Reynolds stress, which is anisotropic and has a non-diffusive component because the Coriolis force affects the convection pattern. Likewise, the convective heat transport is not strictly radial but slightly tilted towards the rotation axis, causing the polar caps to be slightly warmer than the equator. This drives a flow opposite to that caused by differential rotation and so allows the system to avoid the Taylor-Proudman state. Our model reproduces the rotation pattern in the solar convection zone and allows predictions for other stars with outer convection zones. The surface shear turns out to depend mainly on the spectral type and only weakly on the rotation rate. We present results for stars of spectral type F which show signs of very strong differential rotation in some cases. Stars just below the mass limit for outer convection zones have shallow convection zones with short convective turnover times. We find solar-type rotation and meridional flow patterns at much shorter rotation periods and horizontal shear much larger than on the solar surface, in agreement with recent observations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
11.
Bernard R. Durney 《Solar physics》2000,196(1):1-18
The torsional oscillations at the solar surface have been interpreted by Schüssler and Yoshimura as being generated by the Lorentz force associated with the solar dynamo. It has been shown recently that they are also present in the upper half of the solar convection zone (SCZ). With the help of a solar dynamo model of the Babcock–Leighton type studied earlier, the longitudinal component of the Lorentz force, L
, is calculated, and its sign or isocontours, are plotted vs. time, t, and polar angle, (the horizontal and vertical axis respectively). Two cases are considered, (1) differential rotation differs from zero only in the tachocline, (2) differential rotation as in (1) in the tachocline, and purely latitudinal and independent of depth in the bulk of the SCZ. In the first case the sign of L
is roughly independent of latitude (corresponding to vertical bands in the t, plot), whereas in the second case the bands show a pole–equator slope of the correct sign. The pattern of the bands still differs, however, considerably from that of the helioseismic observations, and the values of the Lorentz force are too small at low latitudes. It is all but certain that the toroidal field that lies at the origin of the large bipolar magnetic regions observed at the surface, must be generated in the tachocline by differential rotation; the regeneration of the corresponding poloidal field, B
p has not yet been fully clarified. B
p could be regenerated, for example, at the surface (as in Babcock–Leighton models), or slightly above the tachocline, (as in interface dynamos). In the framework of the Babcock-Leighton models, the following scenario is suggested: the dynamo processes that give rise to the large bipolar magnetic regions are only part of the cyclic solar dynamo (to distinguish it from the turbulent dynamo). The toroidal field generated locally by differential rotation must contribute significantly to the torsional oscillations patterns. As this field becomes buoyant, it should give rise, at the surface, to the smaller bipolar magnetic regions as, e.g., to the ephemeral bipolar magnetic regions. These have a weak non-random orientation of magnetic axis, and must therefore also contribute to the source term for the poloidal field. Not only the ephemeral bipolar regions could be generated in the bulk of the SCZ, but many of the smaller bipolar regions as well (at depths that increase with their flux), all contributing to the source term for the poloidal field. In contrast to the butterfly diagram that provides only a very weak test of dynamo theories, the pattern of torsional oscillations has the potential of critically discriminating between different dynamo models. 相似文献
12.
A. A. Ruzmaikin 《Solar physics》1985,100(1-2):125-140
The basic features of the solar activity mechanism are explained in terms of the dynamo theory of mean magnetic fields. The field generation sources are the differential rotation and the mean helicity of turbulent motions in the convective zone. A nonlinear effect of the magnetic field upon the mean helicity results in stabilizing the amplitude of the 22-year oscillations and forming a basic limiting cycle. When two magnetic modes (with dipole and quadrupole symmetry) are excited nonlinear beats appear, which may be related to the secular cycle modulation.The torsional waves observed may be explained as a result of the magnetic field effect upon rotation. The magnetic field evokes also meriodional flows.Adctual variations of the solar activity are nonperiodic since there are recurrent random periods of low activity of the Maunder minimum type. A regime of such a magnetic hydrodynamic chaos may be revealed even in rather simple nonlinear solar dynamo models.The solar dynamo gives rise also to three-dimensional, non-axisymmetric magnetic fields which may be related to a sector structure of the solar field. 相似文献
13.
V. A. Dogiel 《Solar physics》1983,82(1-2):427-436
A model of velocity field oscillations in the solar convective zone is suggested. The system of convective equations is investigated for a thin rotating spherical envelope when the rotation velocity is depended on the coordinates. It is shown that two different structures of convective cells (longitudinal, or latitudinal) can exist in the envelope depending on gradients values of the rotation velocity and Prandtl number. It is supposed that two different regimes of convection (stationary and autofluctuating) are possible in the envelope when the angular velocity gradients are determined by the convection itself. In the case of autofluctuating regime the alternation of longitudinal and latitudinal structure of convection is realized. If one assumes that on the Sun there exists an autooscillating convection regime, then the periods of the existence of latitudinal convection structure may be associated with long periods of activity minima since according to Cowling's theorem, the action of the axisymmetric magnetic field generation mechanism is impossible under conditions of axisymmetric velocity structures. 相似文献
14.
The time and spatial characteristics of 324 large sunspots (S50 millionths of the solar hemisphere) selected from the Abastumani Astrophysical Observatory photoheliogram collection (1950–1990) have been studied. The variations of sunspot angular rotation velocity residuals and oscillations of sunspot tilt angle were analyzed. It has been shown that the differential rotation rate of selected sunspots correlates on average with the solar cycle. The deceleration of differential rotation of large sunspots begins on the ascending arm of the activity curve and ends on the descending arm reaching minimum near the epochs of solar activity maxima. This behavior disappears during the 21st cycle. The amplitudes and periods of sunspot tilt-angle oscillations correlate well with the solar activity cycle. Near the epochs of activity maximum there appear sunspots with large amplitudes and periods showing a significant scatter while the scatter near the minimum is rather low. We also found evidence of phase difference between the sunspot angular rotation velocity and the amplitudes and periods of tilt-angle oscillations. 相似文献
15.
In this paper we study the interaction of rotation with convection in a deep compressible spherical shell as the Sun's convection zone. We examine how the energy transport and the large scale motions can be affected by rotation. In particular we study how a large scale meridional circulation can give rise to variations of angular velocity with latitude and depth.It is assumed that the energy transport is only due to convection and that the mixing-length theory gives an adequate representation of it. Furthermore we assume that rotation acts as a perturbation of the turbulent convective flux through its transport coefficient.The equations involved in the model are integrated numerically in the limit of large viscosity and slow rotation. After having expanded all physical quantities to the first order in terms of Legendre polynomials, the fitting with the observed solar differential rotation gives the expansion parameter, which represents the coupling constant between rotation and convection.The results show a three-cell circulation extending from the poles to the equator. The first one is located in the lower half of the convection zone with the fluid rising at the equator and sinking at the poles. In the second one the direction of the motion is reversed while the third one, located in a thin upper layer, shows the same characteristics of the first one. The meridional velocities at the surface are directed towards the poles and are about 20 cm s-1. In the other cells the meridional velocities are typically of a few cm s-1 while the radial velocities are of the order of a few tenths of cm s-1.The heat flux relative variation at the surface is about 10-4 (3 × 10-3 at the bottom) with a polar excess. The temperature variation at the surface is of the same order, with an equatorial excess however. The convection seems to be stabilized stronger at the equator. The angular velocity increases inwards and varies about 6% between the surface and the bottom of the convection zone.An attempt is made for explaining the picture which emerges. In particular the negligible flux and temperature variations at the surface are explained in terms of equalization by the particular structure of the latitudinal flow. This configuration of large scale circulation is attributed to the high stratification of the convection zone with depth. 相似文献
16.
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
17.
We examine the effects of rotation about a vertical axis on thermal convection with a simple model in which an inviscid, incompressible fluid of zero thermal conductivity and electrical resistivity is contained in a thin annulus of rectangular cross-section. The initial steady state assumed is one of no motion relative to the rotating frame with constant (unstable) vertical temperature gradient and uniform toroidal magnetic field. Small periodic disturbances are then introduced and the linearized perturbation equations solved. We also determine the second-order mean circulations and magnetic fields that are forced by non-zero Reynolds and thermal stresses and magnetic field transports.The solutions have several properties which are relevant to large-scale solar phenomena if giant long-lived convection cells exist on the sun. In particular, the convective cells are tilted in latitude in the same sense as bipolar magnetic regions, and induce vertical magnetic fields with the same tilt. They transport momentum across latitude circles through Reynolds stresses and induced meridional circulations thus setting up a differential rotation. Cells which grow slowly compared to the rotation rate and have comparable dimensions in latitude and longitude transport momentum toward the equator. The cells also form a poloidal magnetic field from initial toroidal field, in a manner similar to that put forth by Parker.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
18.
A method is described for constructing artificial data that realistically simulate photospheric velocity fields. The velocity fields include rotation, differential rotation, meridional circulation, giant cell convection, supergranulation, convective limb shift, p-mode oscillations, and observer motion. Data constructed by this method can be used for testing algorithms designed to extract and analyze these velocity fields in real Doppler velocity data. 相似文献
19.
Standing wave torsional oscillations of wavenumber 1/2 and 1 hemisphere–1 are studied using an improved fit to Mount Wilson magnetograph data. These oscillations are seen to be in phase with each other and with the magnetic activity cycle, and seem best represented as a flexing of the differential rotation curve. Superposing them gives a differential rotation which at solar minimum is slightly flattened at the equator but considerably ( 5%) steepened at the poles, and also tends to produce a travelling wave with wavenumber 1 hemisphere–1 that moves from pole to equator as the cycle progresses. 相似文献
20.
M.S. Miesch 《Astronomische Nachrichten》2007,328(10):998-1001
In the outer envelope of the Sun and in other stars, differential rotation and meridional circulation are maintained via the redistribution of momentum and energy by convective motions. In order to properly capture such processes in a numerical model, the correct spherical geometry is essential. In this paper I review recent insights into the maintenance of mean flows in the solar interior obtained from high-resolution simulations of solar convection in rotating spherical shells. The Coriolis force induces a Reynolds stress which transports angular momentum equatorward and also yields latitudinal variations in the convective heat flux. Meridional circulations induced by baroclinicity and rotational shear further redistribute angular momentum and alter the mean stratification. This gives rise to a complex nonlinear interplay between turbulent convection, differential rotation, meridional circulation, and the mean specific entropy profile. I will describe how this drama plays out in our simulations as well as in solar and stellar convection zones. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献