共查询到20条相似文献，搜索用时 15 毫秒
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Abstract It is suggested that reverse currents seen in recent numerical reconnection experiments (Biskamp, 1986; Forbes and Priest, 1983) are caused by the choice of outflow boundary conditions. The specification of the normal velocity at the outflow boundary may result in a mismatch in velocity at the diffusion region which is manifested as a spike of reverse current. 相似文献
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应用数值方法研究了日冕多层电流片中电阻撕裂模不稳定性的非线性演化和磁场重联过程,结果表明,计算区域顶部附近两侧电流片中的磁岛和等离子体团向上抛射,并携出大量的磁能和热能;中心电流片中的磁岛向下运动,逐渐演变成底部含有３个磁闭合区的冕流结构。进而在中心电流片中再次发生磁场重联,多次形成向下运动的小型磁岛,并与底部磁闭合区发生结合不稳定性。同时在磁闭合区中也有磁场重联发生,导致中心小磁闭合区的湮灭。初始电流片之间的距离趋近,上述演化过程越快。日冕多层电流片中的磁场重联过程可能对日冕物质抛射和日冕加热有着重要影响。 相似文献
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应用数值方法研究了日冕多层电流片中电阻撕裂模不稳定性的非线性演化和磁场重联过程,结果表明,计算区域顶部附近两侧电流片中的磁岛和等离子体团向上抛射,并携出大量的磁能和热能;中心电流片中的磁岛向下运动,逐渐演变成底部含有３个磁闭合区的冕流结构。进而在中心电流片中再次发生磁场重联,多次形成向下运动的小型磁岛,并与底部磁闭合区发生结合不稳定性。同时在磁闭合区中也有磁场重联发生,导致中心小磁闭合区的湮灭。初始电流片之间的距离趋近,上述演化过程越快。日冕多层电流片中的磁场重联过程可能对日冕物质抛射和日冕加热有着重要影响。 相似文献
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等离子体系统中存在两个或多个电流片时，电流片中发生的不稳定性可能会相互作用．行星际磁场北向时，背阳面碰层顶电流片与磁尾等离子体片之间可能发生相互作用，高纬边界层强烈的流场剪切可能促进磁场重联，产生磁层亚暴．本文运用二维可压缩磁流体模拟研究具有强流场剪切的多个电流片系统中磁场重联的演化．结果表明，Kelvin－Helmholtz不稳定性使多电流片系统的磁场重联过程明显加快；相邻电流片之间的距离越近，两者相互作用越强，重联增长率越大；在三电流片系统中，超Alfven速度强流场导致外侧两个电流片中出现强烈的磁场重联，并引发中心电流片的磁场重联．行星际磁场北向时，也可能发生磁层亚暴． 相似文献
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In the present work, an integral equation approach is developed to solve two-dimensional incompressible resistive magnetohydrodynamic equations. This approach is examined by simulating the magnetic reconnection driven by the Orszag–Tang vortex and the doubly periodic coalescence instability. The results show that when the viscosity and magnetic resistivity of the plasma are reduced, the current sheet forming in the magnetic reconnection driven by the Orszag–Tang vortex becomes thinner. In comparison with the spectral method, the integral equation approach has much better accuracy and convergence. 相似文献
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The results of a three-dimensional MHD simulation and data obtained using specialized spacecraft made it possible to construct
an electrodynamic model of solar flares. A flare results from explosive magnetic reconnection in a current sheet above an
active region, and electrons accelerated in field-aligned currents cause hard X rays on the solar surface. In this review,
we considered works where the boundary and initial conditions on the photosphere were specified directly from the magnetic
maps, obtained by SOHO MDI in the preflare state, in order to simulate the formation of a current sheet. A numerical solution
of the complete set of MHD equations, performed using the new-generation PERESVET program, demonstrated the formation of several
current sheets before a series of flares. A comparison of the observed relativistic proton spectra and the simulated proton
acceleration along a magnetic field singular line made it possible to estimate the magnetic reconnection rate during a flare
(∼107 cm s−1). Great flares (of the X class) originate after an increase in the active region magnetic flux up to 1022 Mx. 相似文献
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E. N. Parker 《地球物理与天体物理流体动力学》2013,107(3-4):161-182
Abstract The relatively large resistivity in the solar photosphere and chromosphere softens the ideal tangential discontinuities of magnetostatic equilibrium into continuous transitions in field direction over scales of 0.1–10 km. This softening is communicated upward at the Alfvén speed into the active solar corona. The degree of softening is a vital part of the theory of magnetic heat input to the active X-ray corona, because the very low resistivity of the coronal gas provides effective dissipation only if the current sheets are reduced to a thickness of 10?2km. A close examination of the problem shows that the Alfvén transit time up into the corona is large compared to the characteristic time of 1 sec in which the coronal tangential discontinutities are formed. It also shows that the principal effect of the resistivity is to create a thin surface layer of fluid on adjacent flux bundles, which causes a general drift of the flux but does not directly broaden the current sheets higher up in the field. In fact the motions of the surface layers do not extend upwards beyond the first winding pattern at each end of a coronal loop. It appears that the photospheric and chromospheric resistivity is without striking consequences for magnetic heating in the corona. 相似文献
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Magnetic reconnection at a three-dimensional null point is a natural extension of the familiar two-dimensional X-point reconnection. A model is set up here for reconnection at a null point with current directed parallel to the fan plane, by solving the kinematic, steady, resistive magnetohydrodynamic equations in its vicinity. The magnetic field is assumed to be steady, and a localised diffusion region surrounding the null point is also assumed, outside which the plasma is ideal. Particular attention is focussed on the way that the magnetic flux changes its connections as a result of the reconnection. The resultant plasma flow is found to cross the spine and fan of the null, and thus transfer magnetic flux between topologically distinct regions. Solutions are also found in which the flow crosses either the spine or fan only. 相似文献
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应用二维磁流体动力学模拟方法数值研究了三层电流片中电阻撕裂模不稳定性的特征及磁场重联过程．结果表明，这是一种复杂的非稳态磁场重联．在初期阶段，三个电流片中分别由撕裂模不稳定性引起磁场重联，形成薄而长的磁岛．随着撕裂模不稳定性的非线性发展，每个磁岛的宽度都逐步增大，以至导致新的磁场重联发生．同时，三个电流片的强度都逐渐减弱，且原中心反向电流区最终消失．部分磁能不断地转化为等离子体的热能和动能，引起等离子体的加热和加速．多层电流片中撕裂模不稳定性引起的自发重联，可能对太阳耀斑、日冕加热、太阳风与磁层耦合等有重要影响． 相似文献
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Laura F. Morales Sergio Dasso Daniel O. Gmez Pablo Mininni 《Journal of Atmospheric and Solar》2005,67(17-18):1821
We analyze in situ observations of magnetic reconnection at the Earth magnetopause to estimate the importance of the Hall current during the merging of interplanetary and magnetospheric magnetic field lines. The reconnection process is studied through numerical simulations, integrating the Hall MHD equations in 2.5 dimensions. A large influence of the Hall effect is found, which can be measured by a significant increase of the reconnection rate. 相似文献
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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
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L. M. B. C. Campos 《地球物理与天体物理流体动力学》2013,107(4):193-215
Abstract We deduce the dissipative Alfvén wave equation in a medium stratified in one direction, with a transverse magnetic field, in the presence of dissipation by fluid viscosity and electrical resistance; the dissipative Alfvén wave equation generalizes earlier results for homogeneous (Cowling, 1960) and inhomogeneous (Campos, 1983a) media, and corrects an error in the literature (Heyvaerts and Priest, 1983). The wave equation is solved exactly in two cases: a uniform magnetic field, and a magnetic field decreasing with height. In both cases the mean state is assumed to be isothermal, with a constant rate of ionization, so that the magnetic diffusivity is constant, but the dynamic viscosity increases with height. There are therefore two regions, a low- (high-) altitude region where electrical resistance dominates fluid viscosity (or vice versa), and an asymptotic regime relevant to the uppermost (lowermost) layers. The two regions are separated by a transition layer, across which the wave field is continuous and whose structure is expressible by hypergeometric functions, with different arguments in the low- and high-altitude regions, and over the whole altitude range. These exact solutions allow the amplitude and phase of the wave field to be plotted as a function of height for a variety of magnetoatmospheric mean states. They show that wave dissipation is more localized and intense when the magnetic field decreases with height than when it is uniform. 相似文献
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A. D. Sneyd 《地球物理与天体物理流体动力学》2013,107(1-3):141-151
Abstract Formation of electric current sheets in the corona is thought to play an important role in solar flares, prominences and coronal heating. It is therefore of great interest to identify magnetic field geometries whose evolution leads to variations in B over small length-scales. This paper considers a uniform field B 0[zcirc], line-tied to rigid plates z = ±l, which are then subject to in-plane displacements modeling the effect of photospheric motion. The force-free field equations are formulated in terms of field-line displacements, and when the imposed plate motion is a linear function of position, these reduce to a 4 × 4 system of nonlinear, second-order ordinary differential equations. Simple analytic solutions are derived for the cases of plate rotation and shear, which both tend to form singularities in certain parameter limits. In the case of plate shear there are two solution branches—a simple example of non-uniqueness. 相似文献
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Manuel Núñez 《地球物理与天体物理流体动力学》2013,107(6):525-535
It is generally admitted that a plasma in the absence of forcing will relax to a minimum energy state compatible with appropriate constraints. Usually this is a force-free state, which, in two dimensions, implies a potential magnetic field except by the possible presence of current sheets. The precise mechanism of this relaxation, and in particular the plasma velocity, are generally ignored. There exists, however, a physically well-defined process that should produce magnetic relaxation: ion--neutral (or ambipolar) friction. While there is no guarantee of the existence of a limit of this process when t?→?∞, there exists a family of sequential limits for whom the Lorentz force tends to zero. To analyze the configuration of these limit states, we study the evolution of several moments of the magnetic energy. We prove that for as long as the enstrophy remains bounded, the current density energy also remains bounded in two dimensions: this excludes all classical configurations of current sheets across which the magnetic field reverses direction. Hence, these sheets cannot be the limit of ion--neutral diffusion unless the flow becomes increasingly irregular over time. 相似文献
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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