Non-local memory effects of the electromotive force by fluid motion with helicity and two-dimensional periodicity     

Kumiko Hori  Shigeo Yoshida 《地球物理与天体物理流体动力学》2013,107(6):601-632

In mean-field dynamo theory, the electromotive force term 〈u′ × B′〉 due to small-scale fields connects the small-scale magnetic field with the large-scale field. This term is usually approximated as the α-effect, assumed to be instantaneous in time and local in space. However, the approximation is valid only when the magnetic Reynolds number Rm is much less than unity, and is inappropriate when Rm ? 1, which is the condition satisfied in the Earth's core or solar convection zone. We introduce a function φ _qr as a non-local and non-instantaneous generalization of the usual α-effect and examine its behaviour as a function of Rm in the range 1/64 ≤ Rm ≤ 10 for a kinematic dynamo model. We use the flow of G.O. Roberts 1972 Roberts, GO. 1972. Dynamo action of fluid motions with two-dimensional periodicity. Phil. Trans. Roy. Soc. London Ser. A, 271: 411–454. [Crossref], [Web of Science ®] , [Google Scholar] (Phil, Trans. Roy. Soc. London Ser. A, 1972, 271, 411–454), which is steady and has non-zero helicities and two-dimensional periodicity. As a result, we identify three regions in Rm space according to the behaviour of the function φ _qr : (i) Rm ? 1/4, where the function φ _qr is local and instantaneous and can be approximated by the traditional α and β effects, (ii) 1/4 ? Rm ? 4, where the deviation from the traditional α and β effects increases and non-localness and non-instantaneousness increase, and (iii) Rm ? 4, where boundary layers develop fully and non-localness and non-instantaneousness are prominent. We show that the non-local memory effect for Rm ? 4 strongly affects the dynamo action and explains an observed augmentation of the growth rate in the dispersion relation. The results imply that the non-local memory effect of the electromotive force should be important in the geodynamo or the solar dynamo.  相似文献   

On the generation,propagation and radiation of magneto–acoustic–gravity waves with application to stars     

L. M. B. C. Campos 《地球物理与天体物理流体动力学》2013,107(1):51-74

Abstract

The study of the mechanisms controlling the stratification in closed fluid regions is an important branch of geophysical fluid dynamics. Part of this subject can be handled with a simple linear model, consisting of a buoyancy layer at the non-horizontal boundaries of a container and an advective-diffusive interior coupled by volume continuity. The model is valid under the following conditions: firstly, the buoyancy-frequency characterizing the solution must be sufficiently large to give rise to a flow pattern of boundary layer type and, secondly, the non-horizontal walls must not have too large thermal conductivity.

The main purpose of the present paper is to summarise previous work done by the authors in this field and to present some consequences of their theory not previously discussed.

Three important cases are discussed; certain stationary solutions, the decay of a given stratification and the build up of a stratification in a homogeneous fluid. The experimental results concerning the afore-mentioned cases are presented.  相似文献   

Small-scale magnetic buoyancy and magnetic pumping effects in a turbulent convection     

Igor Rogachevskii  Nathan Kleeorin 《地球物理与天体物理流体动力学》2013,107(3):243-263

We determine the nonlinear drift velocities of the mean magnetic field and nonlinear turbulent magnetic diffusion in a turbulent convection. We show that the nonlinear drift velocities are caused by three kinds of the inhomogeneities; i.e., inhomogeneous turbulence, the nonuniform fluid density and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear drift velocities of the mean magnetic field cause the small-scale magnetic buoyancy and magnetic pumping effects in the turbulent convection. These phenomena are different from the large-scale magnetic buoyancy and magnetic pumping effects which are due to the effect of the mean magnetic field on the large-scale density stratified fluid flow. The small-scale magnetic buoyancy and magnetic pumping can be stronger than these large-scale effects when the mean magnetic field is smaller than the equipartition field. We discuss the small-scale magnetic buoyancy and magnetic pumping effects in the context of the solar and stellar turbulent convection. We demonstrate also that the nonlinear turbulent magnetic diffusion in the turbulent convection is anisotropic even for a weak mean magnetic field. In particular, it is enhanced in the radial direction. The magnetic fluctuations due to the small-scale dynamo increase the turbulent magnetic diffusion of the toroidal component of the mean magnetic field, while they do not affect the turbulent magnetic diffusion of the poloidal field.  相似文献   

Dynamic non-null magnetic reconnection in three dimensions–II: composite solutions     

A.L. Wilmot-Smith  G. Hornig  E.R. Priest 《地球物理与天体物理流体动力学》2013,107(6):515-534

In this series of papers we examine magnetic reconnection in a domain where the magnetic field does not vanish and the non-ideal region is localised in space so that the reconnection is fully three dimensional. In a previous paper we presented a technique for obtaining analytical solutions to the full set of stationary resistive MHD equations and examined specific examples of non-ideal reconnective solutions. Here we further develop the model, noting that certain ideal solutions may be superimposed onto the fundamental non-ideal solutions. This provides the first analytical demonstration of a lack of coupling between reconnective and non-reconnective flows. We examine the effect of imposing various such ideal flows. Significant implications are found for the evolution of magnetic flux in the reconnection process so that several reconnection solutions may have the same reconnection rate, as defined by the integral of the parallel electric field along the reconnection line, but each appear quite different in terms of their global effect. It is shown that, in contrast to the two-dimensional case, in three dimensions there is a very wide variety of physically different steady reconnection solutions.  相似文献   

Influence of a multiplicative noise on the threshold of helical dynamo     

Christiane Normand 《地球物理与天体物理流体动力学》2013,107(1):90-108

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Nonlinear alfvénic fluctuations in uniform magnetic background fields     

E. Weisshaar 《地球物理与天体物理流体动力学》2013,107(1-2):141-170

Abstract

The two dimensional incompressible MHD equations describing the decay of a random initial velocity field in the presence of a uniform magnetic background field are solved numerically by a Chebyshev spectral method. The nonlinear interactions of standing Alfvén-waves of a given energy are studied for various Reynolds numbers and field strengths of the magnetic background field. Small scale structures are generated by these interactions, which increase the energy dissipation, however, the uniform background field suppresses the production of arbitrary small scales. Thus energy dissipation is found to be insignificant at sufficiently high Reynolds numbers. Anisotropies of the fluctuating field components are also studied. In the temporal evolution they appear first in the magnetic field. This is explained by the conservation of mean square vector potential in the limit of infinite conductivity.  相似文献   

Hamiltonian description of almost geostrophic flow     

Henry D. I. Abarbanel 《地球物理与天体物理流体动力学》2013,107(1-4):145-171

Abstract

Geostrophic flow in the theory of a shallow rotating fluid is exactly analogous to the drift approximation in a strongly magnetized electrostatic plasma. This analogy is developed and exhibited in detailed to derive equations for the slow nearly geostrophic motion. The key ingredient in the theory is the isolation, to whatever order in Rossby number desired, of the fast motion near the inertial frequency. One of the remaining degrees of freedom represents a new approximate constant of the motion for nearly geostrophic flow. This is the analogue of the familiar magnetic moment adiabatic invariant in the plasma problem.

The procedure is a Rossby number expansion of the Hamiltonian for the fluid expressed in Lagrangian, rather than Eulerian variables. The fundamental Poisson brackets of the theory are not expanded so desirable properties such as energy conservation are maintained throughout.  相似文献   

Helicity–vorticity turbulent pumping of magnetic fields in the solar convection zone     

V.V. Pipin 《地球物理与天体物理流体动力学》2013,107(1-2):185-206

We study the effect of turbulent drift of a large-scale magnetic field that results from the interaction of helical convective motions and differential rotation in the solar convection zone. The principal direction of the drift corresponds to the direction of the large-scale vorticity vector. Thus, the effect produces a latitudinal transport of the large-scale magnetic field in the convective zone wherever the angular velocity has a strong radial gradient. The direction of the drift depends on the sign of helicity and it is defined by the Parker–Yoshimura rule. The analytic calculations are done within the framework of mean-field magnetohydrodynamics using the minimal τ-approximation. We estimate the magnitude of the drift velocity and find that it can be a few m/s near the base of the solar convection zone. The implications of this effect for the solar dynamo are illustrated on the basis of an axisymmetric mean-field dynamo model with a subsurface shear layer. The model shows that near the bottom of the convection zone the helicity–vorticity pumping results mostly from the kinetic helicity contributions. We find that the magnetic helicity contributions to the pumping effect are dominant at the subsurface shear layer. There the magnitude of the drift velocity is found to be a few cm/s. We find that the helicity–vorticity pumping effect can have an influence on the features of the sunspot time–latitude diagram, producing a fast drift of the sunspot activity maximum at the rise phase of the cycle and a slow drift at the decay phase of the cycle.  相似文献   

某砖砌体教学楼结构抗震加固设计     

王晓星  李慧剑 《华北地震科学》2013,31(2):59-61,66

某中小学教学楼为典型的80年代建造的砌体教学楼,鉴定结果显示基础稳定,上部结构材料强度不足,部分构造设置不合理。在结构抗震加固再设计中,从概念上分析结构加固的合理性;合理地确定地震作用影响系数,对加固前后的结构进行整体计算分析;对重要及复杂的部分节点进行补充设计。  相似文献   

格尔木地磁Z分量静日变化特征与前兆异常分析     

李延峰  朱胜伟  杨广华 《高原地震》2013,25(1):35-41

研究了格尔木地震台1996-2006年地磁模拟资料和2008-2011年的数字化资料,通过对台站磁静日低点时间、日变幅度、日变形态的分析,确认地磁z分量磁静日低点时间在地方时12时左右;日变幅月均值呈现明显的波浪式起伏变化,其年均值与太阳黑子数保持一致;地磁日变形态较为规则,有1个低点和2个高点;在前兆异常分析中,认为低点时间异常和日变形态双低点异常与周边强震具有一定的对应关系。  相似文献