共查询到20条相似文献，搜索用时 109 毫秒
1.
Stefanie Mehren-Baehr Wolfgang Glatzel 《Monthly notices of the Royal Astronomical Society》1999,310(2):387-397
A systematic study of the dependence on disc parameters and input physics, such as opacity and the treatment of convection, of strange-mode instabilities in thin accretion discs, which have been discovered recently, is presented. The instabilities are found to exist for a wide range of parameters, are partly very robust, and their growth rates can reach the dynamical range. Even discs on galactic scales around massive black holes are affected by them. Two groups of instabilities can be distinguished, the first of which is related to the radiation-pressure-dominated part of the disc, and the second to helium/hydrogen ionization. By application of the NAR approximation, both of them can be shown to be of mechanical origin, and the classical κ -mechanism can be excluded as the instability mechanism. A heuristic model for strange-mode instabilities proposed in the context of stellar strange-mode instabilities in luminous stars seems to be applicable to the group associated with dominant radiation pressure. 相似文献
2.
3.
We present a survey of different kinds of instabilities in the context of radiative colliding flows which greatly contribute
to structure formation. In particular, this includes analytical results for different kinds of thin shell instabilities (DI,
NDI, NTSI). New numerical results for the non-linear evolution of such instabilities in two dimensions, and their coupling
with the thermal cooling instability are presented. The astrophysical implications are briefly outlined, in particular the
formation of knots and filaments.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Daniel Pfenniger 《Celestial Mechanics and Dynamical Astronomy》1998,72(1-2):37-67
An elementary review about stellar and galactic dynamics is presented. Despite involving extremely classical Newtonian physics,
stellar dynamics presents some fundamental difficulties rarely discussed in the literature, such as why the phase space distribution
is assumed to be a smooth function of coordinates. Many systems are found to be unstable over intermediate time-scales, as
more instabilities have been discovered over the years, so the old aim of describing equilibrium stable systems shifts presently
toward understanding evolutive systems. From the linearized variational Boltzmann equation a distinction can be made between
instabilities triggered by the chaotic part of phase space, and instabilities caused by steep gradients in the velocity part
of the distribution function. The new challenges to include evolutive systems can presently only be studied efficiently with
computer techniques. Future studies are likely to involve orders of magnitude more advanced computers in which parallelism
will play a major role.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
《Icarus》1987,69(3):387-422
The theoretical framework for modeling the primordial solar nebula is presented in which convection is assumed to be the sole source of turbulence that causes the nebula to evolve. We use a new model of convective turbulence that takes into account the important effects of radiative dissipation, rotation, and anisotropy of convective motions. This model is based on a closure for the nonlinear interactions that employs the growth rates of hydrodynamic instabilities, a procedure that allows one to compute turbulence coefficients for instabilities other than convection. The vertical structure equations in the thin-disk approximation are developed for this new model, and a detailed comparison and critique of previous convective models of the solar nebula are presented. Numerical results are presented in a subsequent paper. 相似文献
6.
It is shown that dust-neutral and ion-neutral drags can produce instabilities of electrostatic waves in multicomponent self-gravitating dusty plasmas. Explicit expressions for the growth rates of the instabilities are presented. The relevance of our investigation to fragmentation of molecular dust clouds is discussed. 相似文献
7.
Steven A. Balbus 《Monthly notices of the Royal Astronomical Society》2009,395(4):2056-2064
The solar convective zone, or SCZ, is nearly adiabatic and marginally convectively unstable. But, the SCZ is also in a state of differential rotation, and its dynamical stability properties are those of a weakly magnetized gas. This renders it far more prone to rapidly growing rotational baroclinic instabilities than a hydrodynamical system would be. These instabilities should be treated on the same footing as convective instabilities. If isentropic and isorotational surfaces coincide in the SCZ, the gas is marginally (un)stable to both convective and rotational disturbances. This is a plausible resolution for the instabilities associated with these more general rotating convective systems. This motivates an analysis of the thermal wind equation in which isentropes and isorotational surfaces are identical. The characteristics of this partial differential equation correspond to isorotation contours, and their form may be deduced even without precise knowledge of how the entropy and rotation are functionally related. Although the exact solution of the global SCZ problem in principle requires this knowledge, even the simplest models produce striking results in broad agreement with helioseismology data. This includes horizontal (i.e. quasi-spherical) isorotational contours at the poles, axial contours at the equator and approximately radial contours at mid-latitudes. The theory does not apply directly to the tachocline, where a simple thermal wind balance is not expected to be valid. The work presented here is subject to tests of self-consistency, among them the prediction that there should be a good agreement between isentropes and isorotational contours in sufficiently well-resolved large-scale numerical magnetohydrodynamics simulations. 相似文献
8.
A pair of coupled equations governing the nonlinear interaction between Langmuir and acoustic waves in partially-ionized plasmas are presented. Three-wave decay interactions, modulatinal instabilities, as well as the nucleation of coupled Langmuir and acoustic waves can then be studied. The results are of relevance to the lower part of the ionosphere, radio-frequency driven gas discharges, as well as the interstellar medium. 相似文献
9.
P.S. Cally 《Solar physics》2001,199(2):231-249
A spectral method is used to explore the nonlinear evolution of known linear instabilities in a 2D differentially rotating magneto-hydrodynamic shell, representing the solar tachocline. Several simulations are presented, with a range of outcomes for the magnetic field configuration. Most spectacularly, the `clam instability', which occurs for solar differential rotation and a strong broad toroidal magnetic field structure, results in the field tipping over by 90° and reconnecting. A common characteristic of all the simulations though is that the nonlinear instabilities produce a strong angular momentum mixing effect which pushes the rotation towards a solid body form. It is argued that this may be the mechanism required by the model of Spiegel and Zahn to limit the tachocline's thickness. 相似文献
10.
We consider the stability of a compressible shear flow separating two streams of different speeds and temperatures. The velocity and temperature profiles in this mixing layer are hyperbolic tangents. The normal mode analysis of the flow stability reduces to an eigenvalue problem for the pressure perturbation. We briefly describe the numerical method that we used to solve this problem. Then, we introduce the notions of the absolute and convective instabilities and examine the effects of Mach number, and the velocity and temperature ratios of each stream on the transition between convective and absolute instabilities. Finally, we discuss the implication of the results presented in this paper for the heliopause stability. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
11.
We have addressed the problem of combined magnetic and velocity shear driven instabilities in the context of the heliospheric plasma. New high-order numerical methods have been used to analyze the instability dynamics of the heliospheric current-sheet interacting with the structure determined by the slow component of the solar wind on the solar equatorial plane above the helmet streamers. Preliminary results are presented. 相似文献
12.
G. I. Ogilvie 《Monthly notices of the Royal Astronomical Society》2006,365(3):977-990
The non-linear dynamics of a warped accretion disc is investigated in the important case of a thin Keplerian disc with negligible viscosity and self-gravity. A one-dimensional evolutionary equation is formally derived that describes the primary non-linear and dispersive effects on propagating bending waves other than parametric instabilities. It has the form of a derivative non-linear Schrödinger (DNLS) equation with coefficients that are obtained explicitly for a particular model of a disc. The properties of this equation are analysed in some detail and illustrative numerical solutions are presented. The non-linear and dispersive effects both depend on the compressibility of the gas through its adiabatic index Γ. In the physically realistic case Γ < 3, non-linearity does not lead to the steepening of bending waves but instead enhances their linear dispersion. In the opposite case Γ > 3, non-linearity leads to wave steepening and solitary waves are supported. The effects of a small effective viscosity, which may suppress parametric instabilities, are also considered. This analysis may provide a useful point of comparison between theory and numerical simulations of warped accretion discs. 相似文献
13.
Sébastien Lépine 《Astrophysics and Space Science》1998,260(1-2):101-112
I review various observations which suggest that the winds of hot stars are inhomogeneous because of instabilities in the
wind flow. On large scales, local wind overdensities are indirectly detected in the form of excess in the infra-red (IR) and
radio free-free continuum. The X-ray detection of a hot (T ∼ 106) wind component suggests that the wind is pervaded with strong shocks. The small-scale density structure of the wind can
be studied from observations of Line-Profile Variations (LPVs) in optical and UV spectral lines, which are formed close to
the stellar surface.
LPVs in lines of the P Cygni type consist of blue-edge variations in saturated profiles, and Discrete Absorption Components
(DACs) and Periodic Absorption Modulations (PAMs) in unsaturated profiles. These LPVs are shown to be recurrent, and thought
to result from instabilities propagating through the wind and generated at the stellar surface. LPVs in recombination lines
appear as stochastic subpeaks, which suggest that wind instabilities have a clump-like, rather than shell-like, structure.
The kinematics of LPVs in both line types is consistent with wind propagating shocks generated from radiative instabilities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
G. Chanmugam 《Astrophysics and Space Science》1976,42(1):83-87
A number of models have been proposed for the observed cosmic gamma-ray bursts. A class of such models involves the use of magnetic energy as the principal source of energy required for the bursts. In this case, arguments are presented to show that degenerate stars are favored. Mechanisms for magnetohydrodynamic instabilities in white dwarfs and neutron stars are discussed. Preliminary work indicates that magnetic white dwarfs can (but neutron stars probably cannot) account for many of the observed features of the bursts.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975. 相似文献
15.
In the equatorial plasmasphere, plasma waves are frequently observed. To improve our understanding of the mechanism generating plasma waves from instabilities, a comparison of observations, linear growth-rate calculations, and simulation results is presented. To start the numerical experiments from realistic initial plasma conditions, we use the initial parameters inferred from observational data obtained around the plasma-wave generation region by the Akebono satellite. The linear growth rates of waves of different modes are calculated under resonance conditions, and compared with simulation results and observations. By employing numerical experiments by a particle code, we first show that upper hybrid-, Z-, and whistler-mode waves are excited through instabilities driven by a ring-type velocity distribution. The simulation results suggest a possibility that energetic electrons with energies of some tens of keV confined around the geomagnetic equator are responsible for the observed enhancements of Z- and whistler-mode waves. While the comparison between linear growth-rate calculations and observations shows the different tendency of wave amplitude of Z-mode and whistler-mode waves, the wave amplitude of these wave modes in the simulation results is consistent with the observation. 相似文献
16.
17.
The MHD instabilities of a temperature-anisotropic coronal plasma are considered. We show that aperiodic mirror instabilities
of slow MHD waves can develop under solar coronal conditions for weak magnetic fields (B < 1 G) and periodic ion-acoustic instabilities can develop for strong magnetic fields (B > 10 G). We have found the instability growth rates and estimated the temporal and spatial scales of development and decay
of the periodic instability. We show that the instabilities under consideration can play a prominent role in the energy balance
of the corona and may be considered as a large-scale energy source of the wave coronal heating mechanism. 相似文献
18.
The radiative thermal instability is investigated taking into account finite-, or electromagnetic, effects. The two-fluid model for magnetized plasmas together with the Maxwell equations are used to derive a general dispersion relation valid for compressional perturbations with frequency below the electron-cyclotron frequency. The growth rates of the radiative thermal instabilities involving fast magnetosonic flute-like and low-frequency hydromagnetic perturbations are presented. 相似文献
19.
In this paper we review the possibilities for
magnetohydrodynamic processes to handle the angular momentum transport
in accretion disks. Traditionally the angular momentum transport has
been considered to be the result of turbulent viscosity in the disk,
although the Keplerian flow in accretion disks is linearly stable towards
hydrodynamic perturbations. It is on the other hand linearly unstable
to some magnetohydrodynamic (MHD) instabilities.
The most important instabilities are the Parker and Balbus-Hawley
instabilities that are related to the magnetic buoyancy and the shear
flow, respectively. We discuss these instabilities not only in the
traditional MHD framework, but also in the context of slender flux
tubes, that reduce the complexity of the problem while keeping most of
the stability properties of the complete problem. In the non-linear
regime the instabilities produce turbulence. Recent numerical
simulations describe the generation of magnetic fields by a dynamo in
the resulting turbulent flow. Eventually such a dynamo may generate a
global magnetic field in the disk. The relation of the MHD-turbulence
to observations of accretion disks is still obscure. It is commonly
believed that magnetic fields can be highly efficient in transporting
the angular momentum, but emission lines, short-time scale variability
and non-thermal radiation, which a stellar astronomer would take as
signs of magnetic variability, are more commonly observed during periods
of low accretion rates.
Received October 12, 1995 / Accepted November 16, 1995 相似文献
20.
The sequence of evolution of the protoplanetary gas-and-dust disk around the parent star includes, according to modern concepts, its compression in the central plane and decay into separate dust condensations (clusters) due to the occurrence of various types of instabilities. The interaction of dust clusters of a fractal structure during their collisions is considered as a key mechanism for the formation and growth of primary solids, which serve as the basis for the subsequent formation of planetesimals and embryos of planets. Among the mechanisms contributing to the formation of planetesimals, an important place belongs, along with gravitational instability, hydrodynamic instabilities, in particular, the socalled streaming instability of the two-phase gas-dust layer due to its ability to concentrate dispersed particles in dense clots. In contrast to a number of existing models of streaming instability, in which dust particles are considered structurally compact and monodisperse, this paper proposes a more realistic model of polydisperse particles of fractal nature, forming dust clusters as a result of coagulation. The instability of the dust layer in the central plane of the protoplanetary disk under linear axisymmetric perturbations of its parameters is considered. A preliminary conclusion can be drawn that the proposed model of dust fractal aggregates of different scales increases the efficiency of linear growth of hydrodynamic instabilities, including the streaming instabilities associated with the difference between the velocities of the dust and gas phases.
相似文献