摘要:

The propagation of nonlinear quasimonocromatic Alfvén waves with small but finite amplitude into an ideal astrophysical plasma is studied. Using the derivative expansion method in the third order MHD approximation, the perturbation components for the case in study are determined. It was also shown that, under certain conditions, the nonlinear Alfvén waves are responsible for the appearance of shock waves into an astrophysical plasma.

ISSN:0309-1929    

DOI:10.1080/03091929808221144

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

It is shown that the magnetic moment μ is a conserved quantity not only in MHD, but also in general electrodynamics under certain not very restrictive conditions. The propagation of magnetic moment from a regionDwith an evolving current system (e.g., due to dynamo action) is discussed for the two cases of vacuum and a conducting medium, respectively, surroundingD. In the case of vacuum, the MHD approximation no longer holds and the weak electromagnetic wave emitted fromDis important, as was pointed out by Sokoloff (1997). In the case of an unbounded conducting medium, the classical definition of μ is generalised and μ is shown to propagate diffusively, undisturbed by the newly generated magnetic field.

ISSN:0309-1929    

DOI:10.1080/03091929808221145

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The numerical evaluation of Taylor integrals in their natural geometry of a cylinder has long been a problem when considering the dynamics of the Earth's dynamo, as the Earth's natural coordinate system is a sphere. We describe an exact new method of dealing with the Taylor integrals while remaining in a spherical geometry. The procedure explained in this paper is valid for a full sphere and only the axisymmetric case is considered here.

ISSN:0309-1929    

DOI:10.1080/03091929808221146

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Recent investigations by Zhang and Roberts (1997) have shown that the onset of convection in a rapidly rotating fluid layer at sufficiently small Prandtl number and heated from below is via modes which are essentially thermal inertial waves. The detailed structure of these modes is determined by balancing buoyancy forces against weak viscous dissipation and the objective here is to extend the results of Zhang and Roberts into a finite amplitude regime. This paper presents the first study of nonlinear thermoinertial waves in a rapidly rotating layer using multiple timescales analysis and double expansion perturbations in terms of small Ekman number and small amplitude. Some solutions of the nonlinear problem are given and the implications of our study for related problems are considered.

ISSN:0309-1929    

DOI:10.1080/03091929808221147

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

There have been extensive previous laboratory studies of thermal convection in a vertical cylindrical annulus of fluid that rotates about its axis with angular velocity Ω (say) with respect to an inertial frame and is subject to an axisymmetric horizontal temperature gradient, as well as associated theoretical and numerical work. The relative flow produced by concomitant buoyancy forces is strongly influenced by Coriolis forces, which give rise to azimuthal circulations and promote, through the process of “baroclinic instàbility”, regimes of non-axisymmetric sloping convection which can be spatially and temporally regular or irregular (“chaotic geostrophic turbulence”). It is also known from previous work that such flows are changed dramatically by the presence of a thin rigid impermeable radial barrier blocking the cross-section of the annulus, and capable of supporting a net azimuthal pressure gradient and associated net azimuthal temperature gradient within the fluid. The presence of the barrier can thus render convective heat transport across the fluid annulus (as measured by the Nusselt number, Nu) virtually independent of Ω (as measured by the so-called Ekman or Taylor number) and dependent only on the Grashof number, G. The present study reports further systematic determinations of heat transport and of velocity and temperature fields in the presence of a radial barrier, with emphasis on the Ω-dependence of the crucially-important net azimuthal temperature gradient supported by the barrier and the physical interpretation of that dependence.

ISSN:0309-1929    

DOI:10.1080/03091929808221148

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A recent theory of Nycander (1996) establishing the analogy between the steady westward drift of geophysical vortices and the precession of a rigid-body (in particular a disc spinning about its symmetry axis) is rederived within a Lagrangian framework. This formulation allows the time-dependent properties (e.g., trajectory, spinetc.) of the rigidbody, represented by three coupled ODEs, to be studied. Numerical results for a variety of initial zonal velocities and latitudes are presented. Typically the trajectory consists of a mean westward precession coupled with periodic behaviour which, in the terminology of rigid-body dynamics, is known as nutation. In the limit that β-plane physics are appropriate, it is shown that the nutation is exactly analogous to harmonic inertial oscillations at the local Coriolis frequency.

ISSN:0309-1929    

DOI:10.1080/03091929808221149

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The question of how deep ocean eddies can cross the equator is addressed with the aid of analytical and numerical models. We focus on the possibility that deep ocean (lens-like) eddies can cross the equator via deep cross equatorial channels on the ocean floor. We first examine the behavior of solid balls (i.e., free particles) in a meridional parabolic channel on a β plane. Such balls are subject to similar topographical forcing and inertial forces that a lens is subject to, except that pressure forces and friction are absent. We examine both single isolated balls and a “cloud” of (noninteractive) balls. In general, the balls' trajectories have a chaotic character; a fraction of the cloud crosses the equator and ends up in the northern hemisphere, and a fraction is left behind. More realistic numerical experiments (with a fully nonlinear reduced-gravity isopycnic model of the Bleck and Boudra type) show similar behavior. In all cases the equator acts as an “eddy smasher” in the sense that it breaks the lens into at least two parts, one crosses the equator and ends up in the northern hemisphere, and the other is left behind. Here, however, the system is not chaotic. Despite the obvious differences between clouds of balls and eddies, there is a remarkable similarity between the percentage of balls that penetrate into the opposite hemisphere and the percentage of eddies' mass that ends up in the other hemisphere. This suggests that the geometry of the channel and the presence of the equator determine how the fluid will be partitioned among the two hemispheres.

ISSN:0309-1929    

DOI:10.1080/03091929808221150

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor