Anatoly K. Nekrasov

COMPRESSIBLE STREAMING INSTABILITIES IN ROTATING THERMAL VISCOUS OBJECTS

We study electromagnetic streaming instabilities in thermal viscous regions of rotating astrophysical objects, such as protostellar and protoplanetary magnetized accretion disks, molecular clouds, their cores, and elephant trunks. The obtained results can also be applied to any regions of interstellar medium, where different equilibrium velocities between charged species can arise. We consider a weakly and highly ionized three-component plasma consisting of neutrals and magnetized electrons and ions. The vertical perturbations along the background magnetic field are investigated. The effect of perturbation of collisional frequencies due to density perturbations of species is taken into account. The growth rates of perturbations are found in a wide region of wave number spectrum for media, where the thermal pressure is larger than the magnetic pressure. It is shown that in cases of strong collisional coupling of neutrals with ions the contribution of the viscosity is negligible.

ELECTROMAGNETIC STREAMING INSTABILITIES OF MAGNETIZED ACCRETION DISKS WITH STRONG COLLISIONAL COUPLING OF SPECIES

Electromagnetic streaming instabilities of multicomponent collisional magnetized accretion disks are studied. Sufficiently ionized regions of the disk are explored where there is strong collisional coupling of neutral atoms with both ions and dust grains simultaneously. The steady state is investigated in detail and the azimuthal and radial background velocities of species are calculated. The azimuthal velocity of ions, dust grains, and neutrals is found to be less than the Keplerian velocity. The radial velocity of neutrals and dust grains is shown to be directed inward of the disk. The general solution for the perturbed velocities of species taking into account collisions and thermal pressure is obtained. The effect on the collisional frequencies, due to density perturbations of charged species and neutrals, is included. It is shown that dust grains can be involved in the fast electromagnetic perturbations induced by the ions and electrons through the strong collisions of these grains with neutrals that in turn have a strong collisional coupling with the ions. The dispersion relation for the vertical perturbations is derived and its unstable solutions due to different background velocities of ions and electrons are found. The growth rates of the streaming instabilities considered can be much larger than the Keplerian frequency.

Nonlinear plasma density modification by the ponderomotive force of ULF pulsations at the dayside magnetosphere

We investigate analytically and numerically a nonlinear modification of the magnetospheric plasma density under the action of the ponderomotive force induced by ULF traveling waves, using the nonlinear stationary force balance equation. This equation is applied to both the dipole and dayside magnetosphere having one and two minima of the geomagnetic field near the magnetospheric boundary. The separate and joint actions of the ponderomotive, centrifugal, and gravitational forces on the density distribution are shown.

Multicomponent theory of buoyancy instabilities in magnetized plasmas: the case of magnetic field parallel to gravity

We investigate electromagnetic buoyancy instabilities of the electron-ion plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations, but using the multicomponent plasma approach when the momentum equations are solved for each species. We consider a geometry in which the background magnetic field, gravity, and stratification are directed along one axis. The nonzero background electron thermal flux is taken into account. Collisions between electrons and ions are included in the momentum equations. No simplifications usual for the one-fluid MHD-approach in studying these instabilities are used. We derive a simple dispersion relation, which shows that the thermal flux perturbation generally stabilizes an instability for the geometry under consideration. This result contradicts to conclusion obtained in the MHD-approach. We show that the reason of this contradiction is the simplified assumptions used in the MHD analysis of buoyancy instabilities and the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. Our dispersion relation also shows that the medium with the electron thermal flux can be unstable, if the temperature gradients of ions and electrons have the opposite signs. The results obtained can be applied to the weakly collisional magnetized plasma objects in laboratory and astrophysics.

Electromagnetic instabilities in rotating magnetized viscous objects

In this paper, we study electromagnetic streaming instabilities in the thermal viscous regions of rotating astrophysical objects, such as magnetized accretion discs, molecular clouds, their cores and elephant trunks. The results obtained can also be applied to any regions of interstellar medium, where different equilibrium velocities between charged species can arise. We consider a weakly ionized multicomponent plasma consisting of neutrals and magnetized electrons, ions and dust grains. We take into account the effect of perturbation of collisional frequencies as a result of the density perturbations of species. We obtain general expressions for the perturbed velocities of species involving the thermal pressure and viscosity when perturbations propagate perpendicular to the background magnetic field. The dispersion relation is derived and investigated for axisymmetric perturbations. New compressible instabilities generated as a result of different equilibrium velocities of different charged species are found in the cold and thermal limits either when the viscosity of neutrals can be neglected or when it is important. The viscosity of magnetized charged species is negligible for the perturbations considered that have wavelengths much larger than the Larmor radius of species. At the same time, the neutrals are shown to be immobile in electromagnetic perturbations when their viscosity is sufficiently large.

ELECTROMAGNETIC THERMAL INSTABILITY WITH MOMENTUM AND ENERGY EXCHANGE BETWEEN ELECTRONS AND IONS IN GALAXY CLUSTERS

Thermal instability in an electron-ion magnetized plasma, which is relevant in the intragalactic medium of galaxy clusters, solar corona, and other two-component plasma objects, is investigated. We apply the multicomponent plasma approach where the dynamics of all species are considered separately through electric field perturbations. General expressions for the dynamical variables obtained in this paper can be applied over a wide range of astrophysical and laboratory plasmas also containing neutrals and dust grains. We assume that background temperatures of electrons and ions are different and include the energy exchange in thermal equations for electrons and ions along with the collisional momentum exchange in equations of motion. We take into account the dependence of collision frequency on density and temperature perturbations. The cooling-heating functions are taken for both electrons and ions. A condensation mode of thermal instability has been studied in the fast sound speed limit. We derive a new dispersion relation including different electron and ion cooling-heating functions and other effects mentioned above and find its simple solutions for growth rates in limiting cases. We show that the perturbations have an electromagnetic nature and demonstrate the crucial role of the electric field perturbation along the background magnetic field in the fast sound speed limit. We find that at the conditions under consideration, condensation must occur along the magnetic field while the transverse scale sizes can be both larger and smaller than the longitudinal ones. The results obtained can be useful for interpretating observations of dense cold regions in astrophysical objects.

MULTICOMPONENT THEORY OF BUOYANCY INSTABILITIES IN ASTROPHYSICAL PLASMA OBJECTS: THE CASE OF MAGNETIC FIELD PERPENDICULAR TO GRAVITY

We develop a general theory of buoyancy instabilities in the electron-ion plasma with the electron heat flux based not upon magnetohydrodynamic (MHD) equations, but using a multicomponent plasma approach in which the momentum equation is solved for each species. We investigate the geometry in which the background magnetic field is perpendicular to the gravity and stratification. General expressions for the perturbed velocities are given without any simplifications. Collisions between electrons and ions are taken into account in the momentum equations in a general form, permitting us to consider both weakly and strongly collisional objects. However, the electron heat flux is assumed to be directed along the magnetic field, which implies a weakly collisional case. Using simplifications justified for an investigation of buoyancy instabilities with electron thermal flux, we derive simple dispersion relations for both collisionless and collisional cases for arbitrary directions of the wave vector. Our dispersion relations considerably differ from that obtained in the MHD framework and conditions of instability are similar to Schwarzschilds criterion. This difference is connected with simplified assumptions used in the MHD analysis of buoyancy instabilities and with the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. The results obtained can be applied to clusters of galaxies and other astrophysical objects.

STREAMING COLD COSMIC-RAY BACK-REACTION AND THERMAL INSTABILITIES ALONG THE BACKGROUND MAGNETIC FIELD

We investigate the streaming and thermal instabilities of the electron-ion plasma with homogeneous cold cosmic rays drifting perpendicular to the background magnetic field in the multi-fluid approach. One-dimensional perturbations along the magnetic field are considered. The induced return current of the background plasma and back-reaction of cosmic rays are taken into account. It is shown that the cosmic ray back-reaction results in the streaming instability having considerably larger growth rates than that due to the return current of the background plasma. This increase is by a factor of the square root of the ratio of the background plasma mass density to the cosmic ray mass density. The maximal growth rates and corresponding wave numbers are found. The thermal instability is shown to be not subject to the action of cosmic rays in the model under consideration. The dispersion relation for the thermal instability includes ion inertia. In the limit of fast thermal energy exchange between electrons and ions, the isobaric and isochoric growth rates are derived. The results obtained can be useful for the investigation of the electron-ion astrophysical objects such as galaxy clusters including the dynamics of streaming cosmic rays.

Influence of energy exchange of electrons and ions on the long‐wavelength thermal instability in magnetized astrophysical objects

We investigate the thermal instability in an electron–ion magnetized plasma relevant to galaxy clusters, solar corona and other two-component astrophysical objects. We apply the multicomponent plasma approach when the dynamics of all the species are considered separately through electric field perturbations. General expressions for perturbations obtained in this paper can be applied to a wide range of multicomponent astrophysical and laboratory plasmas also containing the neutrals, dust grains and other species. We assume that background temperatures of electrons and ions are different and include the energy exchange in thermal equations. We take into account the dependence of the collision frequency on density and temperature perturbations. The cooling–heating functions are taken as different ones for electrons and ions. As a specific case, we consider a condensation mode of the thermal instability of long-wavelength perturbations when the dynamical time is smaller than a time during which the particles cover the wavelength along the magnetic field due to the thermal velocity. We derive a general dispersion relation taking into account the effects mentioned above and obtain simple expressions for growth rates in limiting cases. Perturbations are shown to have an electromagnetic nature. We find that at conditions under consideration transverse scale sizes of unstable perturbations can have a wide spectrum relatively to longitudinal scale sizes and, in particular, form very thin filaments. The results obtained can be useful for the interpretation of observations of dense cold regions in astrophysical objects.

INFLUENCE OF THE BACKREACTION OF STREAMING COSMIC RAYS ON MAGNETIC FIELD GENERATION AND THERMAL INSTABILITY

Using a multifluid approach, we investigate streaming and thermal instabilities of the electron-ion plasma with homogeneous cold cosmic rays propagating perpendicular to the background magnetic field. Perturbations are also considered to be across the magnetic field. The backreaction of cosmic rays resulting in strong streaming instabilities is taken into account. It is shown that, for sufficiently short wavelength perturbations, the growth rates can exceed the growth rate of cosmic-ray streaming instability along the magnetic field, found by Nekrasov & Shadmehri, which is in turn considerably larger than the growth rate of the Bell instability. The thermal instability is shown not to be subject to the action of cosmic rays in the model under consideration. The dispersion relation for the thermal instability has been derived, which includes sound velocities of plasma and cosmic rays and Alfven and cosmic-ray streaming velocities. The relation between these parameters determines the kind of thermal instability ranging from the Parker to the Field instabilities. The results obtained can be useful for a more detailed investigation of electron-ion astrophysical objects, such as supernova remnant shocks, galaxy clusters, and others, including the dynamics of streaming cosmic rays.