共查询到20条相似文献，搜索用时 31 毫秒
1.
Y. C. Whang 《Solar physics》1994,149(2):347-362
We study a nonplanar model of magnetic reconnection associated with conical slow shocks, assuming that the shock surfaces are two identical cones with circular cross sections symmetrical about the ±x-axis. In the inflow region upstream of the shocks, two oppositely directed magnetic fields are separated by a current sheet. The model treats the current sheet as a tangential discontinuity and treats shocks and tangential discontinuity as surfaces of zero thickness. The dynamical structure of the global magnetic field in the continuous regions is studied using compressible, non-resistive MHD equations. In the inflow region, nonplanar magnetic field lines first move toward the current sheet. Near the sheet, the middle sections of the field lines become highly flattened, almost parallel to the sheet. Eventually, then oppositely directed field lines merge across the tangential discontinuity between the two shocks, and the magnetic lines are reconnected at the intersection of the shock and the tangential discontinuity. Reconnected magnetic lines are carried away at high speeds by the MHD flow in the outflow region, downstream of the shocks. 相似文献
2.
Bhimsen K. Shivamoggi 《Astrophysics and Space Science》1984,103(2):219-232
An extension of Sonnerup's model for the magnetic field-line reconnection for a compressible plasma is given. The plasma is considered to be only slightly compressible so that the leading wave in Sonnerup's model can still be taken to be a thin discontinuity. The flow is assumed to occur under adiabatic conditions, and de Hoffmann-Teller jump conditions are used to connect the state variables across the shocks. The compressibility effects are found to increase the reconnection rate. The signaling problem is finally considered to study the evolution of MHD waves in a compressible, dissipative plasma so as to investigate the conditions under which the assumption of MHD waves in a compressible plasma to be thin discontinuities is valid. 相似文献
3.
S. S. Komissarov 《Monthly notices of the Royal Astronomical Society》2005,359(3):801-808
In this paper we report the results of axisymmetric relativistic magnetohydrodynamic (MHD) simulations for the problem of a Kerr black hole immersed in a rarefied plasma with 'uniform' magnetic field. The long-term solution shows properties that are significantly different from those of the initial transient phase studied recently by Koide. The topology of magnetic field lines within the ergosphere is similar to that of the split-monopole model with a strong current sheet in the equatorial plane. Closer inspection reveals a system of isolated magnetic islands inside the sheet and ongoing magnetic reconnection. No regions of negative hydrodynamic 'energy at infinity' are seen inside the ergosphere and the so-called MHD Penrose process does not operate. However, the rotational energy of the black hole continues to be extracted via the purely electromagnetic Blandford–Znajek mechanism. In spite of this, no strong relativistic outflows from the black hole are seen to be developing. Combined with results of other recent simulations, our results signal a potential problem for the standard MHD model of relativistic astrophysical jets should they be found at distances as small as a few tens of gravitational radii from the central black hole. 相似文献
4.
It has recently been shown that adiabatic solutions of the one-dimensional current sheet problem exist provided that magnetically trapped particles are included in the model together with the current-carrying untrapped “beam” particles. We show here that a formulation of the problem in terms of particle velocity and pitch angle is advantageous, and we derive some general properties of the solutions. In particular it is shown that there is, in general, no discontinuity in the value of the particle distribution function ? across the boundary in velocity space between “beam” and trapped particles, but that there will be a discontinuity in the gradients of ?. An example is given in which the beam population is of bi-Maxwellian form at the outer boundary of the current sheet. 相似文献
5.
For the 1D radial hypersonic flow the development of the configuration with two shocks and contact discontinuity is considered.
At small and large moments of time solutions in explicit form are found. As follows from these solutions the contact surface
accelerates in time. This acceleration makes possible the Rayleigh-Taylor instability to develop. The 2D numerical investigation
of the problem has confirmed the instability of the 1D solution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Two-dimensional stationary magnetic reconnection models that include a thin Syrovatskii-type current sheet and four discontinuous
magnetohydrodynamic flows of finite length attached to its endpoints are considered. The flow pattern is not specified but
is determined from a self-consistent solution of the problem in the approximation of a strong magnetic field. Generalized
analytical solutions that take into account the possibility of a current sheet discontinuity in the region of anomalous plasma
resistivity have been found. The global structure of the magnetic field in the reconnection region and its local properties
near the current sheet and attached discontinuities are studied. In the reconnection regime in which reverse currents are
present in the current sheet, the attached discontinuities are trans-Alfvénic shock waves near the current sheet endpoints.
Two types of transitions from nonevolutionary shocks to evolutionary ones along discontinuous flows are shown to be possible,
depending on the geometrical model parameters. The relationship between the results obtained and numerical magnetic reconnection
experiments is discussed. 相似文献
7.
A. I. Podgorny 《Solar physics》1995,156(1):41-64
Current sheet (CS) creation in a region with anX-type zero magnetic field line in plasma was simulated by numerically solving the 3D MHD equations for conditions which were close to the solar corona: the disturbance propagated from the photosphere boundary under which the magnetic field sources were situated. Some of values (B,,V) were set on the photosphere boundary, while others were determined from the conditions inside the region. Several Alfvén times after its creation, the CS motion practically ceased, and the plasma velocity changed its direction above the sheet, so that the plasma flow was directed into the CS from both sides. 相似文献
8.
This paper examines the consequences of the assumption that substorm-associated growth of magnetosphere-ionosphere current systems is triggered by the incidence, on the ionosphere, of a large amplitude Alfvén wave generated in the distant magnetotail. It is pointed out that there is a large body of evidence suggesting that, in the acceleration region near 1 RE, one is likely to find a major discontinuity in mass density. Following the approach of Cohen and Kulsrud (1974) who studied the steepening of large amplitude hydromagnetic waves into shocks, we demonstrate that the character of the background plasma and magnetic field in the auroral acceleration region near 1 RE can be ideal for the generation of MHD shocks and that these shocks can lead to the acceleration of ions and electrons as reported by investigators using S3-3 satellite data. 相似文献
9.
Collisionless shocks in turbulent space plasmas accelerate particles by the Fermi mechanism to ultrarelativistic energies. The interaction of accelerated particles with the plasma inflow produces extended supersonic MHD flows of multicomponent plasma. We investigate the instabilities of a flow of three-component turbulent plasma with relativistic particles against long-wavelength perturbations with scales larger than the accelerated particle transport mean free path and the initial turbulence scales. The presence of turbulence allows us to formulate the system of single-fluid equations, the equation of motion for the medium as a whole, and the induction equation for the magnetic field with turbulent magnetic and kinematic viscosities. The current of accelerated particles enters into the induction equation with an effective magnetic diffusion coefficient. We have calculated the local growth rates of the perturbations related to the nonresonant long-wavelength instability of the current of accelerated particles for MHD perturbations in the WKB approximation. The amplification of long-wavelength magnetic field perturbations in the flow upstream of the shock front can affect significantly the maximum energies of the particles accelerated by a collisionless shock and can lead to the observed peculiarities of the synchrotron X-ray radiation in supernova remnants. 相似文献
10.
The question about the interpretation of numerical experiments on magnetic reconnection in solar flares is considered. A correspondence
between the standard classification of magnetohydrodynamic discontinuities and the parameters characterizing the mass flux
through a discontinuity and the magnetic field configuration has been established within a classical formulation of the problem
on discontinuous magnetohydrodynamic flows. A pictorial graphical representation of the relationship between the angles of
the magnetic field vector relative to the normal to the discontinuity plane on both its sides has also been found. The relations
between the parameters of a two-dimensional discontinuous flow have the simplest form in a frame of reference where the magnetic
field lines (B) are parallel to the matter velocity (u)—the deHoffmann-Teller frame. The question about the transformation of the magnetic field configuration when passing to a
“laboratory” frame of reference where (v · B) ≠ 0, i.e., an electric field is present, is considered in this connection. The result is applied to the analytical solution
of the problem on the magnetic field structure in the vicinity of a reconnecting current sheet obtained previously by Bezrodnykh
et al. The regions of nonevolutionary shocks are shown to appear near the endpoints of a current sheet with reverse currents. 相似文献
11.
We numerically simulate the evolution of the plane two-dimensional deformations of a contact discontinuity that is impulsively accelerated by a shock wave. We take into account the effects of radiative cooling and perturbation scale lengths on the dynamics and shape of the forming density inhomogeneities. For moderately intense shocks in a stellar wind and for strong shocks from a supernova, we show that the radiative cooling processes do not affect significantly the growth rate of the initial perturbations and the total mass of the forming condensations. However, the density of the matter compressed by the transmitted shock wave increases dramatically. At the same time, the contribution from long-wavelength perturbations to the deformation of the contact surface decreases significantly. In the case of shock propagation from a supernova, the initial conditions have been found to be a factor that can affect the morphology of the shocked interstellar medium. 相似文献
12.
A longitudinal stability is considered for the quasi-steady current sheet which is uniform along the current. In the MHD approximation, the stability problem is solved for the plane neutral sheet and small disturbances propagating along the current. The current sheet is shown to break-up into the system of cooler and more dense filaments due to radiative cooling. The filaments are parallel to magnetic field lines. This process corresponds to the condensation mode of a thermal instability and can play a trigger role for a solar flare. Moreover, at the nonlinear stage of development, it can lead to the formation of very dense cold filaments surrounded by high-temperature low-density plasma inside the current sheet. Flowing into the filaments, hot plasma is cooled by radiation and compressed. Then the cold dense plasma flows out from the current sheet along the filaments. We think that the process under consideration is responsible for the often observed picture of an arcade of cold loops in the solar corona.The text of this paper was written by B. V. Somov after the death of Prof. S. I. Syrovatskii. 相似文献
13.
Shaohua Zhang A. M. Du Xueshang Feng Xin Cao Quanming Lu Liping Yang Gengxiong Chen Ying Zhang 《Solar physics》2014,289(5):1607-1623
Electron acceleration in a drastically evolved current sheet under solar coronal conditions is investigated via the combined 2.5-dimensional (2.5D) resistive magnetohydrodynamics (MHD) and test-particle approaches. Having a high magnetic Reynolds number (105), the long, thin current sheet is torn into a chain of magnetic islands, which grow in size and coalesce with each other. The acceleration of electrons is explored in three typical evolution phases: when several large magnetic islands are formed (phase 1), two of these islands are approaching each other (phase 2), and almost merging into a “monster” magnetic island (phase 3). The results show that for all three phases electrons with an initial Maxwell distribution evolve into a heavy-tailed distribution and more than 20 % of the electrons can be accelerated higher than 200 keV within 0.1 second and some of them can even be energized up to MeV ranges. The lower-energy electrons are located away from the magnetic separatrices and the higher-energy electrons are inside the magnetic islands. The most energetic electrons have a tendency to be around the outer regions of the magnetic islands or to appear in the small secondary magnetic islands. It is the trapping effect of the magnetic islands and the distributions of E p that determine the acceleration and spatial distributions of the energetic electrons. 相似文献
14.
B. Vršnak 《Solar physics》1989,120(1):79-92
The properties and development of a high-temperature current sheet characterized by increasing merging velocity are studied and related to the early phases of solar flares. It is shown that the system can be described by the Petschek-type geometry for a wide range of merging velocities. In the diffusion region and the standing MHD shocks a certain low-frequency plasma microturbulence is generated from the very beginning of the reconnection process. We present qualitative solutions for the case of ion-acoustic turbulence in marginally stable state, which provide a comparison with observations. The increasing merging velocity leads to the appearance of the soft X-ray precursor. The precursor temperature maximum should appear during the current sheet formation, before the Petschek regime is established. In the Petschek regime the temperature of the hot plasma decreases due to the decrease of the magnetic field strength at the diffusion region boundary, while the soft X-ray radiation still increases, reaching precursor maximum for merging velocities about 1% of the external Alfvén velocity. The precursor phase ends when the value of the merging velocity surpasses the upper limit for the Petschek regime and the system enters into the pile-up regime, causing a new increase of plasma temperature and soft X-ray radiation.It is shown that Alfvén velocities in the range 800–1200 km s –1 are sufficient to explain typical soft X-ray precursors. Cases of low merging velocities and low Alfvén velocities are discussed and can be applied to describe the properties of spotless flares. 相似文献
15.
Isao Okamoto 《Monthly notices of the Royal Astronomical Society》2001,327(1):55-68
It is argued that for steady, axisymmetric, non-relativistic magneto-centrifugal winds, not only the boundary and criticality conditions but also the current-closure condition are of crucial significance as global conditions in resolving the acceleration-collimation problem. In Sakurai's numerical models, the split-monopole field adopted at the surface of the source provided the most favourable condition for global collimation of the flow, by making the domain of anti -collimating flow with outgoing electric current degenerate into an infinitely thin boundary layer at the equator, and hence suppressing the explicit appearance of the current-closure condition.
For more general or realistic boundary conditions at the source, it is shown that the current-closure condition yields a two-component structure (with the return current at least in part in a volume current, not totally a sheet current) as a natural consequence of the transfield equation in the asymptotic domain. This equation, combined with the Bernoulli (and other) integrals, requires the wind to tend asymptotically to a 'quasi-conical' structure, as a natural consequence of the flow particles' becoming more and more ballistic as a result of the magnetohydrodynamic (MHD) acceleration. This is a result that the Poynting energy flux diminishes to zero along each field line. The criticality problem is solved for magneto-centrifugal winds, to give the eigenvalues of the Alfvénic distance and other quantities at the fast magnetosonic surface, situated somewhere between the subasymptotic and asymptotic domains. 相似文献
For more general or realistic boundary conditions at the source, it is shown that the current-closure condition yields a two-component structure (with the return current at least in part in a volume current, not totally a sheet current) as a natural consequence of the transfield equation in the asymptotic domain. This equation, combined with the Bernoulli (and other) integrals, requires the wind to tend asymptotically to a 'quasi-conical' structure, as a natural consequence of the flow particles' becoming more and more ballistic as a result of the magnetohydrodynamic (MHD) acceleration. This is a result that the Poynting energy flux diminishes to zero along each field line. The criticality problem is solved for magneto-centrifugal winds, to give the eigenvalues of the Alfvénic distance and other quantities at the fast magnetosonic surface, situated somewhere between the subasymptotic and asymptotic domains. 相似文献
16.
17.
S. S. Komissarov 《Monthly notices of the Royal Astronomical Society》2006,367(1):19-31
In this paper we present the results of time-dependent simulations of the dipolar axisymmetric magnetospheres of neutron stars carried out within the frameworks of both relativistic magnetohydrodynamics (MHD) and resistive force-free electrodynamics. The results of force-free simulations reveal the inability of our numerical method to accommodate the equatorial current sheets of pulsar magnetospheres, and raise a question mark about the robustness of this approach. On the other hand, the MHD approach allows us to make significant progress. We start with a non-rotating magnetically dominated dipolar magnetosphere and follow its evolution as the stellar rotation is switched on. We find that the time-dependent solution gradually approaches a steady state that is very close to the stationary solution of the pulsar equation found in 1999 by Contopoulos, Kazanas & Fendt. This result suggests that other stationary solutions that have the Y-point located well inside the light cylinder are unstable. The role of particle inertia and pressure on the structure and dynamics of MHD magnetospheres is studied in detail, as well as the potential implications of dissipative processes in the equatorial current sheet. We argue that pulsars may have differentially rotating magnetospheres which develop noticeable structural oscillations, and that this may help to explain the nature of the subpulse phenomena. 相似文献
18.
M.F. Heyn 《Astrophysics and Space Science》1997,256(1-2):343-348
Magnetic reconnection causes the local non-linear decay of an infinitely thin current sheet, i.e., a tangential discontinuity, into large amplitude magnetohydrodynamic waves. This decay process propagates along the initial current sheet and induces linear wave perturbations in the surroundings. The Greens function for the self-consistent linear compressible plane wave problem is constructed and it is shown how to perform the convolution with some source function in form of a local reconnection pulse in an efficient way. 相似文献
19.
The possibility that the type of magnetohydrodynamic (MHD) discontinuity changes as the plasma flow conditions gradually change is investigated in a general form. The conservation laws in MHD admit such transitions if there exist the so-called transition solutions that simultaneously satisfy two types of discontinuities. These solutions have been sought for. The system of possible transitions between MHD discontinuities obtained on their basis is presented in a clear schematic form. The ultimate general scheme of transitions includes all of the previously described schemes of transitions known to us. The system of discontinuities and transitions between them is studied in a self-consistent solution of the analytical problem of reconnection in a strong magnetic field. 相似文献
20.
本文在中子星磁层与吸积盘之间引入了一个速度、密度、压强和磁场都连续变化的有限厚度的剪切层,以代替Anzer理论中的切向间断面,用磁流体力学方法讨论了中子星磁层与吸积盘交界处等离子体可压缩情况下平面波扰动的K-H不稳定性。结果表明,K-H不稳定性依然存在,径向波矢扰动成为不稳定的主要模式。文中特别讨论了剪切层厚度取值对中子星自转的影响,表明适当调节剪切层厚度就可解释X射线脉冲星周期的变化。将此模型应用到脉冲X射线源Her X-1上,得到较好的结果。 相似文献