共查询到20条相似文献，搜索用时 15 毫秒
1.
L. L. Kitchatinov 《Astronomy Letters》2016,42(5):339-345
Helioseismology revealed an increase in the rotation rate with depth just beneath the solar surface. The relative magnitude of the radial shear is almost constant with latitude. This rotational state can be interpreted as a consequence of two conditions characteristic of the near-surface convection: the smallness of convective turnover time in comparison with the rotation period and absence of a horizontal preferred direction of convection anisotropy. The latter condition is violated in the presence of a magnetic field. This raises the question of whether the subphotospheric fields can be probed with measurements of near-surface rotational shear. The shear is shown to be weakly sensitive to magnetic fields but can serve as a probe for sufficiently strong fields of the order of one kilogauss. It is suggested that the radial differential rotation in extended convective envelopes of red giants is of the same origin as the near-surface rotational shear of the Sun. 相似文献
2.
Ogilvie 《Monthly notices of the Royal Astronomical Society》1998,297(1):291-314
The theory of waves and instabilities in a differentially rotating disc containing a poloidal magnetic field is developed within the framework of ideal magnetohydrodynamics. A continuous spectrum, for which the eigenfunctions are localized on individual magnetic surfaces, is identified but is found not to contain any instabilities associated with differential rotation. The normal modes of a weakly magnetized thin disc are studied by extending the asymptotic methods used previously to describe the equilibria. Waves propagate radially in the disc according to a dispersion relation which is determined by solving an eigenvalue problem at each radius. The dispersion relation for a hydrodynamic disc is re-examined and the modes are classified according to their behaviour in the limit of large wavenumber. The addition of a magnetic field introduces new, potentially unstable, modes and also breaks up the dispersion diagram by causing avoided crossings. The stability boundary to the magnetorotational instability in the parameter space of polytropic equilibria is located by solving directly for marginally stable equilibria. For a given vertical magnetic field in the disc, bending of the field lines has a stabilizing effect and it is shown that stable equilibria exist which are capable of launching a predominantly centrifugally driven wind. 相似文献
3.
We summarize studies of helical properties of solar magnetic fields such as current helicity and twist of magnetic fields in solar active regions (ARs), that are observational tracers of the alpha-effect in the solar convective zone (SCZ). Information on their spatial distribution is obtained by analysis of systematic mag-netographic observations of active regions taken at Huairou Solar Observing Station of National Astronomical Observatories of Chinese Academy of Sciences. The main property is that the tracers of the alpha-effect are antisymmetric about the solar equator. Identifying longitudinal migration of active regions with their individual rotation rates and taking into account the internal differential rotation law within the SCZ known from helioseismology, we deduce the distribution of the effect over depth. We have found evidence that the alpha-effect changes its value and sign near the bottom of the SCZ, and this is in accord with the theoretical studies and numerical simulations. We discuss 相似文献
4.
M. Lazrek A. Pantel E. Fossat B. Gelly F. X. Schmider D. Fierry-Fraillon G. Grec S. Loudagh S. Ehgamberdiev I. Khamitov J. T. Hoeksema P. L. Pallé C. Régulo 《Solar physics》1996,166(1):1-16
The Sun is not a rigid body and it is well known that its surface rotation is differential, the polar regions rotating substantially slower than the equator. This differential rotation has been demonstrated by helioseismology to continue down to the base of the convective zone, below which it becomes closer to a rigid body rotation. Far deeper, inside the energy generating core, the rotation has generally been assumed to be much faster, keeping memory of the presumably high speed of the young Sun. However, several recent results of helioseismology have decreased this likelihood more and more, so that the core rotation could be suspected to be only marginally, or even not at all faster than the envelope. Certain results would even imply a core rotation slower than the envelope, an interesting but unlikely possibility. We present here a complete analysis of the rotational splitting of the low degree modes measured in three different time series obtained in 1990, 1991, and 1992 by the IRIS full-disk network. With a time of integration slightly longer than 4 months, the splitting has been measured by 4 different global methods on 42 doublets of l
= 1, 35 triplets of l = 2, and 30 quadruplets of l = 3. With a high level of confidence, our result is consistent with a rigid solar core rotation. 相似文献
5.
Observations demonstrate a nearly 22-year periodic zonal flow superimposed on general solar differential rotation (LaBonte and Howard, 1982) and some meridional motions (e.g., Tuominen, Tuominen, and Kyrolänen, 1983). Such flows can be excited by the magnetic wave generated by the dynamo in the solar convective zone.An approximate analytical solution for the zonal and meridional flows for a given magnetic wave is constructed. This approach is justified by the fact that the magnetic field is generated by differential rotation and mean helicity, and the magnetic field in the time interval under consideration does not affect much this main flow; it can, however, strongly influence the perturbations of this flow.The density gradient in the convective zone is taken into account as an essential point in the solution construction. The solution agreed well with observational features and, in particular, it gives a phase shift between the rotational (zonal) wave and solar activity. A polar branch of the rotational wave can be described as an effect created by a poleward moving dynamo wave.Secular variations in the symmetrical part of the differential rotation and in the asymmetry between the north and south hemispheres are predicted.The alternative approaches to the explanation of the origin of the observed large-scale flows are discussed. 相似文献
6.
《Icarus》1987,69(3):387-422
The theoretical framework for modeling the primordial solar nebula is presented in which convection is assumed to be the sole source of turbulence that causes the nebula to evolve. We use a new model of convective turbulence that takes into account the important effects of radiative dissipation, rotation, and anisotropy of convective motions. This model is based on a closure for the nonlinear interactions that employs the growth rates of hydrodynamic instabilities, a procedure that allows one to compute turbulence coefficients for instabilities other than convection. The vertical structure equations in the thin-disk approximation are developed for this new model, and a detailed comparison and critique of previous convective models of the solar nebula are presented. Numerical results are presented in a subsequent paper. 相似文献
7.
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
8.
The aim of the present investigation has been to consider rotational evolution of solar-type stars simulated by a polytropic model that possesses differential rotation of Clement's type. A properly defined reduction factor moderates the effects of such a rotation. The present treatment is based upon the general Eulerian equation, governing nonuniform (i.e., nonrigid-body) rotation, which has been set up in a previous investigation. Nonconservative terms, arising when stellar wind torque is under consideration, are taken into account. Data available for the viscosity of the Sun are used to construct a plausible viscosity model. Certain assumptions are made that remove the mathematical difficulties and simplify the physical ground. The obtained results are compared to corresponding estimates of recent observations. 相似文献
9.
L. L. Kitchatinov 《Astronomy Letters》2002,28(9):626-631
We propose a solution to one of the oldest problems in the solar-dynamo theory: explaining the equatorward drift of magnetic activity in the solar cycle. The well-known suggestion that the dynamo waves propagate along the surfaces of constant angular velocity is shown to be restricted to an isotropic medium. Allowance for the rotation-induced anisotropy in turbulent diffusion leads to an equatorward deviation of the wave phase velocity from the isorotational surface. Estimates for the dynamo waves are illustrated with two-dimensional numerical models in a spherical geometry. The model with anisotropic diffusion also shows an equatorward drift of the toroidal magnetic field when the rotation is radially uniform. 相似文献
10.
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
11.
The mechanism of the solar differential rotation usually ascribed to an anisotropic viscosity action is shown to be caused by Coriolis forces which influence anisotropic convective elements in a stratified medium. The estimation of an anisotropy parameters as a function of the convective zone depth is given. The value of (s–1) is positive near the solar surface and negative at the convective zone base, which is in good agreement with observations and the dynamo theory. 相似文献
12.
This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be trapped between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds. 相似文献
13.
Günther Rüdiger Rainer Hollerbach Manfred Schultz Detlef Elstner 《Monthly notices of the Royal Astronomical Society》2007,377(4):1481-1487
We consider the effect of toroidal magnetic fields on hydrodynamically stable Taylor–Couette differential rotation flows. For current-free magnetic fields a non-axisymmetric m = 1 magnetorotational instability arises when the magnetic Reynolds number exceeds O (100) . We then consider how this 'azimuthal magnetorotational instability' (AMRI) is modified if the magnetic field is not current-free, but also has an associated electric current throughout the fluid. This gives rise to current-driven Tayler instabilities (TIs) that exist even without any differential rotation at all. The interaction of the AMRI and the TI is then considered when both electric currents and differential rotation are present simultaneously. The magnetic Prandtl number Pm turns out to be crucial in this case. Large Pm have a destabilizing influence, and lead to a smooth transition between the AMRI and the TI. In contrast, small Pm have a stabilizing influence, with a broad stable zone separating the AMRI and the TI. In this region the differential rotation is acting to stabilize the TIs, with possible astrophysical applications (Ap stars). The growth rates of both the AMRI and the TI are largely independent of Pm , with the TI acting on the time-scale of a single rotation period, and the AMRI slightly slower, but still on the basic rotational time-scale. The azimuthal drift time-scale is ∼20 rotations, and may thus be a (flip-flop) time-scale of stellar activity between the rotation period and the diffusion time. 相似文献
14.
R.A. GarcÍa T. Corbard W.J. Chaplin S. Couvidat A. Eff-Darwich S.J. Jiménez-Reyes S.G. Korzennik J. Ballot P. Boumier E. Fossat C.J. Henney R. Howe M. Lazrek J. Lochard P.L. Pallé S. Turck-Chièze 《Solar physics》2004,220(2):269-285
In the modern era of helioseismology we have a wealth of high-quality data available, e.g., more than 6 years of data collected
by the various instruments on board the SOHO mission, and an even more extensive ground-based set of observations covering
a full solar cycle. Thanks to this effort a detailed picture of the internal rotation of the Sun has been constructed. In
this paper we present some of the actions that should be done to improve our knowledge of the inner rotation profile discussed
during the workshop organized at Saclay on June 2003 on this topic. In particular we will concentrate on the extraction of
the rotational frequency splittings of low- and medium-degree modes and their influence on the rotation of deeper layers.
Furthermore, for the first time a full set of individual |m|-component rotational splittings is computed for modes ℓ≤4 and 1<ν<2 mHz, opening new studies on the latitudinal dependence
of the rotation rate in the radiative interior. It will also be shown that these splittings have the footprints of the differential
rotation of the convective zone which can be extremely useful to study the differential rotation of other stars where only
these low-degree modes will be available. 相似文献
15.
Hirokazu Yoshimura 《Solar physics》1971,18(3):417-433
The magnetic field pattern associated with large scale convective motions, which are much larger than the supergranules and have been conceived as a source of maintenance of the solar differential rotation, is calculated in the framework of a slowly and differentially rotating thin spherical shell, including the effects of thermal conductivity and viscosity. The approximations of Boussinesq are used and the initial state of the magnetic field is assumed to be purely toroidal.The resulting magnetic field pattern rotates rigidly on the differentially rotating Sun with some phase delay to the convective pattern, if it is assumed that only the predominant mode with the maximum growth rate is actually realized in the solar convection zone. The obtained magnetic and convective patterns and their properties seem to explain naturally the various aspects of large scale ordering of solar activity such as the existence and behavior of complexes of activity, the rigid body rotation of proton flare active longitudes, their association with UMR's, the existence of ghost and mirror image of UMR's themselves and the fact that the rotational period derived from sunspot data is shorter than that derived spectroscopically from fluid velocity. 相似文献
16.
It has been suggested that the solar differential rotation might be maintained by nearly horizontal non-spherical convective circulation called the Rossby-type waves (the wave motions characterized by the close balance of the Coriolis force and pressure gradient in horizontal motions). In this paper, such Rossby-type waves which could be excited in the upper solar convection zone are considered, and the possibility of maintenance of the solar differential rotation by such waves is examined. A numerical estimate, in terms of the rate of conversion of the kinetic energy of such wave motions into the mean rotational motion, indicates this possibility. The implications and limitations of the results are also discussed.Visiting Scientist to the High Altitude Observatory on leave of absence from the Department of Astronomy, University of Tokyo, Japan.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
17.
Measurements from the Galileo probe suggest that the zonal winds are deep rooted. Jupiter's high rotation rate makes it likely that the whole outer molecular H/He layer is involved in these long-lived jet flows. Assuming that the primary flows are geostrophic, and that the banded surface structure stretches right through the molecular H/He layer, we examine the conditions for such flows to be stable. As a first step, the linear stability of some prescribed banded zonal flows in a rotating spherical shell is explored. Incompressibility is assumed for simplicity, and the boundary condition is stress-free. We compare solutions for two aspect ratios, appropriate for the molecular H/He layers of Jupiter and Saturn, and two Ekman numbers (E=10−2 and E=10−4). Convective and shear flow instabilities compete in our system. The convective instabilities are of the well-known columnar structure. Shear flow instabilities for the larger Ekman number are similar to the Taylor-Couette instability in rotating annuli. At the lower Ekman number, shear flow instabilities adopt a geostrophic character, assuming the form of rotating columns, similar to the convective instabilities. While the convective instability always sets in outside the tangent cylinder, shear instability can become unstable inside the tangent cylinder. If even a weak zonal flow is present inside the tangent cylinder, the flow is unstable to shear instability. This offers an explanation why the jovian zonal jet structure is much weaker at the higher latitudes that correspond to locations inside the tangent cylinder. 相似文献
18.
C. G. Campbell 《Monthly notices of the Royal Astronomical Society》2003,345(1):123-143
A semi-analytic method is presented for solving for the radial and vertical structures of an accretion disc, with a magnetically channelled wind flowing from its surfaces. Both magnetic and turbulent viscous effects are taken into account, and the essential wind properties are related to the disc structure. The angular momentum removed by the wind plays a major part in driving the inflow through the disc, with photospheric temperatures being sufficient to generate the required wind mass flux. The magnetic field is generated by an αω-dynamo, but the method of solution should have application with other magnetic field sources. Self-consistent disc-wind solutions result, with rms turbulent Mach numbers which are in good agreement with those found in simulations of turbulence generated from magnetic shearing instabilities. 相似文献
19.
Olav L. Hansen 《Icarus》1975,26(1):24-29
Infrared (1.5–5 μm) albedos and rotation curves of the Galilean satellites have been obtained. The data suggest that the rotational variation in the infrared is less than ±10% for all four satellites. While no conclusion about rotational variation could be reached for Io, the 1.57 μm data for the outer three satellites marginally suggest phase correlation with the visual variation. The geometric albedos obtained are in general agreement with earlier results. For Io, the absorption feature near 1.5 μm found by Pilcher et al. (1972) is confirmed, thus contradicting the flat spectrum measured by Fink et al. (1973). Io and Ganymede were observed in the 1.57 μm bandpass as they reappeared from eclipse. The curve for Io shows a slight (<10%) overshoot similar to those sometimes reported for visual measurements. This result is based on a single reappearance, and is extremely tentative. 相似文献
20.
C. Richard DeVore 《Solar physics》1987,112(1):17-35
In the absence of new bipolar sources of flux, the large-scale magnetic field at the solar photosphere decays due to differential rotation, meridional flow, and supergranular diffusion. The rotational shear quickly winds up the nonaxisymmetric components of the field, increasing their latitudinal gradients and thus the rates of diffusive mixing of their flux. This process is particularly effective at mid latitudes, where the rotational shear is largest, so that eventually low- and high-latitude remnants of the initial, nonaxisymmetric field pattern survive. In this paper I solve analytically the transport equation describing the evolution of the large-scale photospheric field, to study its time-asymptotic behavior. The solutions are rigidly rotating, uniformly decaying distributions of flux, wound up by differential rotation and localized near either the equator or the poles. A balance between azimuthal transport of flux by the rotational shear and meridional transport by the diffusion gives rise to the rigidly rotating field patterns. The time-scale on which this balance is achieved, and also on which the nonaxisymmetric flux decays away, is the geometric mean of the short time-scale for shearing by differential rotation and the long time-scale for dispersal by supergranular diffusion. A poleward meridional flow alters this balance on its own, intermediate time-scale, accelerating the decay of the nonaxisymmetric flux at low latitudes. Such a flow also hastens the relaxation of the axisymmetric field to a modified dipolar configuration. 相似文献