摘要:

This Special Issue was originally conceived as an opportunity to publish together papers presented at or submitted for the symposium “Models of the Geodynamo and Core-Mantle Coupling” as part of the International Association of Geomagnetism and Aeronomy (IAGA) 7th Scientific Assembly held in Buenos Aires during August 1993. This Symposium (1 .l) was convened by Michael Proctor and Ivan Cupal. While a large proportion of the papers fall into this category, in the interests of a more harmonious whole it has been decided to adjoin some others. These latter, while not directly connected with the meeting, nonetheless address problems in the same area of geomagnetism, and their presence will, it is hoped, enhance the impact of the Issue.

ISSN:0309-1929    

DOI:10.1080/03091929408203671

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

This is a sequel to an earlier paper [Roberts,Geophys. Astrophys. Fluid Dynam.v. 49, p. 143 (1989)] in which one of us claimed, on the basis of two sequences of integrations of a particular intermediate model of the geodynamo, that, as the dynamo number increases, a smooth transition occurs from Taylor-like behavior to model-Z-type behavior. A more complete survey of parameter space for this model is presented here which tends to corroborate this conclusion. Also, the relationship provided by this model between the external dipole moment of the field and the heat flux from the core is examined. The asymptotic dependence of solutions in the large dynamo number limit is considered.

ISSN:0309-1929    

DOI:10.1080/03091929408203672

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A solution of the momentum equation in the outer fluid core of the Earth is obtained, in presence of inner solid core which includes its rotation rate. It is proposed to use this solution for development of a computer program for numerical solution of the generation problem. An advantage of such a scheme, which distinguishes explicitly the boundary layers, should be the increased speed of calculations and smaller values of the Ekman number. We believe that the expressions for the meridional convection obtained here will induce development of a model of the mass-transfer in the core. This would allow to give up the model with prescribed Archimedean velocity, used widely nowadays, in favor of more realistic models in which the latter is a result of the solution of the problem.

ISSN:0309-1929    

DOI:10.1080/03091929408203673

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The combined action of rotation and buoyancy generally imparts helicity to turbulent motions. If the fluid is electrically conducting a mean electromotive force (or emf) results that is proportional to the components of the prevailing mean magnetic field. This is known as “the alpha-effect”. In the simplest (isotropic) case, the emf is parallel to the mean field, and this convenient ansatz is the one most commonly adopted. Nevertheless, an isotropic α-effect is unrealistic, and this has led a number of authors, e.g. Busse and Miin (1979), Rüdiger (1980), Olson and Hagee (1990), and Rüdiger and Kitchatinov (1993) to examine dynamos with an anisotropic α-effect. In this paper, we study electromagnetic induction by an α-effect transverse to the rotation axis and by an α-effect parallel to it. We formulate the problem using the method of Bullard and Gellman (1954), and obtain expressions suitable for solving both the general α-model and the general αω-model. We present results of calculations for kinematic α-dynamos which were obtained by this method.

ISSN:0309-1929    

DOI:10.1080/03091929408203674

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Suggested approximation of a strong generation reduces the kinematic dynamo problem to one simple scalar equation which has a local analytical solution near the maximum of generation. For α-dynamo, the maximum is at the extremum of the α-effect. For αω-dynamo, it is at the extremum of the αω-effect, that is the product of α-effect and differential rotation. These dynamos are named ‘limited’ dynamos, because our parameterQrepresenting a generation regime tends to zero for α-dynamo and has an infinite limit for pure αω-dynamo. In a realistic situation,Qis not in any limit and there is αω-dynamo, which is well worth pursuing.

ISSN:0309-1929    

DOI:10.1080/03091929408203675

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We have used techniques of nonlinear dynamics to compare a special model for the reversals of the Earth's magnetic field with the observational data. Although this model is rather simple, there is no essential difference to the data by means of well-known characteristics, such as correlation function and probability distribution. Applying methods of symbolic dynamics we have found that the considered model is not able to describe the dynamical properties of the observed process. These significant differences are expressed by algorithmic complexity and Renyi information.

ISSN:0309-1929    

DOI:10.1080/03091929408203676

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0309-1929    

DOI:10.1080/03091929408203677

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0309-1929    

DOI:10.1080/03091929408203678

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Instabilities in the form of slow azimuthally travelling hydrodynamic waves in a rapidly rotating, stress-free, electrically conducting spherical fluid shell are investigated. The instabilities are generated by the toroidal decay mode of the lowest order or a combination of toroidal decay modes. It is found that the Elsasser number Λcat the onset of instability is always Λc= O(10) for various profiles of the basic magnetic field. It is also found that the hydromagnetic waves of the preferred instability propagate eastward [i.e. for solutions proportional to expi(m+ ωt), ω < 0] and are characterized by nearly two-dimensional columnar fluid motions attempting to satisfy the Proudman-Taylor theorem, indicating that the most rapidly growing magnetic disturbance arranges itself in such a way that the corresponding magnetic forces balance only the ageostrophic component of the Coriolis force. Except for the Stewartson-type velocity boundary layer at the equator of the inner core, the velocity and magnetic field of the most unstable mode are always smooth, with length scale comparable with the shell width. By studying the same system with and without fluid inertia, we show that fluid inertia cannot introduce any new instability of physical relevance or significantly change the main features of the instability whenTm> 105.Tmbeing the magnetic Taylor number. By studying the detailed dependence of the instabilities in the whole range 0 <Tm≤ ∞, we are able to demonstrate that the existence of the Stewartson-type layer is unlikely to affect the primary properties of the instabilities. In the case of the quadruple symmetry mode, there is no indication of a Stewartson-type boundary layer; a result of the symmetry properties. For sufficiently largeTm. the most rapidly growing instability always has dipole symmetry (which allows the formation of two-dimensional fluid motions). We have compared our resuls (in the limitTm→ ∞) with those found using the magnetostrophic approximation. Calculations using the latter do not impose stress-free boundary conditions and slightly lower values of the critical Elsasser number result. Otherwise, the solutions found are broadly similar.

ISSN:0309-1929    

DOI:10.1080/03091929408203679

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0309-1929    

DOI:10.1080/03091929408203680

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor