共查询到20条相似文献，搜索用时 15 毫秒
1.
R. M. Hueckstaedt S. H. Batha M. M. Balkey N. D. Delamater J. R. Fincke R. L. Holmes N. E. Lanier G. R. Magelssen J. M. Scott J. M. Taccetti C. J. Horsfield K. W. Parker S. D. Rothman 《Astrophysics and Space Science》2005,298(1-2):255-259
Observations of the interstellar medium reveal a dynamic realm permeated by shocks. These shocks are generated on a large
range of scales by galactic rotation, supernovae, stellar winds, and other processes. Whenever a shock encounters a density
interface, Richtmyer-Meshkov instabilities may develop. Perturbations along the interface grow, leading to structure formation
and material mixing. An understanding of the evolution of Richtmyer-Meshkov instabilities is essential for understanding galactic
structure, molecular cloud morphology, and the early stages of star formation. An ongoing experimental campaign studies Richtmyer-Meshkov
mixing in a convergent, compressible, miscible plasma at the Omega laser facility. Cylindrical targets, consisting of a low
density foam core and an aluminum shell covered by an epoxy ablator, are directly driven by fifty laser beams. The aluminum
shell is machined to produce different perturbation spectra. Surface types include unperturbed (smooth), single-mode sinusoids,
multi-mode (rough), and multi-mode with particular modes accentuated (specified-rough). Experimental results are compared
to theory and numerical simulations. 相似文献
2.
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3.
This paper provides an analysis of magneto-sonic eigenwaves travelling in magnetic plasma structures based on the Chew-Goldberger-Low approximation, for which the plasma kinetic pressure is different along and across the magnetic field. The anisotropy does not lead to the emergence of new modes. The dependence of phase velocities of the waves, trapped by a single magnetic surface, on the pressure anisotropy is investigated. For a magnetic slab with field-free surroundings, the dispersion relations for the eigenwaves are obtained. The pressure anisotropy may change dispersion relations of such modes significantly. In particular, backward waves are possible in the case of strong anisotropy. The dependences of the thresholds for the mirror and hose instabilities on the system parameters are obtained. In particular, hose and mirror instabilities of such waves are absent for some wave number regions. The results are used to obtain the eigenwave characteristics in coronal loops and chromospheric flux tubes. 相似文献
4.
S. Mineshige 《Astrophysics and Space Science》1993,210(1-2):83-103
Basic properties of accretion disk instabilities are summarized. We first explain the standard disk model by Shakura and Sunyaev. In this model, the dominant sources of viscosity are assumed to be chaotic magnetic fields and turbulence in gas flow, and the magnitude of viscosity is prescribed by so-called model. It is then possible to build a particular disk model. In the framework of the standard model, accretion disks are stationary, but when some of the basic assumptions are relaxed, various kinds of instabilities appear. In particular, we focus on the thermal limit-cycle instability caused by partial ionization of hydrogen (and helium). We demonstrate that the disk instability model well accounts for the basic observed features of outbursts of dwarf novae and X-ray nova. We then introduce other kinds of instabilities based on the viscosity model. They are suspected to produce time variabilities observed on a wide range of timescales in close binaries and active galactic nuclei. 相似文献
5.
Steven J. DESCH Melissa A. MORRIS Harold C. CONNOLLY
Jr. Alan P. BOSS 《Meteoritics & planetary science》2012,47(7):1139-1156
Abstract— We review a number of constraints that have been placed on the formation of chondrules and show how these can be used to test chondrule formation models. Four models in particular are examined: the “X‐wind” model (sudden exposure to sunlight <0.1 AU from the proto‐Sun, with subsequent launching in a magnetocentrifugal outflow); solar nebula lightning; nebular shocks driven by eccentric planetesimals; and nebular shocks driven by diskwide gravitational instabilities. We show that constraints on the thermal histories of chondrules during their melting and crystallization are the most powerful constraints and provide the least ambiguous tests of the chondrule formation models. Such constraints strongly favor melting of chondrules in nebular shocks. Shocks driven by gravitational instabilities are somewhat favored over planetesimal bow shocks. 相似文献
6.
Rolf Walder 《Astrophysics and Space Science》1998,260(1-2):243-252
The properties of the interaction region of colliding winds in hot star binaries and evolved low mass binaries are discussed
in the frame of 2D and 3D hydrodynamical numerical simulations. The emission of such systems and their nebulae, in particular
in X-rays, is reviewed and new results regarding orbital line profile variations are presented. The connection between colliding
winds, thin shell instabilities and structure formation is outlined.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
Vadim Urpin 《Journal of Astrophysics and Astronomy》2017,38(3):41
The magnetospheres around neutron stars should be very particular because of their strong magnetic field and rapid rotation. A study of the pulsar magnetospheres is of crucial importance since it is the key issue to understand how energy outflow to the exterior is produced. In this paper, we discuss magnetohydrodynamic processes in the pulsar magnetosphere. We consider in detail the properties of magnetohydrodynamic waves that can exist in the magnetosphere and their instabilities. These instabilities lead to formation of magnetic structures and can be responsible for short-term variability of the pulsar emission. 相似文献
8.
Bruce G. Elmegreen 《Astrophysics and Space Science》2003,284(2):819-832
Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source
of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities,
suggesting that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this
turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The
final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently
independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form.
If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can
be calculated and the star formation rate determined from first principles. The results suggest that only 10-4 of the ISM mass actively participates in the star formation process and that this fraction does so because its density is
larger than 105 cm-3, at which point several key processes affecting dynamical equilibrium begin to break down.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
9.
本文采用微扰方法导出色散方程，并在四种情况下详细讨论了薄吸积盘的不稳定性。结果表明：在纯粘滞和纯磁场盘中都存在脉动不稳定性。而且在吸积盘内同时考虑粘滞和磁场时，存在两种不稳定性，一种是脉动不稳定性，另一种是单调不稳定性。同时数值计算还表明，脉动不稳定性更可能存在于盘的内区，而单调不稳定性则只在盘的外区，对短波扰动才有意义。这些结果为解释ＢＬＬａｃ天体、Ｓｅｙｆｅｒｔ星系及类星体等活动星系核的光变现象进一步提供了理论依据。 相似文献
10.
Several authors have claimed for correlations between surges (dark features) and various kinds of solar emissions (radio, microwave, X-ray). In this paper we propose a model to explain such correlations, in particular presenting the properties of the instabilities resulting from the coupling between material flow, connected to the appearance of a surge, and magnetic field topology. As a consequence of such instability a turbulent energy cascade to small characteristic lengths grows up. Depending of the relevant parameters of the surge (dark feature), different regimes can be found, producing different levels of electrons acceleration and mass motion deceleration. We try then to correlate the different developments of the instability with the behavior observed in type I and type III radio bursts related to surges.Proceedings of the Second CERSA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986. 相似文献
11.
D.F. Vogl D. Langmayr N.V. ErkaevH.K. Biernat C.J. FarrugiaS. Mühlbachler 《Planetary and Space Science》2003,51(12):715-722
In this paper, we concentrate on the analysis of the anisotropic Rankine-Hugoniot equations for perpendicular and oblique fast shocks. In particular, as additional information to the anisotropic set of equations, the threshold conditions of the fire-hose and mirror instability are used to bound the range of the pressure anisotropy downstream of the discontinuity. These anisotropic threshold conditions of the plasma instabilities are obtained via a kinetic approach using a generalized Lorentzian distribution function, the so-called kappa distribution function. Depending on up-stream conditions, these instabilities further define stable and unstable regions with regard to the pressure anisotropy downstream of the shock. The calculations are done for different upstream Alfvén Mach numbers. We found that low values of the parameter kappa reduce the pressure anisotropy downstream of the shock. 相似文献
12.
Adi Nusser 《Monthly notices of the Royal Astronomical Society》2005,361(3):977-982
We discuss the stability of galactic discs in which the energy of interstellar clouds is gained in encounters with expanding supernova (SN) remnants and lost in inelastic collisions. Energy gain and loss processes introduce a phase difference between the pressure and density perturbations, making discs unstable on small scales for several recipes of star formation. This is in contrast to the standard stability analysis in which small-scale perturbations are stabilized by pressure. In the limit of small scales, the dispersion relation for the growth rate reduces to that of thermal instabilities in a fluid without gravity. If instabilities lead to star formation, then our results imply a secondary mode of star formation that operates on small scales and feeds on the existence of a primary mode on intermediate scales. This may be interpreted as positive feedback. Further, the standard stability criterion on intermediate scales is significantly modified. 相似文献
13.
R. M. Hueckstaedt J. H. Hunter Jr R. V. E. Lovelace 《Monthly notices of the Royal Astronomical Society》2006,369(3):1143-1151
In order to understand star formation it is important to understand the dynamics of atomic and molecular clouds in the interstellar medium (ISM). Non-linear hydrodynamic flows are a key component to the ISM. One route by which non-linear flows arise is the onset and evolution of interfacial instabilities. Interfacial instabilities act to modify the interface between gas components at different densities and temperatures. Such an interface may be subject to a host of instabilities, including the Rayleigh–Taylor, Kelvin–Helmholtz, and Richtmyer–Meshkov instabilities. Recently, a new density interface instability was identified. This self-gravity interfacial instability (SGI) causes any displacement of the interface to grow on roughly a free-fall time-scale, even when the perturbation wavelength is much less than the Jeans length. In previous work, we used numerical simulations to confirm the expectations of linear theory and examine the non-linear evolution of the SGI. We now continue our study by generalizing our initial conditions to allow the acceleration due to self-gravity to be non-zero across the interface. We also consider the behaviour of the SGI for perturbation wavelengths near the Jeans wavelength. We conclude that the action of self-gravity across a density interface may play a significant role in the ISM either by fuelling the growth of new instabilities or modifying the evolution of existing instabilities. 相似文献
14.
An ongoing debate is how magnetic energy is released in solar flares, which type of magnetic instabilities are responsible for triggering the energy release, and which magnetic topologies are most likely to host the instabilities. In this connection magnetic reconnection has been a general ingredient, with most of the previous work focussing on 2D reconnection. A natural extension to this is to investigate reconnection in 3D topologies, in particular the behaviour of magnetic nulls and the magnetic topology associated with them. This paper investigates the difference in dynamical behaviour of a numerical domain that either contains a double null-point pair connected by a separator or only a fraction of the separator defined by the null-points. The experiments show that nulls can either accumulate current individually, or act together to produce a singular current collapse along the separator. The implication of these results for the interpretation of coronal data is discussed. 相似文献
15.
Alexander Lazarian 《Astrophysics and Space Science》1993,206(1):37-50
Different mechanisms for filament formation are studied with a view to account for the results of Verschuur (1991a, b). It is shown that pinch phenomena can produce a filamentary structure of H I without any pressure contribution from the warm component of the interstellar medium (ISM). The mechanisms of filament formation involving the Parker instability and magnetoconvection are also discussed. It is shown that instabilities corresponding to all these mechanisms may result in a picture similar to that observed by Verschuur. 相似文献
16.
Conventional planet formation models via coagulation of planetesimals require timescales in the range of several 10 or even 100 Myr in the outer regions of a protoplanetary disk. But according to observational data, the lifetime of a protoplanetary disk is limited to about 6 Myr. Therefore the existence of Uranus and Neptune poses a problem. Planet formation via gravitational instability may be a solution for this discrepancy. We present a parameter study of the possibility of gravitationally triggered disk instability. Using a restricted N‐body model which allows for a survey of an extended parameter space, we show that a passing dwarf star with a mass between 0.1 and 1 M⊙ can probably induce gravitational instabilities in the pre‐planetary solar disk for prograde passages with minimum separations below 80‐170 AU. Inclined and retrograde encounters lead to similar results but require slightly closer passages. Such encounter distances are quite likely in young moderately massive star clusters. The induced gravitational instabilities may lead to enhanced planetesimal formation in the outer regions of the protoplanetary disk, and could therefore be relevant for the formation of Uranus and Neptune. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
Electrostatic double layers have been proposed as an acceleration mechanism in solar flares and other astrophysical objects. They have been extensively studied in the laboratory and by means of computer simulations. The theory of steady-state double layers implies several existence criteria, in particular the Bohm criteria, restricting the conditions under which double layers may form. In the present paper several already published theoretical models of different types of double layers are discussed. It is shown that the existence conditions often imply current-driven instabilities in the ambient plasma, at least for strong double layers, and it is argued that such conditions must be used with care when applied to real plasmas. Laboratory double layers, and by implication those arising in astrophysical plasmas often produce instabilities in the surrounding plasma and are generally time-dependent structures. Naturally occuring double layers should, therefore, be far more common than the restrictions deduced from idealised time-independent models would imply. In particular it is necessary to understand more fully the time-dependent behaviour of double layers. In the present paper the dynamics of weak double layers is discussed. Also a model for a moving strong double layer, where an associated potential minimum plays a significant role, is presented.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
18.
The MHD wave instabilities due to non-uniform magnetic field and non-homogeneity of density have been studied. The reference
(coordinate) system considered here is cylindrical type. The General Dispersion Relation (GDR) for the wave propagation in
a gravitating but non-relativistic region has been derived. Similar to common knowledge, the said non-uniformities have been
found to be responsible for the instability of the system. But interestingly many instability factors are produced due to
presence of two types of non-uniformities simultaneously. This theory may add more clues for the event of instabilities, formation
of hot plasma-bed in Galactic Central Region, and mass out-flow from there. Many conditions for instabilities could be obtained
from GDR deduced here. However, a few conditions for critical wavelength of the MHD wave have been obtained in terms of system
parameters (like gradient of magnetic field and rotation).
This theory, in turn, may be helpful for the better understanding of the Explosion Theory of formation of outer structure
of Galaxies like ours. 相似文献
19.
评述了球状星团系统研究中的比频、金属度和质量谱三大问题，就目前的认识和存在的问题进行了讨论。指出不同星系球状星团比频之间的差别表明了需要有各种不同的球状星团形成模型；球状星团金属度的分布表明球状星团可能有三个形成时期，分别与三类形成模型效应；尽管对数正态初始质量的数值模拟初步结果与观测结果符合得更好，然而幂函数的初始质量谱在物理上能很好地与球状星团形成联系起来。 相似文献
20.
Direct photographs of Comet Ikeya-Seki obtained on four consecutive days from October 29 to November 1, 1965, are used for an analysis of the multiple helical structures in the ionised tail. The formation of these structures is explained on the basis of plasma instabilities excited in the tail containing twisted magnetic fields. The growth rate of the modes excited at the mode rational surface agrees well with the observed results. This model also accounts for the presence of harmonic structures seen in the tail of the comet. 相似文献