Some Exact Solutions of String Cosmological Models in Bianchi Type-V Space-Time

The Bianchi type-V cosmological solutions of massive strings in the presence and absence of the magnetic field are investigated. The physical behavior of these models are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study on Bianchi-IX cosmological model

In this paper the inflationary solutions are studied for the Bianchi-IX space-time in presence of a massless scalar field with a flat potential. Also a class of cosmological solutions of massive strings are obtained following the techniques used by Letelier and Stachel. Some solutions are calculated for pure massive strings following the Takabayashi equation of state =(1+w).     

Bulk viscous barotropic magnetised string cosmological models

Spatially homogeneous and anisotropic LRSBianchi type-I string cosmological models are studied in the frame work of general relativity when the source for the energy momentum tensor is a bulk viscous fluid containing one dimensional strings embedded in a magnetic field. A barotropic equation of state for the pressure and density is assumed to get determinate solutions of the field equations. The bulk viscous pressure is assumed to be proportional to the energy density. The effects of viscosity and electromagnetic field on the properties of the model are investigated. The role of bulk viscosity and electromagnetic field in getting an inflationary phase and in establishing a string phase in the universe is studied.     

String cloud cosmologies for Bianchi type-III models with electromagnetic field

The Saez-Ballester field equations for spatially homogeneous and anisotropic Bianchi type-III cosmological models have been solved for pure geometric cosmic string cloud pervading the universe either in the absence or in presence of electromagnetic field. It has been established here that the model does not survive for geometric cosmic string cloud pervading the universe when there is no electromagnetic field. But in presence of electromagnetic field the model can have plausible solutions fostering the idea that strings forming the surface of the world sheet have to co-exist with electromagnetic field.     

Some string cosmological models in Bianchi Type I space-time

We obtain some cosmological models that are exact solutions of Einstein's field equations. The metric utilized is Marder's metric which is Bianchi Type I and the curvature source is a cloud of strings which are one dimensional objects. Bianchi type cosmological models play an important role in the study of the universe on a scale which anisotropy is not ignored. In this paper we have investigated the effect of cosmic strings on the cosmic microwave background anisotropy. Various physical and geometrical properties of the model are also discussed. The solutions have reported that the cosmic microwave background anisotropy may due to the cosmic strings.     

Anisotropic bulk viscous cosmological models in a modified gravity

A spatially homogeneous Bianchi type-VI₀ space-time is considered in the frame work of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Exact solutions of the field equations are obtained both in the absence and in the presence of cosmic strings under some specific plausible physical conditions. Some physical and kinematical properties of the model are, also, studied.     

Higher Dimensional String Cosmologies in Scale-Covariant theory of Gravitation

Field equations are obtained with the aid of higher dimensional Bianchi type-I cosmological model in scale covariant theory of gravitation in the context of cosmic strings. We present here isotropic and anisotropic solutions of the field equations and some physical implications of these solutions are briefly discussed.     

Primordial magnetic field,inflation and cosmic strings

The possibility of a primordial magnetic field is discussed. The formation of closed, superconducting cosmic strings before the end of inflation is pointed out as a mechanism able to preserve a magnetic field inside the loops which could provide seeds for galactic magnetic fields.     

Analytic Magnetic Reconnection Solutions in Cylindrical Geometry

In this paper we present a new class of exact reconnection solutions in cylindrical geometry. We point out that in the case of planar reconnection there is a natural cylindrical analog to the Cartesian Dawson function model for the magnetic field. Although the resistive energy release scalings of these solutions mimic the Cartesian models an important new feature is the presence of curvature in the current sheet. We go on to show that these solutions can be generalized to three dimensions.     

Similarity solutions for an axially symmetric explosion model in magnetogasdynamics. I

Similarity solutions, for a point explosion in a spheroid with axially symmetric density distribution obeying power laws in the presence of magnetic field, are obtained. A new technique suggested by Bhowmick (1978) has been utilised to study the character of flow variables behind the shock front in an axisymmetric model. The total energy of the wave is constant.     

String Cosmology With Magnetic Field in Cylindrically Symmetric Space-Time

In this paper, the Einstein field equations have been solved for inhomogeneous cylindrically symmetric space with string source in the presence of magnetic field. Certain physical and geometrical properties of the models have been examined. Also, the string solutions in which magnetic field are absent are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydromagnetic unsteady flow due to an unsteady plate

The unsteady flow near an infinite flat plate, which is oscillating harmonically in its own plane, is studied in the presence of a magnetic field subjected to suction or injection. The magnetic field is perpendicular to the plate and the flow of viscous incompressible and electrically conducting fluid is regarded as being initially at rest. Exact solutions for velocity and skin-friction are obtained for the magnetic field fixed to the fluid or to the plate.     

Strong-plane shock waves in optically-thin atmospheres in magnetogasdynamics

In this paper, similarity solutions for the propagation of strong-plane shock waves in optically-thin grey atmospheres are obtained in the presence of a magnetic field. Density and magnetic field are constant in the undisturbed gaseous medium in front of the shock. Planck's diffusion approximation has been taken into account in this problem and a comparative study has been made between the results of ordinary gasdynamics and magnetogasdynamics.     

Maximum mass of magnetic white dwarfs

We revisit the problem of the maximum masses of magnetized white dwarfs(WDs).The impact of a strong magnetic field on the structure equations is addressed.The pressures become anisotropic due to the presence of the magnetic field and split into parallel and perpendicular components.We first construct stable solutions of the Tolman-Oppenheimer-Volkoff equations for parallel pressures and find that physical solutions vanish for the perpendicular pressure when B(?) 10~(13) G.This fact establishes an upper bound for a magnetic field and the stability of the configurations in the(quasi) spherical approximation.Our findings also indicate that it is not possible to obtain stable magnetized WDs with super-Chandrasekhar masses because the values of the magnetic field needed for them are higher than this bound.To proceed into the anisotropic regime,we can apply results for structure equations appropriate for a cylindrical metric with anisotropic pressures that were derived in our previous work.From the solutions of the structure equations in cylindrical symmetry we have confirmed the same bound for B ~ 10~(13) G,since beyond this value no physical solutions are possible.Our tentative conclusion is that massive WDs with masses well beyond the Chandrasekhar limit do not constitute stable solutions and should not exist.     

Bianchi type-V cosmological model with purely magnetic solution

In the present paper we study some new aspects of the Bianchi type-V space time. The Electric and Magnetic parts of Weyl tensors are calculated in terms of tilted congruence and discussed the purely magnetic Weyl tensor. Einstein field equations for purely magnetic space time are obtained and solution of such field equations called purely magnetic solution. To get deterministic solutions of the field equations we consider a new law of variation of average scale factor which yields time dependent deceleration parameter. Certain physical and geometrical properties of the model are also discussed.     

Current sheet models for inverse polarity prominences in twisted flux tubes

This paper treats the prominence model of Low (1993) to examine more complicated sheet currents than those used in the original model. Nonlinear force-free field solutions, in Cartesian coordinates, invariant in a given direction, are presented to show the possibility of an inverse-polarity prominence embedded in a large twisted flux tube. The force-free solution is matched to an external, unsheared, potential coronal magnetic field. These new solutions are mathematically interesting and allow an investigation of different profiles of the current intensity, magnetic field vector and mass density in the sheet. These prominence models show a general increase in magnetic field strength with height in agreement with observations. Other prominence properties are shown to match the observed values.     

Hydromagnetic flow near an oscillating wall

The flow of a viscous incompressible and electrically conducting fluid produced by harmonically oscillating wall of infinite extent in presence of a transverse magnetic field is considered. Exact solutions for velocity, induced magnetic field, electrical current density and skin-friction are obtained when the magnetic Prandtl number is unity. It is shown that the velocity has a phase lag with respect to the oscillations of the wall. This phase lag is found to be significantly affected by the applied magnetic field.On study-leave from Defence Science Laboratory, Delhi, India.     

Self-similar flows behind shock wave with increasing energy in magnetogasdynamics,I

Self-similar solutions for adiabatic and isothermal flows driven out by a propelling contact surface, moving into a quiet solar wind region, are investigated in the presence of magnetic field. The total energy of the flow between the shock and the contact surface is taken to be time-dependent obeying a power-law. The shock is assumed to be strong and propagating into a perfect gas at rest with non uniform density and magnetic field.     

The solar corona as a minimum energy system?

This paper is an exploration of the possibility that the large-scale equilibrium of plasma and magnetic fields in the solar corona is a minimum energy state. Support for this conjecture is sought by considering the simplest form of that equilibrium in a dipole solar field, as suggested by the observed structure of the corona at times of minimum solar activity. Approximate, axisymmetric solutions to the MHD equations are constructed to include both a magnetically closed, hydrostatic region and a magnetically open region where plasma flows along field lines in the form of a transonic, thermally-driven wind. Sequences of such solutions are obtained for various degrees of magnetic field opening, and the total energy of each solution is computed, including contributions from both the plasma and magnetic field. It is shown that along a sequence of increasingly closed coronal magnetic field, the total energy curve is a non-monotonic function of the parameter measuring the degree of magnetic field opening, with a minimum occurring at moderate field opening.For reasonable choices of model parameters (coronal temperature, base density, base magnetic field strength, etc.), the morphology of the minimum energy solution resembles the observed quiet, solar minimum corona. The exact location energy minimum along a given sequence depends rather sensitively on some of the adopted parameter values. It is nevertheless argued that the existence of an energy minimum along the sequences of solutions should remain a robust property of more realistic coronal wind models that incorporate the basic characteristics of the equilibrium corona- the presence of both open and closed magnetic regions.The National Center for Atmospheric Research is sponsored by the National Science Foundation.