Magnetic fields and the dynamics of spiral galaxies

We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H  i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength.
Furthermore, we find that the introduction of a cold H  i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.     

Impact of magnetic field on the gas mass fraction of galaxy clusters

Magnetic fields have been observed in galaxy clusters with strengths of the order of  ~ μG. The non-thermal pressure exerted by magnetic fields also contributes to the total pressure in galaxy clusters and can in turn affect the estimates of the gas mass fraction, f_gas. In this paper, we have considered a central magnetic field strength of 5μG, motivated by observations and simulations of galaxy clusters. The profile of the magnetic field has also been taken from the results obtained from simulations and observations. The role of magnetic field has been taken into account in inferring the gas density distribution through the hydrostatic equilibrium condition (HSE) by including the magnetic pressure. We have found that the resultant gas mass fraction is smaller with magnetic field as compared to that without magnetic field. However, this decrease is dependent on the strength and the profile of the magnetic field. We have also determined the total mass using the NFW profile to check for the dependency of f_gas estimates on total mass estimators. From our analysis, we conclude that for the magnetic field strength that galaxy clusters seem to possess, the non-thermal pressure from magnetic fields has an impact of  ≈ 1 % on the gas mass fraction of galaxy clusters. However, with upcoming facilities like Square Kilometre Array (SKA), it can be further expected to improve with more precise observations of the magnetic field strength and profile in galaxy clusters, particularly in the interior region.     

Modelling of self-similar cylindrical shock wave in radiating magnetogasdynamics,I

Self-similar flows of a perfect gas behind a cylindrical blast wave with radiation heat flux in the presence of an azimuthal magnetic field have been investigated. The effects of radiation flux and magnetic field together on the other flow variables have been studied in the region of interest. The magnetic field and density distribution vary as an inverse power of radial distance from the axis of symmetry. The electrical conductivity of the gas is taken to be infinite. The total energy of the flow between the inner expanding surface and the shock is assumed to be constant. We also have supposed the gas to be grey and opaque and the shock to be transparent and isothermal.     

3D global simulations of galactic magnetic fields and gas flows

The evolution of three-dimensional (3D), dynamo excited galactic magnetic fields under the influence of a time-dependent gas flow in spiral arms is already well investigated. Our principal goal is to check how the dynamo-driven turbulent magnetic fields affect the gas flows. Numerical solutions of the full set of 3D MHD equations for dynamos in spiral galaxies are presented. Further we try to investigate the nonlinear evolution of magnetic instabilities in a global galactic model. The model includes differential rotation, eddy diffusivity and tensorial alpha-effect. In a first step the flow is driven by a prescribed gravitational potential. The vertical density stratification and the radial-azimutal spiral pattern are taken closely to observational data. We use a modified variant of the highly parallelized time-stepping ZeusMP code for the simulations of global galactic magnetic fields and gas flows. This revised version was published online in August 2006 with corrections to the Cover Date.     

Propagation of magneto-radiative shock waves,II

Self-similar motion of a perfect gas behind a cylindrical shock wave with radiation heat flux in the presence of an azimuthal magnetic field have been discussed. The shock is assumed to be propagating in a medium at rest with non-uniform density. The conductivity of the gas is infinite and magnetic permeability is one everywhere. Also, the shock is assumed to be transparent and isothermal.     

Effect of suspended particles on the thermal convection instability in hydromagnetics

The effect of suspended particles in a finitely conducting gas on the thermal convection instability is studied. The critical Rayleigh number at which instability sets in is reduced by the presence of suspended particles. The effect of vertical magnetic field is stabilizing. We also study the effect of conducting particles suspended in a non-conducting gas. It is found that the stabilizing effect of the magnetic field is reduced by the electrically conducting suspended particles.     

Current-Sheet Buildup and Magnetic Reconnection in Weakly Ionized Solar Lower Atmosphere

Solar observations show that magnetic reconnection can occur in the Sun's weakly ionized lower atmosphere (magnetic cancellation, Ellerman bombs and type II white-light flares). Unlike what the usual reconnection models have predicted, such a reconnection is accompanied by temperature enhancements which are less than 10%. To overcome this difficulty, we have reexamined the reconnection in a two-fluid model using a 2D numerical simulation. The numerical solutions demonstrate the following results: (1) Under the influence of Lorentz force, ionized gas carries the magnetic field into a diffusion region where part of the field is annihilated, and the current-sheet scaling laws for the weakly ionized plasma are basically the same as in the fully ionized case. (2) Though the neutral gas is not directly affected by the magnetic field due to frictional forces, its motion is almost the same as the ionized gas except in the region near stagnation point where the streamlines of both species differ appreciably. (3) The pressure of neutrals which governs the distribution of total pressure and temperature varies slightly. So the temperature of the whole domain is nearly uniform in space and constant in time. These results support the idea that magnetic cancellation, Ellerman bombs, and type II white-light flares are due to magnetic reconnection in the Sun's lower atmosphere.     

Element separation effects in the boundary region of a plasma surrounded by neutral gas

A one-dimensional model is being considered where a fully ionized plasma is separated from a neutral gas by a homogeneous magnetic field directed along the plasma boundary. The plasma and the neutral gas consist of two different types of ions and neutral particles. In a stationary state the outflux of plasma by diffusion across the magnetic field is compensated by an influx of neutrals which are ionized in a partially ionized boundary region. It is found that the ratio between the ion densities in the fully ionized region will in general differ from the density ratio of the two types of neutrals being present in the gas region. This provides a separation mechanism with applications both to cosmical and laboratory plasmas, such as in the following cases:

1. The abundance anomalies in magnetic variable stars and in the solar wind.
2. Separation processes of non-identical ions and neutral atoms in gas blanket systems.

     

Star formation induced by supersonic turbulence in interstellar molecular cloud

We studied fragmentation process of the interstellar molecular cloud which is predominated by supersonic turbulence with special regard to collisions of turbulent gas elements and formation of a shock-compressed layer by receding shock waves. The propagation of the shock waves and the evolution of the compressed layer are followed by one-dimensional gas dynamical simulation until self-gravity becomes significant, taking account of the effects of thermal properties of the molecular gas and magnetic fields. It is shown that the efficient cooling by CO molecules and its sensitive dependence on gas density make the shock-compressed layer so cold and dense that the layer becomes gravitationally unstable and breaks into fragments even if the gas elements are gravitationally stable prior to the collision. The mass of the unstable fragment is estimated to be about two solar masses or less, irrespective of the presence of the magnetic field. The stars formed by collisions of supersonic turbulent gas elements accelerate the surrounding gas in T Tauri stage and replenish the turbulent energy to maintain the mechanical equilibrium of the molecular cloud.     

Neutral Points at the Cometary Diamagnetic Cavity Boundary as Possible Source of Cometary Rays

The point source of neutral gas undergoing ionization and expanding into an uniform magnetic field is considered. Friction between the neutral and ionized particles results in the formation of the magnetic field barrier and diamagnetic cavity surrounding the source. At least one neutral point inevitably arises at the boundary of the cavity. When the neutral gas production rate grows, two neutral points may arise at this boundary. In the vicinity of these points magnetic field lines converge, along with the plasma flow which is magnetic field aligned in the steady state. As a result, two plasma jets originate from the neutral points. Possible relation of these jets to cometary rays is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The physics of strong magnetic fields in neutron stars

In this paper we present a new result, namely that the primal magnetic field of the collapsed core during a supernova explosion will, as a result of the conservation of magnetic flux, receive a massive boost to more than 90 times its original value by the Pauli paramagnetization of the highly degenerate relativistic electron gas just after the formation of the neutron star. Thus, the observed super-strong magnetic field of neutron stars may originate from the induced Pauli paramagnetization of the highly degenerate relativistic electron gas in the interior of the neutron star. We therefore have an apparently natural explanation for the surface magnetic field of a neutron star.     

3D numerical models of magnetic field evolution in galaxies interacting with the ICM

A fully three-dimensional (3D) MHD model is applied to simulate the evolution of large-scale magnetic field in galaxies interacting with the intra-cluster medium (ICM). As the model input we use a time dependent velocity field of gas clouds (HI) resulting from 3D N-body sticky-particle model of a galaxy. These clouds are affected by ram pressure due to their rapid motion through the ICM. The gas evolves in an analytically given gravitational potential which includes a dark matter halo, a disk, and a bulge component. We found that due to the interaction with the ICM the resultant magnetic field correctly reproduces the observed structures of the magnetic field forming peculiar spiral arms and magnetic features widely observed in cluster spiral galaxies. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Lifshitz-Kosevich-Shoenberg theory of relativistic electronic gas in neutron stars

Similar to the de Haas-van Alphen magnetic oscillatory in some normal metals when the Landau quantization is predominant, the magnetic oscillation can also occur in highly degenerate and relativistic electron gas in neutron stars. At large Landau quantum number (Landau quantum number r≥2), we generalize the Lifshitz-Kosevich-Shoenberg theory in non-relativistic electron gas to relativistic gas. At small Landau quantum number (r<2), we expand the grand potential into Fourier series and get similar harmonic oscillatory formula of magnetization. These results indicate that magnetic phase transition similar as Condon transition observed in metals can appear in neutron stars when the differential susceptibility exceeds 1/4π.     

Localized magnetic reconnection as a cause of extraplanar diffuse ionized gas in the Galactic halo

Many observations indicate the occurrence of ionized gas in the distant haloes of galaxies (including our own). Since photoionization by stars (mainly O stars, young stars or evolved low-mass stars depending on the kind of galaxy) does not seem to be exclusively responsible for the ionization of the hydrogen filaments that should otherwise cool fast and recombine quickly, the question arises which extra energy source can produce the quasi-stationary ionization. We show that stationary localized magnetic reconnection in current filaments may contribute to the ionization of the extraplanar halo gas. In these filaments magnetic energy is dissipated. Consequently, the ionized as well as the neutral component is heated and re-ionized on a time-scale significantly shorter than the recombination time-scale. The amount of energy required for efficient re-ionization can in principle easily be provided by the free magnetic energy. We present quasi-static models that are characterized by plasma temperatures and densities that agree well with the observed values for the diffuse ionized gas component of the interstellar medium. Plasma–neutral gas fluid simulations are made to show that the recombination-induced dynamical reconnection process indeed works in a self-regulatory way.     

An accurate solution in magnetic shearing dynamics

In this paper an exact solution of magnetic shearing dynamics in the stage of linear development under constant gas density is obtained. Numerical results in the cases of constant and gravity-stratified densities show that their difference is very small when the gas/magnetic pressure ratio β is of the order of 10⁻². Therefore our results are applicable to the actual solar atmosphere. It is shown that the amplitude of velocities grows almost linearly with tine and falls slowly with height, the fall getting slower all the time. This peculiar behaviour makes the magnetic arch rise higher and higher while the feet of the magnetic loops remain nearly fixed. Thus, a closed magnetic arch system may eventually become open.     

The circumstellar gas of sigma orionis E

In order to explain the variable H emission and the eclipse-like light variation of Ori E, we investigated the circumstellar gas trapped by the stellar magnetic field and corotating with the star. By considering the potential along the magnetic field line, we found that the gas concentrates to a potential minimum. The circumstellar gas forms either two condensations or a disk, depending on the inclination of the magnetic dipole to the stellar rotation axis. The geometrical thickness of the circumstellar disk, of about 0.2 stellar radii, and the distance from the center of the star to the inner edge of the disk, of about 3 stellar radii, were obtained. The H emission line profile at its maximum phase and the amplitude of light variation were calculated by assuming the isothermal gas in LTE with the maximum gas density which the magnetic field can hold. The model gives good agreement with observation in the low obliquity case, and also explains the phase correlation among the H emission maximum, the light minimum, and the magnetic extreme. The model, however, failed to explain the large IR excess in theM band.     

Origin of quiescent prominences

For stable equilibrium, prominences must be supported with magnetic lines of force leaning upon the photosphere and concave in their tops; however the general structure may be more complicated. If such a field appears in the corona, the heating of the gas near the upper pit should be low, because Alfvén and slow waves do not propagate across magnetic lines and fast mode waves attenuate because of refraction. The gas of the corona, distributed along the magnetic lines tube, cannot keep balance, it should flow down in the pit, condense there and fall down into the chromosphere in some places. The prominence, therefore, originates in the matter of the chromosphere which is situated at the other end of the magnetic lines and flows through the corona under the effect of a siphon-type mechanism. A similar mechanism for chromospheric structures was earlier suggested by Meyer and Schmidt. A stationary stream along the tube has been calculated with allowance for the heat conductivity and radiative cooling of the corona gas. The stream is subsonic and is about 10¹⁵ cm⁻² sec⁻¹ which corresponds to the prominence formation time of the order of a day.     

The importance of chemical data for the study of interstellar shocks

Shock waves perturb the chemical state of the interstellar gas. We consider the effects of C-shocks on the composition of molecular clouds, for a range of values of the pre-shock gas density and magnetic induction. The time required to re-establish equilibrium in the post-shock gas depends on the initial conditions and can become very large. The significance of the two known chemical phases of dark clouds and of bistability is considered in this context.     

Energy characteristics of a relativistic Fermi gas in a magnetic field

We derive expressions for the chemical potential, pressure, and mean total energy of an extremely degenerate ideal relativistic gas of charged fermions. We take into account their static anomalous magnetic moments in the presence of a quantizing magnetic field. We examine the cases of ultra-strong magnetic fields and of weak fields in which one does not need to take into account Landau quantization when describing a Fermi gas in ultra-dense matter.Translated from Astrofizika, Vol. 37, No. 1, pp. 161–165, January–March, 1994.     

Magnetic fields and chemical evolution in the disk of the Milky Way

An alternative non-infall model for the chemical evolution in the solar neighbourhood is proposed. The evolution of the disk is divided into two phases. In phaseA, the magnetic field and the gas viscosity produced an outward flux of gas, forming and maintaining the ring observed today. This flux balanced the star formation in the ring. The number of stars increased until the beginning of phaseB, during which stellar viscosity generated an inward flux of stars towards the inner disk, while the magnetic fields continued supplying gas to the ring. The combination of these two effects brought the ring to a quasi-steady state, with a constant mass of gas and stars which we assume has continued till the present. A coherent picture is obtained in which the observational restrictions are explained without introducing any arbitrary hypothesis. The inward flux of stars in phaseB has transported the metal-poor G-dwards to the inner region, thus explaining their absence in the solar neighbourhood.