Structure and stability of magnetized accretion disks with anomalous viscosity

The structure and stability of a magnetized accretion disk are numerically examined with anomalous viscosity. The temperature, surface density and radial velocity all decrease with increasing radius r. The results show that the existence of the magnetic field B has an impact on the structure of the disk, which directly results in the variation of the growth rate and the damping rate of the unstable and stable modes. For Inward-moving mode, the magnetic field greatly enhances the instability at short wavelength and acts as a factor of stability at long wavelength. The growth rate of outward-moving unstable mode decreases, while the damping rate of thermally stable mode increases significantly owing to the magnetic field.     

The spontaneous magnetic field direction in an anisotropic MHD dynamo

The phenomenon of magnetic field generation in an astrophysical plasma in the frame of developed magnetohydrodynamic (MHD) turbulence is considered. The functional quantum field renormalization group approach is applied to helical anisotropic MHD developed turbulence which is stabilized by the self-generated homogeneous magnetic field. The purpose of the study is to calculate the value as well as direction of the magnetic field in the stochastic dynamo model. The generated magnetic field is determined by ignoring divergent rotor part of Green function of the magnetic field. It is shown that the magnetic field direction is connected with unique existing vector n describing the anisotropic turbulence forcing.     

Non-linear damping of visco-resistive Alfvén waves in solar spicules

Interaction of Alfvén waves with plasma inhomogeneities generates phase mixing which can lead to dissipate Alfvén waves and to heat the solar plasma. Here we study the dissipation of Alfvén waves by phase mixing due to viscosity and resistivity variations with height. We also consider nonlinear magnetohydrodynamic (MHD) equations in our theoretical model. Non-linear terms of MHD equations include perturbed velocity, magnetic field, and density. To investigate the damping of Alfvén waves in a stratified atmosphere of solar spicules, we solve the non-linear MHD equations in the x–z plane. Our simulations show that the damping is enhanced due to viscosity and resistivity gradients. Moreover, energy variations is influenced due to nonlinear terms in MHD equations.     

Stability of thermally conducting plasma in a variable magnetic field

The flow of an infinitely extending homogenous thermally conducting plasma permeated by a variable magnetic field is considered. The combined effect of several physical parameters, namely Hall currents, finite conductivity, ion viscosity and thermal conductivity on plasma instability is studied in the framework of Tsallis statistics. A new Jeans Criterion is derived, which depends explicitly on the nonextensive parameter q. The standard values are obtained in the limiting case q=1.     

The role of toroidal magnetic field in the stability of accretion disks with alpha viscosity and other viscosities

The standard thin accretion disk model predicts that the inner regions of alpha model disks, where radiation pressure is dominant, are thermally and viscously unstable. However, observations show that the bright X-ray binaries and AGN accretion disks, corresponding to radiation-pressure thin disks, are stable. In this paper, we reconsider the linear and local instability of accretion disks in the presence of a toroidal magnetic field. In the basic equations, we consider physical quantities such as advection, thermal conduction, arbitrary viscosity, and an arbitrary cooling function also. A fifth order diffusion equation is obtained and is solved numerically. The solutions are compared to non-magnetic cases. The results show that the toroidal magnetic field can make the thermal instability in radiation pressure-dominated slim disks disappear if ? _m ≥0.3. However, it causes a more thermal instability in radiation pressure alpha disks without advection. Also, we consider the thermal instability in accretion disks with other values of the viscosity and obtain a general criterion for thermal instability in the long-wavelength limit and in the presence of a toroidal magnetic field.     

磁塌缩不稳定与自生间歇磁流

太阳和天体物理吸积盘中的场是一种空间间歇的磁流。在整个太阳上都可发现这种间歇磁流片，其中光球上90%以上的磁流呈现为强场形态，其强度为0.1-0.2T，大小为50-300km；在吸积盘中，已知脉动磁场比宏观磁场强几个数量级。磁场的重联湮灭，导致在薄电流片区形成小尺度的磁环胞以及同涨的横等离激元。磁流和等离激元之间的非线性相互作用引起自类似塌缩，形成更为空间间歇的塌缩的磁环元胞。而横等离激元诱发的自生磁场具有调制不稳定性，导致磁场塌缩，形成高度间歇的磁流。分别在磁流力学和等离子体动力论两种情况下，分析了这种磁塌缩不稳定性，并用于解释太阳上的间歇磁流以及寻求天体物理吸积盘中的反常粘滞。     

A possible current system associated with the Sqp variation

It is suggested that the quiet day daily magnetic variation in the polar cap region, S_q^p, results partly from the short-circuit effect of the magnetotail current by the polar ionosphere. This implies that there is an inward field-aligned current from the dawnside magnetopause to the forenoon sector of the auroral oval (positively charged) and an outward field-aligned current to the duskside magnetopause from the afternoon sector of the oval (negatively charged), together with the ionospheric (Pedersen and Hall) currents. The distribution of the magnetic field vectors of both combined current systems agrees with the observed S_q^pvector distribution. The space charges provide an electric field distribution which is similar to that which has been observed by polar orbiting satellites.     

Es-q layer at huancayo during the March 1970 geomagnetic storm

The paper describes a comparison of vertical electron drift in the F-region (V_z) measured by VHP incoherent scatter radar at Jicamarca with the corresponding variations of geomagnetic horizontal field (H) and the maximum frequency reflected from The E_s layer (E_s) at Huancayo during the geomagnetic storm period 7–9 March, 1970. The V_z is generally upward during the daytime at the equator, but during 7–9, March, 1970, V_z was negative for brief periods associated with negative bays in H. These periods of abnormally low or of downward V_z correspond closely with the period of complete disappearance of the q type of E_s layer. The magnetic bays associated with the intensification of ring current do not affect the equatorial E_s- q and it is only the negative bays in H at the equator due to the ionospheric current flowing westward, that cause sudden disappearance of E_s? q. It is suggested that the q type of E_s is due to cross-field instability created in the electrojet region due to interaction of northward magnetic field and vertical upward Hall polarization electric field when the plasma density gradient is upward. The sudden disappearances of E_s? q are due to the reversal of the horizontal electric field in the equatorial ionosphere and thereby due to the reversal of the equatorial electrojet currents. These reversals of electric field may be due to the imposition on the normal S_q field of another westward electric field.     

Solution of one-fluid model equations with short range retarding magnetic forces for the quiet solar wind

The discrepancy of the low predicted versus the observed coronal particle densities is investigated by considering radial magnetic forces acting at the base of the corona in the one fluid model equations with anomalous thermal conductivity for the quiet solar wind. If the short range retarding magnetic force is taken to fall asr ^?5,r being the heliocentric distance, then in order to obtain satisfactory agreement between the predicted and observed (about 3×10⁸ cm^?3 at 1R _⊙) coronal densities, the strength of the retarding magnetic force at 1R _⊙ should be 1.2 times that of the gravitational force.     

The Relative Umbral Area in Spot Groups as an Index of Cyclic Variation of Solar Activity

The Greenwich series of data was used to study the ratio [q] of the total umbra area to the total area of the sunspot group (for brevity “relative umbral area”) for the period 1874?–?1976. It was revealed that the annual mean value of q varied in time from 0.15 to 0.28 and reached its maximum in the early 1930s. The dependence of q on the sunspot group area [S] was considered to show that the smallest groups, of area less than 100 m.v.h. (millionths of the visible hemisphere), contributed most significantly to the temporal variation of q. In contrast to the earlier results, the dependence obtained proved to be rather complicated. The coefficients of the linear expansion q(S) are themselves dependent on the sunspot-group area and time [t]; i.e. the relation of q to both S and t is nonlinear. Only in sunspot groups with a large area does dependence disappear, and q becomes constant, equal to 0.18. This is the value given in textbooks. The relations obtained show that the relive umbral area and the relative number of small groups are important parameters of the secular variation of solar activity. In particular, they may account for variations in the mean magnetic field in active regions, the complexity of a group according to the magnetic classification, the flare activity of a sunspot group, and its geophysical impact. It is conjectured that the parameter q describes the time-varying relative contribution from the interior and subsurface dynamo mechanisms.     

Magneto-convection in sunspot penumbrae

Finite amplitude convection in the presence of a horizontal magnetic field has been investigated in a region where thermal diffusivity (κ) is less than magnetic diffusivity (η) and whenκ/η > 1,Q ≤Q _c, where $$Q_c = \frac{{(1 + \sigma _1 )(\pi ^2 + q_c^2 )^2 }}{{q_c^2 (\sigma _2 - \sigma _1 )}}$$ ,Q is the Chandrasekhar number,σ ₁ the Prandtl number,σ ₂ the magnetic Prandtl number, andq _c the critical wave number at the onset of stationary convection. We have derived a nonlinear time-dependent Landau—Ginzburg equation near the onset of supercritical stationary convection and a nonlinear, second-order equation at the Takens—Bogdanov bifurcation. We have obtained steady-state solutions of these equations, which describe the nonlinear behaviour near the onset of stationary convection.     

Swirling hydrodynamic jets in viscous accretion flows

A study is made of axisymmetric, low sonic-Mach-number flows of a viscous fluid with angular momentum outside of a black-hole. The viscosity is an eddy viscosity due to turbulence in the sheared flows. Self-similar solutions arise naturally, reducing the Navier-Stokes equations to a set of nonlinear ordinary differential equations. These equations are solved analytically for flows of constant specific angular momentum and numerically for more general flows. For flows with non-constant specific angular momentum, the momentum flux density includes a planar discontinuity which is interpreted as an accretion disc. In general, two flow regions appear on each side of the disk, corresponding to accretion onto the disk and jet-like outflows along the ±z-axes. Physical interpretations of the solutions show that these flows arise in response to point sources of axial momentum at the origin directed in the ±z-directions. The power needed to maintain this momentum input is assumed to come from the mass accretion onto the black hole.The hydrodynamic flows are generalized to include a magnetic field. In the limit of infinite electrical conductivity, the possible types of flow patterns are the same as in hydrodynamic case. The magnetic field alters the relative amounts of reversible and irreversible momentum and angular momentum transport by the flow. For a flow with turbulent viscosity, the magnetic field acts to reduce the level of the turbulence and the effective value of the eddy viscosity.     

Modification of the Jeans and Toomre instability criteria for astrophysical fractal objects within nonextensive statistics

Within the formalism of Tsallis nonextensive statistics designed to describe the behavior of anomalous systems, systems with a strong gravitational interaction between their individual parts and the fractal nature of phase space, we have obtained linearized equations for the oscillations of a rigidly rotating disk by taking into account dissipative effects and give a derivation of the dispersion equation in the WKB approximation. Based on the previously derived modified Navier—Stokes hydrodynamic equations (the so-called equations of q-hydrodynamics), we have analyzed the axisymmetric oscillations of an astrophysical, differentially rotating gas—dust cosmic object and obtained modified Jeans and Toomre gravitational instability criteria for disks with a fractal phase-space structure.     

String cosmology with magnetized bulk viscous fluid in Bianchi I universe

The objective of the present paper is to study an anisotropic Bianchi-I cosmological model filled with bulk viscous fluid and magnetic field in string cosmology. The magnetic field is due to an electric current produced along the x-axis. The expansion in the model is considered to be proportional to one of the components of the shear tensor. We obtain two different quadrature forms of volume scale factor by considering two different relations between bulk viscosity and expansion scalar. We discuss the behavior of the classical potential with respect to the volume scale factor in the presence or absence of magnetic field and bulk viscosity in each case. We observe the role of bulk viscosity on the classical potential and also on the choices of bulk viscous pressure. By introduction of magnetic field or bulk viscosity or both into the model it results in changes in the potential as well as in volume scale factors. The physical and geometrical aspects of the solutions are discussed in detail.     

Viscous-resistive ADAF with a general large-scale magnetic field

We have studied the structure of hot accretion flow bathed in a general large-scale magnetic field. We have considered magnetic parameters , where are the Alfvén sound speeds in three direction of cylindrical coordinate (r,φ,z). The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence and viscosity in the accretion flow. Also, we adopt a more realistic model for kinematic viscosity (ν=αc _s H), with both c _s and H as a function of magnetic field. As a result in our model, the kinematic viscosity and magnetic diffusivity (η=η ₀ c _s H) are not constant. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. It is found that the existence of magnetic resistivity will increase the radial infall velocity as well as sound speed and vertical thickness of the disk. However the rotational velocity of the disk decreases by the increase of magnetic resistivity. Moreover, we study the effect of three components of global magnetic field on the structure of the disk. We found out that the radial velocity and sound speed are Sub-Keplerian for all values of magnetic field parameters, but the rotational velocity can be Super-Keplerian by the increase of toroidal magnetic field. Also, Our numerical results show that all components of magnetic field can be important and have a considerable effect on velocities and vertical thickness of the disk.