共查询到20条相似文献，搜索用时 31 毫秒
1.
Jonathan Ferreira 《Astrophysics and Space Science》2004,293(1-2):83-90
The goal of this contribution is not to describe results on accretion discs launching jets obtained within the self-similar framework. Rather, I would like (1) to emphasize the huge difficulties one has to face when attempting to perform magnetohydrodynamic (MHD) numerical simulations of such discs; (2) to show how self-similar solutions can help to solve these difficulties. In particular, they allow to choose suitable boundary conditions and/or relevant initial conditions. In all cases anyway, they provide a framework for understanding and testing the outcome of numerical simulations. 相似文献
2.
We present numerical simulations and explore scalings and anisotropy of compressible magnetohydrodynamic (MHD) turbulence. Our study covers both gas-pressure-dominated (high β) and magnetic-pressure-dominated (low β) plasmas at different Mach numbers. In addition, we present results for super-Alfvénic turbulence and discuss in what way it is similar to sub-Alfvénic turbulence. We describe a technique of separating different magnetohydrodynamic modes (slow, fast and Alfvén) and apply it to our simulations. We show that, for both high- and low-β cases, Alfvén and slow modes reveal a Kolmogorov k −5/3 spectrum and scale-dependent Goldreich–Sridhar anisotropy, while fast modes exhibit a k −3/2 spectrum and isotropy. We discuss the statistics of density fluctuations arising from MHD turbulence in different regimes. Our findings entail numerous astrophysical implications ranging from cosmic ray propagation to gamma ray bursts and star formation. In particular, we show that the rapid decay of turbulence reported by earlier researchers is not related to compressibility and mode coupling in MHD turbulence. In addition, we show that magnetic field enhancements and density enhancements are marginally correlated. Addressing the density structure of partially ionized interstellar gas on astronomical-unit scales, we show that the viscosity-damped regime of MHD turbulence that we reported earlier for incompressible flows persists for compressible turbulence and therefore may provide an explanation for these mysterious structures. 相似文献
3.
Michel Tagger 《Astrophysics and Space Science》2004,292(1-4):377-381
In this contribution I review the main challenges for theory and numerical simulation of accretion turbulence in disks. I then present briefly a solution we have elaborated in recent years to a part of these questions: we have found an MHD instability which occurs in the inner region of a disk in the configuration (poloidal field of the order of equipartition with the gas pressure) used for MHD jet models. This instability has the unique characteristic that it re-emits toward the corona a fraction of the energy and angular momentum it extracts from the disk. It is a good candidate to explain the low-frequency Quasi-Periodic Oscillation observed in X-ray binaries, and we believe that it might occur also in YSO. 相似文献
4.
Matthew R. Bate 《Astrophysics and Space Science》2004,292(1-4):297-307
In this paper, we review the results from numerical simulations that investigate the evolution of molecular cores into stars and brown dwarfs. We begin by discussing the four main processes involved in this evolution, fragmentation, accretion, interactions with discs, and stellar dynamical interactions. We then discuss large-scale calculations in which all of these processes are model simultaneously. Finally, we review results that have been obtained from past magnetohydrodynamical (MHD) calculations of fragmentation and look towards future MHD calculations. 相似文献
5.
Alexander A. Schekochihin Steven C. Cowley Samuel F. Taylor Jason L. Maron James C. McWilliams 《Astrophysics and Space Science》2004,292(1-4):141-146
We consider the problem of incompressible, forced, nonhelical, homogeneous, isotropic MHD turbulence with no mean magnetic field. This problem is essentially different from the case with externally imposed uniform mean field. There is no scale-by-scale equipartition between magnetic and kinetic energies as would be the case for the Alfvén-wave turbulence. The isotropic MHD turbulence is the end state of the turbulent dynamo which generates folded fields with small-scale direction reversals. We propose that the statistics seen in numerical simulations of isotropic MHD turbulence could be explained as a superposition of these folded fields and Alfvén-like waves that propagate along the folds. 相似文献
6.
Ralph E. Pudritz 《Astrophysics and Space Science》2004,292(1-4):471-478
This article summarizes recent theoretical, numerical and observational progress on the nature of jets and outflows in regions of star formation. The emphasis is placed on the role of hydromagnetic phenonmena in understanding these new results. The confluence of sophisticated 3D numerical MHD simulations and the recent observations of rotating jets makes it possible, for the first time, to rigorously test jet models. 相似文献
7.
《Planetary and Space Science》2007,55(5):589-597
We present three-dimensional numerical simulations of the interaction between a Hot Jupiter and the stellar wind plasma of its host star in the framework of resistive magnetohydrodynamics (MHD). In a first step, we investigate the numerical realization of the plasma flow around the planet and the planetary magnetic field using a simplified model, before we simulate more realistic scenarios on the basis of the stellar wind model by Weber and Davis. A main goal is to understand the magnetic interaction between star and planet. In analogy to the well-known Jupiter Io scenario, we study the development of a magnetic field-aligned current system in different parameter regimes. 相似文献
8.
Rony Keppens Hubert Baty Jeroen Bergmans Fabien Casse 《Astrophysics and Space Science》2004,293(1-2):217-224
A suitable model for the macroscopic behavior of accretion disk-jet systems is provided by the equations of MagnetoHydroDynamics (MHD). These equations allow us to perform scale-encompassing numerical simulations of multidimensional nonlinear magnetized plasma flows. For that purpose, we continue the development and exploitation of the Versatile Advection Code (VAC) along with its recent extension which employs dynamically controlled grid adaptation. In the adaptive mesh refinement AMRVAC code, modules for simulating any-dimensional special relativistic hydro- and magnetohydrodynamic problems are currently operational. Here, we review recent 3D MHD simulations of fundamental plasma instabilities, relevant when dealing with cospatial shear flow and twisted magnetic fields. Such magnetized jet flows can be susceptible to a wide variety of hydro (e.g. Kelvin-Helmholtz) or magnetohydrodynamic (e.g. current driven kink) instabilities. Recent MHD computations of 3D jet flows have revealed how such mutually interacting instabilities can in fact aid in maintaining jet coherency. Another breakthrough from computational magnetofluid modeling is the demonstration of continuous, collimated, transmagnetosonic jet launching from magnetized accretion disks. Summarizing, MHD simulations are rapidly gaining realism and significantly advance our understanding of nonlinear astrophysical magnetofluid dynamics. 相似文献
9.
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
10.
Various radio observations have shown that the hot atmospheres of galaxy clusters are magnetized. However, our understanding of the origin of these magnetic fields, their implications on structure formation and their interplay with the dynamics of the cluster atmosphere, especially in the centres of galaxy clusters, is still very limited. In preparation for the upcoming new generation of radio telescopes (like Expanded Very Large Array, Low Wavelength Array, Low Frequency Array and Square Kilometer Array), a huge effort is being made to learn more about cosmological magnetic fields from the observational perspective. Here we present the implementation of magnetohydrodynamics (MHD) in the cosmological smoothed particle hydrodynamics (SPH) code gadget . We discuss the details of the implementation and various schemes to suppress numerical instabilities as well as regularization schemes, in the context of cosmological simulations. The performance of the SPH–MHD code is demonstrated in various one- and two-dimensional test problems, which we performed with a fully, three-dimensional set-up to test the code under realistic circumstances. Comparing solutions obtained using athena , we find excellent agreement with our SPH–MHD implementation. Finally, we apply our SPH–MHD implementation to galaxy cluster formation within a large, cosmological box. Performing a resolution study we demonstrate the robustness of the predicted shape of the magnetic field profiles in galaxy clusters, which is in good agreement with previous studies. 相似文献
11.
12.
AMR Simulations of Magnetohydrodynamic Problems by the CESE Method in Curvilinear Coordinates 总被引：1，自引：0，他引：1
The objective of this paper is to present new extensions of the space – time conservation element and solution element (CESE)
method for simulations of magnetohydrodynamic (MHD) problems in general curvilinear coordinates by using an adaptive mesh
refinement (AMR) grid system. By transforming the governing MHD equations from the physical space (x,y,z) to the computational space (ξ,η,ζ) while retaining the form of conservation, the CESE method is established for MHD in the curvilinear coordinates. Utilizing
the parallel AMR package PARAMESH, we present the first implementation of applying the AMR CESE method for MHD (AMR-CESE-MHD)
in both Cartesian and curvilinear coordinates. To show the validity and capabilities of the AMR-CESE-MHD code, a suite of
numerical tests in two and three dimensions including ideal MHD and resistive MHD are carried out, with two of them in both
Cartesian and curvilinear coordinates. Numerical tests show that our results are highly consistent with those obtained previously
by other authors, and the results under both coordinate systems confirm each other very well. 相似文献
13.
R. J. R. Williams 《Astrophysics and Space Science》2007,307(1-3):179-182
Low mass star formation may be triggered by the dynamical effects of radiation fields and winds from massive stars on nearby molecular material. The columns of neutral material observed at the edges of many Hii regions may be the tracers of this process. Magnetic fields are dynamically important in the molecular clouds from which new stars form, but their effect on the development of molecular columns has not been studied in detail. In this paper, I present initial MHD simulations of this process. 相似文献
14.
Following a brief overview of the two main approaches to investigate the interaction between magnetic fields and convective flows near the solar surface layers by numerical simulation, namely idealized model problems and ‘realistic’ large‐eddy simulations, we present first results obtained with a newly developed MHD code. The first example concerns the realistic simulation of the magnetic field dynamics in a solar plage region while the second example demonstrates small‐scale dynamo action in idealized compressible convection. 相似文献
15.
太阳大气中磁重联的MHD数值模拟 总被引：1，自引：0，他引：1
回顾了近３０年太阳大气中磁重联过程的ＭＨＤ数值模拟工作取得的进展。着重描述了在验证理论模型,解释观测现象,以及研究各种因素对重联的影响三个方面的成果,如快速磁重联,太阳耀斑机制及色球,日冕中的各种爆发现象等。指出了在数值模拟中应注意的几个问题,并对该领域今后的发展作了简要的展望。 相似文献
16.
De-Fu Bu Feng Yuan Fu-Guo Xie 《Monthly notices of the Royal Astronomical Society》2009,392(1):325-331
Observations and numerical magnetohydrodynamic (MHD) simulations indicate the existence of outflows and ordered large-scale magnetic fields in the inner region of hot accretion flows. In this paper, we present the self-similar solutions for advection-dominated accretion flows (ADAFs) with outflows and ordered magnetic fields. Stimulated by numerical simulations, we assume that the magnetic field has a strong toroidal component and a vertical component in addition to a stochastic component. We obtain the self-similar solutions to the equations describing the magnetized ADAFs, taking into account the dynamical effects of the outflow. We compare the results with the canonical ADAFs and find that the dynamical properties of ADAFs such as radial velocity, angular velocity and temperature can be significantly changed in the presence of ordered magnetic fields and outflows. The stronger the magnetic field is, the lower the temperature of the accretion flow will be and the faster the flow rotates. The relevance to observations is briefly discussed. 相似文献
17.
We discuss the self-consistent time-dependent numerical boundary conditions on the basis of theory of characteristics for magnetohydrodynamics (MHD) simulations of solar plasma flows. The importance of using self-consistent boundary conditions is demonstrated by using an example of modeling coronal dynamic structures. This example demonstrates that the self-consistent boundary conditions assure the correctness of the numerical solutions. Otherwise, erroneous numerical solutions will appear. 相似文献
18.
The theory that magnetic fields are instrumental in the formation and propagation of jets in active galactic nuclei dates
back four decades. Despite a recent growing consensus on this notion stemming from the results of numerical simulations of
magnetohydrodynamic (MHD) flows near black holes, the precise dynamical role of magnetic fields in observed parsec and kiloparsec
jets remains uncertain. Some of the unanswered fundamental questions about extragalactic jets include the location where the
flow becomes relativistic and where acceleration and collimation terminate, as well as the specifics of how the flow interacts
with the ISM. Such observed properties as superluminal motions and wiggled structures based on numerical simulations to constitute
the foundation of an MHD paradigm for extragalactic jets. We particularly focus our attention to the M87 jet, which is one
of the best candidates to investigate relativistic outflows in extragalactic system. 相似文献
19.
We present results of MHD axisymmetrical cylindrical simulations performed to study Hα emission maps from jets, with a toroidal field geometry for the magnetic field. Our code uses a linear Riemann solver, integrates equations in 2.5 dimensions and includes the calculation of the ionization fraction and the effects of the emission due to the collisional excitation of [O I] and [O II] lines, radiative recombination of H and the collisional ionization of H and excitation of Lyman-α. The simulations with variable ejection velocities show that the magnetic field produces an increase in the emission from all of the knots except for the first bow shock. 相似文献
20.
S. Servidio V. Carbone L. Primavera P. Veltri K. Stasiewicz 《Planetary and Space Science》2007,55(15):2239-2243
By direct numerical simulations we investigate the nonlinear dynamics of a compressible Hall Magnetohydrodynamic (MHD) plasma. At small scales, where the Hall effect dominates, we found an increase of the compressibility of the system and the breakdown of the strong link between velocity and magnetic fields, typical of usual MHD. Moreover, we find that small-scale fluctuations are characterized by an anti-correlation between density and magnetic field intensity. These features characterize the excitation of a quasi-perpendicular magnetosonic turbulence that can be interpreted as the small-scale signature of the break-down of the MHD nonlinear energy cascade due to Hall effect. Fluctuations with the same properties, based on measurements by Cluster spacecraft in space plasma turbulence during different magnetopause crossings, have been recently observed. 相似文献