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We present grid-adaptive numerical simulations of magnetized plasma jets, modeled by means of the compressible magnetohydrodynamic
equations. The Adaptive Mesh Refinement strategy makes it possible to investigate long-term jet dynamics where both large-scale
and small-scale effects are at play. We extend recent findings for uniformly magnetized, periodic shear layers to planar and
fully 3D extended jet segments. The jet lengths cover multiple, typically 10, axial wavelengths of the fastest growing Kelvin–Helmholtz
(KH) like modes. The dominant linear MHD instabilities of the jet flows are quantified by means of MHD spectroscopic analysis.
In cases characterized by sonic Mach numbers about unity and large plasma beta values, both single and double shear layers
(planar jets) manifest self-organizing trends to large scales, e.g. by continuous pairing/merging between co-rotating vortices,
simultaneously with the introduction of small-scale features by magnetic reconnection events. The vortices form as a result
of KH unstable shear-flow layers, and their coalescence arises from the growth of subharmonic modes at multiple wavelengths
of the fastest growing KH instability. In extended two-dimensional jet segments, we investigate how varying jet width alters
this coalescence process occurring at both edges, e.g. by introducing Batchelor-like coupling between counter-rotating vortices
formed at opposing weakly magnetized, close shear layers. Finally, periodic segments of supersonic magnetized jets are simulated
in two- and three-dimensional cases, which are characterized by violent shock-dominated transients. 相似文献
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In previous publications (Keppens et al.: 2002, Astrophys. J. 569, L121; Goedbloed et al.: 2004a, Phys. Plasmas
11, 28), we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous
number of different MHD instabilities, with the well-known magneto-rotational instability (Velikhov, E. P.: 1959, Soviet Phys.–JETP Lett. 36, 995; Chandrasekhar, S.: 1960, Proc. Natl. Acad. Sci. U.S.A. 46, 253; Balbus, S. A. and Hawley, J. F.: 1991, Astrophys. J. 376, 214) as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of
the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion
disks, is the class of trans-slow Alfv’en continuum modes, that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually
every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum.
The mode structures rotate, in turn, about the rotating disk. These structures lock and become explosively unstable when the
mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order
of the Alfv’en transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across
magnetic surfaces and jet formation. 相似文献
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We discuss steady-state transonic outflows obtained by direct numerical solution of the hydrodynamic and magnetohydrodynamic
equations. We make use of the Versatile Advection Code, a software package for solving systems of (hyperbolic) partial differential
equations. We model thermally and magneto-centrifugally driven stellar outflows as generalizations of the well-known Parker
and Weber-Davis wind solutions. To obtain steady-state solutions efficiently, we exploit fully implicit time stepping.
Wind solutions containing both a 'wind' and a 'dead' zone are presented. We emphasize the boundary conditions imposed at the
stellar surface. For axisymmetric wind solutions, we use the knowledge of the flux functions to verify the numerical solutions.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
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K.M. Schure J. Vink A. Achterberg R. Keppens 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Observations show that the magnetic field in young supernova remnants (SNRs) is significantly stronger than can be expected from the compression of the circumstellar medium (CSM) by a factor of four expected for strong blast waves. Additionally, the polarization is mainly radial, which is also contrary to expectation from compression of the CSM magnetic field. Cosmic rays (CRs) may help to explain these two observed features. They can increase the compression ratio to factors well over those of regular strong shocks by adding a relativistic plasma component to the pressure, and by draining the shock of energy when CRs escape from the region. The higher compression ratio will also allow for the contact discontinuity, which is subject to the Rayleigh–Taylor (R–T) instability, to reach much further out to the forward shock. This could create a preferred radial polarization of the magnetic field. With an Adaptive Mesh Refinement MHD code (AMRVAC), we simulate the evolution of SNRs with three different configurations of the initial CSM magnetic field, and look at two different equations of state in order to look at the possible influence of a CR plasma component. The spectrum of CRs can be simulated using test particles, of which we also show some preliminary results that agree well with available analytical solutions. 相似文献
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