共查询到20条相似文献，搜索用时 31 毫秒
1.
Mohsen Shadmehri 《Monthly notices of the Royal Astronomical Society》2009,397(3):1521-1527
We study the dynamical effects of cosmic rays (CRs) on thermal instability in the linear regime. CRs and the thermal plasma are treated as two different interacting fluids, in which CRs can diffuse along the magnetic field lines. We show that the growth rate of the magnetothermal condensation mode is reduced because of the existence of CRs, and this stabilizing effect depends on the diffusion coefficient and the ratio of CR pressure to gas pressure. Thus, a slower rate of structure formation via thermal instability is predicted when CRs are considered. 相似文献
2.
We suggest that the fibril structure of prominences may be caused by filamentation during its formation by radiative instability. We also discuss the effects of other types of instability and give a mechanism for the formation of vertical threads. The models indicate that highly inhomogeneous density structures can exist in the presence of smooth profiles for the plasma pressure and magnetic field. In our particular models the plasma pressure of a fibril prominence is higher and the vertical magnetical field is weaker than in a uniform prominence model, while the mass is substantially smaller. 相似文献
3.
The problem of pressure limitations on the rate of flux pile-up magnetic reconnection is studied. We first examine the recent suggestion of Jardine and Allen (1998) for moderating the build-up of magnetic pressure in the current sheet by considering inflows with nonzero vorticity. An analytic argument shows, however, that unbounded magnetic pressures in the limit of small resistivities can be avoided only at the cost of unphysical dynamic pressures in the plasma. Hence, the pressure limitation on the reconnection rate in a low-beta plasma cannot be avoided completely. Nevertheless, we demonstrate that reconnection can be more rapid in a new solution that balances the build-up in dynamic pressure against both the plasma and magnetic pressures. This exact MHD solution has the characteristics of merging driven by the coalescence instability. The maximum energy release rate of the model is capable of explaining a modest solar flare. 相似文献
4.
Kelvin-Helmholtz instability of the interface separating two viscous rotating-conducting fluids has been studied in the presence of finite ion-Larmor radius (FLR) effects. Emloying the normal mode technique, the solutions have been obtained when the fluids are assumed to be permeated by a uniform horizontal magnetic field. For the case of two highly viscous fluids, the dispersion relation has been derived and solved numerically. It is found that the streaming velocity has a stabilizing influence on the potentially unstable arrangement of the fluids. The viscosity and FLR effects are also found to have a stabilizing influence while the Coriolis forces have a destabilizing influence on the system. 相似文献
5.
Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field. 相似文献
6.
The problem of Rayleigh-Taylor instability of superposed viscous magnetized fluids through porous medium is investigated in a partially-ionized medium. The fluid has ionized and neutralized particle components interacting with collisions. The effect of surface tension on R-T instability is also included in the present problem. The magnetohydrodynamic equations are modified for finite-Larmor radius corrections which is in the form of tensor. The equations of problem are linearized and using appropriate boundary condition, general dispersion relation is derived for two superposed fluids separated by horizontal boundary. The first part of the dispersion relation gives stable mode and condition is investigated using Hurwitz conditions. The second part of the dispersion relation shows that the growth rate of unstable system is reduced due to FLR corrections, viscosity, and collisional frequency of the neutrals. The role of surface tension on the system is also discussed. 相似文献
7.
Kelvin-Helmholtz instability of two superimposed fluids has been studied. One of the fluids is non-conducting and the other is conducting with finite resistivity. The fluids are assumed rotating and slipping past each other with a relative velocity. The neutral particles are also incorporated in one of the fluids of the system. A general dispersion relation has been derived and discussed under different conditions. The effect of rotation and the neutral particles, considering an infinitely-conducting system has also been analyzed and it is observed that small rotation and the presence of neutral particles destabilize the system. The effect of neutral particles is to decrease the effect of magnetic field, angular velocity, and the gravitational field. It has been found that an otherwise stable mode becomes overstable and grows exponentially in the presence of finite resistivity. 相似文献
8.
Ajoy K. Dasgupta 《Astrophysics and Space Science》1983,94(1):171-175
Drift instabilities arising when accelerated protons are trapped in the intergalactic medium are examined. If α, the ratio of total (plasma + energetic particles) pressure and magnetic field pressure is larger than some value α?0.1 to 0.3, the magnetic trap is destroyed and protons are released into interstellar medium. If α<α*, the trapped protons exhibit gradient instability due to magnetic drift resonance. This ‘universal’ instability results in rapid development of strong Alfvén wave turbulence with small wavelengths transverse to the magnetic field. Particle diffusion due to the waves has a rather complicated character and appears to be weak as compared to quasilinear diffusion. 相似文献
9.
E. C. Bowers 《Astrophysics and Space Science》1973,24(2):349-369
In an earlier paper, Bowers (1973), ion plasma oscillations were found to be unstable in the steady state developed by Cowley
(1972) for the neutral sheet in the Earth's geomagnetic tail. In this paper a similar stability analysis is carried out but
for a different steady state, suggested by Dungey, with the result that unstable waves with frequencies near the electron
plasma frequency are found. In the Dungey steady state the current necessary for magnetic field reversal is carried by plasma
originating from both the magnetosheath and the lobes of the tail. This modifies the steady state proposed by Alfvén and subsequently
developed by Cowley in which all the current is carried by plasma from the lobes of the tail thereby fixing the cross-tail
potential Φ. With magnetosheath plasma present the value of Φ is no longer fixed solely by parameters in the lobes of the
tail but the cross-tail electric field is still assumed localised in the dusk region of the sheet as in the Cowley model due
to the balance of charge required in the neutral sheet. The value of Φ can be expected to increase as magnetic flux is transported
to the tail which inflates and causes flux annihilation because the magneto-sheath plasma in the neutral sheet has insufficient
pressure to keep the two lobes of the tail apart. The Vlasov-Maxwell set of equations is perturbed and linearised enabling
a critical condition for instability to be found for modes propagating across the tail. Typically, this condition requireseΦ≳KT
m
whereT
m
is the temperature of magnetosheath electrons. The instability occurs in the presence of cold plasma which hasE×B drifted into the neutral sheet from the lobes of the tail. This contrasts with the usual two stream instability which is
stabilised by the cold plasma. Once precipitated the instability may be explosive provided current disruption occurs, for
then a further increase in Φ will result which drives a greater range of wave numbers unstable thereby causing even more turbulence
and an even larger cross-tail electric field. Because of this behaviour the instability may be a trigger for a substorm. 相似文献
10.
Prominences and filaments are thought to arise as a consequence of a magnetized plasma undergoing thermal instability. Therefore, the thermal stability of a magnetized plasma is investigated under coronal conditions. The equilibrium structure of the plasma is approximated by a 1-D slab configuration. This is investigated in thermal instability taking into account optically thin plasma radiation and anisotropic thermal conduction. The thermal conduction perpendicular to the magnetic field is taken to be small but non-zero.The classic rigid wall boundary conditions which are often applied in the literature, either directly on the plasma or indirectly through some other medium, are replaced by a more physical situation in which the plasma column is placed in a low-density background stretching towards infinity. Results for a uniform equilibrium structure indicate the major effect of this change is on the eigenfunctions rather than on the growth rate. Essentially, perpendicular thermal conduction introduces field-aligned fine structure. It is also shown that in the presence of perpendicular thermal conduction, thermal instability in a slab model is only possible if the inner plasma has the shortest thermal instability time scale.Research Assistant of the National Fund for Scientific Research (Belgium). 相似文献
11.
An instability associated with the magnetosonic wave driven unstable due to coupling with electron and ion drift modes has been considered as a potential source for driving the hydromagnetic turbulence observed in Giacobini-Zinner (G-Z) Cometary plasma. The instability has good growth rate for propagation perpendicular to plasma inhomogeneities and exists for all wave numbers. The wave period for waves propagating perpendicular to the gradients is about a few times ion-gyroperiod and higher values of plasma beta (
e
lead to stronger instability. 相似文献
12.
We introduce between the magnetosphere of a neutron star and its accretion disk a sheared layer of finite thickness in which the velocity, density, pressure and magnetic field vary continuously and we discuss the Kelvin-Helmholtz instability of plane wave purturbations for the case of a compressible plasma. The results show that the K-H instability is still present and radial wave vector perturbation is the main mode of instability. We particularly considered the effect of the thickness of the sheared layer on the rotation of the neutron star, showing that by suitably adjusting the thickness we can explain the period changes in the X-ray pulsars. Application of this model to Her X-1 gave a good result. 相似文献
13.
The evolutionary excitation dynamics of the gravitational instability in a self-gravitating viscoelastic non-thermal polytropic complex fluid is semi-analytically explored on the astro-scales of space and time. The polytropic equation of state is well validated for the hydrostatic equilibrium established by a perfect heating-cooling balancing in the uni-component complex fluid. We apply a generalized gravitating hydrodynamic model in the concurrent presence of buoyancy, thermal fluctuations, volumetric expansion, and so forth. A normal mode (local) analysis yields a quadratic linear dispersion relation with a unique set of multi-parametric coefficients. The analytical reliability is checked by comparing with the existing reports on purely ideal inviscid nebular fluids and non-ideal viscoelastic fluids in isolation. It is seen that, unlike the normal instability mechanisms, the instability here remains unaffected due to the thermo-mechanical diffusion processes. The stabilizing (destabilizing) and accelerating (decelerating) factors of the instability are illustratively explored. The instability features are judged in the light of both impure non-ideal viscoelastic fluid and pure ideal inviscid nebular fluid scenarios. The relevancy of our exploration in superdense compact viscoelastic astro-objects and their surrounding atmospheres is summarily outlined. 相似文献
14.
We have shown that a current-carrying plasma loop can be heated by magnetic pinch driven by the pressure imbalance between inside and outside the loop, using a 3-dimensional electromagnetic (EM) particle code. Both electrons and ions in the loop can be heated in the direction perpendicular to the ambient magnetic field, therefore the perpendicular temperature can be increased about 10 times compared with the parallel temperature. This temperature anisotropy produced by the magnetic pinch heating can induce a plasma instability, by which high-frequency electromagnetic waves can be excited. The plasma current which is enhanced by the magnetic pinch can also excite a kinetic kink instability, which can heat ions perpendicular to the magnetic field. The heating mechanism of ions as well as the electromagnetic emission could be important for an understanding of the coronal loop heating and the electromagnetic wave emissions from active coronal regions. 相似文献
15.
Solar mass ejections seem, on the basis of many observations, to be divided into two categories: stable and unstable. We use linear magnetohydrodynamic (MHD) instability of a cylindrical plasma in an attempt to search for a theoretical explanation for this phenomenon. The dispersion relation is obtained and solved numerically. It is found that the initial plasma-flow velocity has a significant effect on the instability criteria and growth rate. Also found is that the instability growth-rate is much larger in those cases where plasma flow exists in comparison with the static case. The wave number range where the instability may occur also becomes wider with plasma flow. Further, it is shown that the region of the instability shifts to the short wavelength region with increasing plasma-flow velocity. Therefore, the plasma column may break into small pieces, resembling the melon seed phenomenon that has been suggested as a mechanism for mass ejection in the solar atmosphere. Under the assumption of a thin-tube approximation we show that gravity has little effect on the instability of quasi-horizontal ejection, but it has considerable effect for the vertical ejection. In order to deal with the gravitational force it is convenient to divide the problem into three cases: horizontal, vertical, and oblique. The exact analytical solution exists only in the vertical case. Asymptotic solutions are given in the horizontal and oblique cases. 相似文献
16.
The effect of compressive viscosity, thermal conductivity and radiative heat-loss functions on the gravitational instability of infinitely extended homogeneous MHD plasma has been investigated. By taking in account these parameters we developed the six-order dispersion relation for magnetohydrodynamic (MHD) waves propagating in a homogeneous and isotropic plasma. The general dispersion relation has been developed from set of linearized basic equations and solved analytically to analyse the conditions of instability and instability of self-gravitating plasma embedded in a constant magnetic field. Our result shows that the presence of viscosity and thermal conductivity in a strong magnetic field substantially modifies the fundamental Jeans criterion of gravitational instability. 相似文献
17.
The Rayleigh-Taylor instability of the plane interface separating the two partially-ionized superposed fluids through porous medium is analysed. The effect of variable horizontal magnetic field, surface tension and rotation along the vertical axis are also incorporated. The relevant linearized perturbation equations are taken and using normal mode analysis the general relation is obtained from which the dispersion relation for two superposed fluids of different densities is derived. It is found that the surface tension and horizontal magnetic field have the stabilizing effect on the R-T-instability. The condition of instability remains unaffected by the permeability of porous medium, presence of neutral particles in the fluids and rotation.It is concluded that the system is unstable only for those positive wave numbers which are less than certain critical value in case of an adverse density gradient. 相似文献
18.
The electrostatic ion-cyclotron instability (EICI) in low β (ratio of plasma to magnetic pressure), anisotropic, inhomogeneous
plasma is studied by investigating the trajectories of the particles using the general loss-cone distribution function (Dory-Guest-Harris
type) for the plasma ions. In particular, the role of the loss-cone feature as determined by the loss-cone indices, in driving
the drift-cyclotron loss-cone (DCLC) instability is analysed. It is found that for both long and short wavelength DCLC mode
the loss-cone indices and the perpendicular thermal velocity affect the dispersion equation and the growth rate of the wave
by virtue of their occurrence in the temperature anisotropy. The dispersion relation for the DCLC mode derived here using
the particle aspect analysis approach and the general loss-cone distribution function considers the ion diamagnetic drift
and also includes the effects of the parallel propagation and the ion temperature anisotropy. It is also found that the diamagnetic
drift velocity due to the density gradient of the plasma ions in the presence of the general loss-cone distribution acts as
a source of free energy for the wave and leads to the generation of the DCLC instability with enhanced growth rate. The particle
aspect analysis approach used to study the EICI in inhomogeneous plasma gives a fairly good explanation for the particle energisation,
wave emission by the wave–particle interaction and the results obtained using this particle aspect analysis approach are in
agreement with the previous theoretical findings using the kinetic approach. 相似文献
19.
The occurrence of oscillatory mode for magnetized fluid in the presence of vertical and horizontal temperature gradients has been studied. In both the cases (highly viscous and low-viscosity fluids) the occurrence of oscillatory solutions to the Bénard problem has been investigated as a function of Prandtl number, magnetic number, resistivity number and the ratio of applied temperature gradients. It is clear that the high Prandtl numbers completely change the character of instability and of the solutions as compared to those appropriate for low Prandtl-number fluids. It has also been shown that small horizontal temperature gradient readily generate oscillatory modes only for real frequencies. The critical Rayleigh number is higher than the critical Rayleigh number of non-magnetized fluid in both the cases-i.e., highly viscous fluid and one of moderate viscosity. 相似文献
20.
The properties of heavy-ion-acoustic (HIA) solitary structures associated with the nonlinear propagation of cylindrical and spherical electrostatic perturbations in an unmagnetized, collisionless dense plasma system has been investigated theoretically. Our considered model contains degenerate electron and inertial light ion fluids, and positively charged static heavy ions, which is valid for both of the non-relativistic and ultra-relativistic limits. The Korteweg-de Vries (K-dV) and modified K-dV (mK-dV) equations have been derived by employing the reductive perturbation method, and numerically examined in order. It has been found that the effect of degenerate pressure and number density of electron and inertial light ion fluids, and positively charged static heavy ions significantly modify the basic features of HIA solitary waves. It is also noted that the inertial light ion fluid is the source of dispersion for HIA waves and is responsible for the formation of solitary waves. The basic features and the underlying physics of HIA solitary waves, which are relevant to some astrophysical compact objects, are briefly discussed. 相似文献