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61.
Magnetic reconnection at a three-dimensional null point is the natural extension of the familiar two-dimensional X-point reconnection. A model is set up here for reconnection at a spiral null point, by solving the kinematic, steady, resistive magnetohydrodynamic equations in its vicinity. A steady magnetic field is assumed, as well as the existence of a localised diffusion region surrounding the null point. Outside the diffusion region the plasma and magnetic field move ideally. Particular attention is focussed on the way that the magnetic flux changes its connections as a result of the reconnection. The resultant plasma flows are found to be rotational in nature, as is the change in connections of the magnetic field lines. 相似文献
62.
Diffusion of a magnetic field through a plasma is discussed in one-, two- and three-dimensional configurations, together with the possibility of describing such diffusion in terms of a magnetic flux velocity, which, when it exists, is in general non-unique. Physically useful definitions of such a velocity include doing so in terms of the energy flow or in such a way that it vanishes in a steady state. Straight field lines (or plane flux surfaces) diffuse as if flux is disappearing at a neutral sheet, whereas circular field lines (or cylindrical flux surfaces) do so as if flux is disappearing at an 0-type neutral line. In three dimensions it is not always possible to define a flux velocity, for example when the magnetic flux through a closed field line is changing in time. However, in at least some such cases it is possible to describe the behaviour of the magnetic field in terms of a pair of quasi-flux-velocities. 相似文献
63.
本文采用特征线方法和激波装配法,对磁流体中间激波在行星际空间的演化过程进行数值模拟。主要结论如下:（1） 2→4型中间激波通过向下游发出后向慢压缩波使下游态磁场减幅,通过向上游发出前向快压缩波使上游态磁场增幅,以致2→4型中间激波迅速经导灭激波向慢激波转化;所发出的前向快压缩波经非线性变陡形成快激波。（2）1→3型中间激波首先通过向下游发出前向慢稀疏波而很快变成1→3=4型临界中间激波,并瞬间转变为由前向快激波和前向2→4型中间激波构成的激波系统。其中,2→4型中间激波可在其前导快激波的下游传播较远的距离,有可能为 IAU 附近的飞船观测到,但最终导灭激波转变为慢激波。 相似文献
64.
Numerical studies with a spherical dynamo model have shown two remarkable phenomena. The model consists of a spherical body of an electrically conducting incompressible uid surrounded by free space. In addition to a rotation of the body an inner motion due to a given forcing is considered which satisfies a no–slip condition at the boundary. The full interaction of magnetic field and motion is taken into account. Starting from a fluid motion capable of dynamo action and a very weak magnetic field it was observed that the growing magnetic field destroys the dynamo property of the motion and then decays, and that the system ends up in a state with another motion incapable of dynamo action and zero magnetic field. In another case with a motion unable to prevent small magnetic fields from decay it proved to be possible that stronger magnetic fields deform it so that a dynamo starts to work which enables the system to approach a steady state with a finite magnetic field. 相似文献
65.
We present an improved formalism for translationally invariant magnetohydrodynamic equilibria with anisotropic pressure and currents with a field aligned component. The derivation of a Grad-Shafranov type equation is given along with a constraint which links the shear field to the parallel pressure. The difficulties of the formalism are discussed and various methods of circumventing these difficulties are given. A simple example is then used to highlight the methods and difficulties involved. 相似文献
66.
地磁场起源及其倒转是地球科学的难题之一。究其原因一方面是由于无法直接观测地球内部发生的物理过程,另一方面是由于缺乏理论与实验相结合的综合研究。本文以磁流体力学为基础,将古地磁学与αω发电机理论结合在一起进行分析和研究。得出了如下新观点:(1)洛仑兹力在地核发电过程起负反馈作用;(2)较差旋转控制着地磁场西向漂移,(3)α作用使地磁极偏离地球自转轴 相似文献
67.
68.
Strong Potential Magnetic Field Influence on Slightly Differentially Rotating Spherical Shell 总被引：1，自引：0，他引：1
An electrically conducting viscous fluid-filled spherical shell is permeated by an axisymmetric strong potential magnetic field with large Elssaser number 2 1. We describe analytically the steady flow driven by a slightly faster rotation of the conducting inner boundary of the shell. The main flow is controlled by Ekman-Hartmann boundary layers with a small thickness /, where 2
is the Ekman number. Asymptotics based on small –1 1 reveal the nature of a free shear layer O((/)1/2) and a super-rotation that allows a part of the fluid to rotate faster than the inner sphere. The free shear is following an imposed field line that is tangent to the inner or outer sphere. Meridional flux is concentrated in the shear and boundary layers. Fluid tends to rotate with the inner sphere and to expel azimuthal magnetic field from an
-region restricted by the free shear in the spherical shell.
For an imposed axial uniform magnetic field, this
-region is outside the cylinder tangent to the inner sphere and rotates with the outer sphere. Weak differential rotation O(/) is inside the cylinder, while almost all difference in rotation rates between spheres is accommodated in the thin O((/)1/2) free shear.
For an imposed dipole magnet, the region
has a shape of a lobe touching the outer equator. Inside
a super-rotation exists; this is the common case for such
when the source of the imposed field is inside. 相似文献
69.
This contribution is aimed at a comparison of two different methods of how to deal with the solid inner core in geodynamo models. The first method, based on a direct application of the non-slip boundary conditions, was frequently used in the past. The second one, developed by the authors of the present paper, is based on an advanced analytical solution within the boundary layers and consequent formulation of new boundary conditions on the flow in the volume of the outer core. As an example we have used the results obtained by Hollerbach (1997) in the study of the influence of an imposed axial magnetic field on the fluid flow in a differentially rotating spherical shell. In the case of a weak imposed magnetic field, our solutions are very similar to those of Hollerbach. This non-trivial correspondence confirms the correctness of both methods, which are different not only in the formulation of boundary conditions, but also in the numerical methods: whereas Hollerbach used spectral methods, our computer code is based on finite differences. The influence of the conductivity of the inner core on the fluid flow was also studied. 相似文献
70.
In broad astrophysical contexts of large-scale gravitational collapses and outflows and as a basis for various further astrophysical
applications, we formulate and investigate a theoretical problem of self-similar magnetohydrodynamics (MHD) for a non-rotating
polytropic gas of quasi-spherical symmetry permeated by a completely random magnetic field. Within this framework, we derive
two coupled nonlinear MHD ordinary differential equations (ODEs), examine properties of the magnetosonic critical curve, obtain
various asymptotic and global semi-complete similarity MHD solutions, and qualify the applicability of our results. Unique
to a magnetized gas cloud, a novel asymptotic MHD solution for a collapsing core is established. Physically, the similarity
MHD inflow towards the central dense core proceeds in characteristic manners before the gas material eventually encounters
a strong radiating MHD shock upon impact onto the central compact object. Sufficiently far away from the central core region
enshrouded by such an MHD shock, we derive regular asymptotic behaviours. We study asymptotic solution behaviours in the vicinity
of the magnetosonic critical curve and determine smooth MHD eigensolutions across this curve. Numerically, we construct global
semi-complete similarity MHD solutions that cross the magnetosonic critical curve zero, one, and two times. For comparison,
counterpart solutions in the case of an isothermal unmagnetized and magnetized gas flows are demonstrated in the present MHD
framework at nearly isothermal and weakly magnetized conditions. For a polytropic index γ=1.25 or a strong magnetic field, different solution behaviours emerge. With a strong magnetic field, there exist semi-complete
similarity solutions crossing the magnetosonic critical curve only once, and the MHD counterpart of expansion-wave collapse
solution disappears. Also in the polytropic case of γ=1.25, we no longer observe the trend in the speed-density phase diagram of finding infinitely many matches to establish global
MHD solutions that cross the magnetosonic critical curve twice.
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