Effects of charge on the stability of thin-shell wormholes

In this paper, we use Visser’s cut and paste approach to construct thin-shell wormholes from charged black string. The dynamics of thin-shell wormholes is analyzed by taking Van der Waals quintessence fluid at the wormhole throat. We investigate the stability of these constructed thin-shell wormholes under linear perturbations preserving the cylindrical symmetry and also study the effects of charge on its stability. It turns out that there exist both unstable and stable wormhole solutions depending on different parameters involved in the equation of state.     

Cylindrical thin-shell wormholes in f(R) gravity

In this paper, we employ cut and paste scheme to construct thin-shell wormhole of a charged black string with f(R) terms. We consider f(R) model as an exotic matter source at wormhole throat. The stability of the respective solutions are analyzed under radial perturbations in the context of R+δR ² model. It is concluded that both stable as well as unstable solutions do exist for different values of δ. In the limit δ→0, all our results reduce to general relativity.     

Linearized stability of Reissner Nordstrom de-Sitter thin shell wormholes

Using the Darmois–Israel formalism the dynamical analysis of Reissner Nordstrom de-Sitter thin shell wormholes, at the wormhole throat, are determined by considering linearized radial perturbations around static solutions.The region of stability in the presence of a large value of charge is significantly increased. Also, the region of stability in the presence of a positive large value of cosmological constant is significantly increased.     

The force-free dipole magnetosphere in nonlinear electrodynamics

Quantum electrodynamics(QED) effects may be included in physical processes of magnetar and pulsar magnetospheres with strong magnetic fields. Involving the quantum corrections, Maxwell electrodynamics is modified to nonlinear electrodynamics. In this work, we study the force-free magnetosphere in nonlinear electrodynamics in a general framework. The pulsar equation describing a steady and axisymmetric magnetosphere is derived, which now admits solutions with corrections. We derive the first-order nonlinear corrections to the near-zone dipole magnetosphere in some popular nonlinear effective theories.The field lines of the corrected dipole tend to converge on the rotational axis so that the fields in the polar region are stronger compared to the pure dipole case.     

Connectance and stability of nonlinear symplectic systems

We have revisited the problem of the transition from ordered to chaotic motion for increasing number of degrees of freedom in nonlinear symplectic maps. Following the pioneer work of Froeschlé (Phys. Rev. A 18, 277–281, 1978) we investigate such systems as a function of the number of couplings among the equations of motion, i.e. as a function of a parameter called connectance since the seminal paper of Gardner and Ashby (Nature 228, 784, 1970) about linear systems. We compare two different models showing that in the nonlinear case the connectance has to be intended as the fraction of explicit dynamical couplings among degrees of freedom, rather than the fraction of non-zero elements in a given matrix. The chaoticity increases then with the connectance until the system is fully coupled.     

Coupled nature of evening-time ionospheric electrodynamics

The F region evening electrodynamics in the equatorial region is characterized by a pre-reversal enhancement (PRE) in the zonal eastward electric field. Although the theoretical mechanisms for PRE are known, its variability, particularly day-to-day variability is not fully resolved. PRE is a large scale phenomenon driven by the F region dynamo after the sunset hours. This paper investigates whether the variability of the E region conductivity (particularly the one associated with the sporadic E, Es) has any influence on the F region dynamo and hence on the PRE of zonal electric field. Interestingly, ionosonde observations have indicated a higher occurrence of the blanketing type Es (Esb) over the low latitude on days with highly suppressed PRE of zonal electric field in comparison with the days with significantly larger PRE. Observational evidences presented in this paper suggests that the formation of the Esb in the evening hours is a sovereign process, not always controlled by the sheared F region vertical electric field of equatorial origin, mapping along the magnetic field line on to the low latitude E region. Model computations of the PRE suppression based on the measured Es densities have further substantiated the observational findings presented in this paper. These results clearly indicate that the low latitude Es has the potential to suppress the PRE of zonal electric field and possibly can play a vital role in explaining the PRE variability, particularly the day-to-day variability. Results have been discussed in light of earlier reports on PRE mechanisms and E-F region coupling processes.     

被动型氢钟流量变化对长期频率稳定度的影响分析

为提高系统的长期频率稳定度(它是被动型氢钟的一个重要指标),设计了一种实验,用以分析在不同微波腔谐振频率下,氢气流量变化对输出频率的影响。通过实验发现微波腔的控制电压存在一个调谐点,在调谐点附近,流量变化对输出频率的影响变小。但是对于单频系统,如何将微波腔锁定在调谐点附近以及如何减小流量的影响和提高系统的长期频率稳定度还需要进一步探索。     

Astrophysical limits on gauge invariance breaking in electrodynamics with torsion

General relativistic electrodynamics in a torsion background can give rise to a situation where the photons can ahve a non-zero rest mass and magnetic moment. Astrophysical limits are used to constrain these parameters.     

The fragmentation of expanding shells – I. Limitations of the thin-shell approximation

We investigate the gravitational fragmentation of expanding shells in the context of the linear thin-shell analysis. We make use of two very different numerical schemes; the flash adaptive mesh refinement code and a version of the Benz smoothed particle hydrodynamics code. We find that the agreement between the two codes is excellent. We use our numerical results to test the thin-shell approximation and we find that the external pressure applied to the shell has a strong effect on the fragmentation process. In cases where shells are not pressure-confined, the shells thicken as they expand and hydrodynamic flows perpendicular to the plane of the shell suppress fragmentation at short wavelengths. If the shells are pressure-confined internally and externally, so that their thickness remains approximately constant during their expansion, the agreement with the analytical solution is better.     

A nonlinear stability problem in the three dimensional restricted three body problem

The question of whether or not there is a transfer of energy between the in-plane motion and out-of-plane motion in the neighborhood ofL ₄ in the restricted problem of three bodies is investigated in this paper. The in-plane motion is assumed to be finite and the out-of-plane motion to be infinitesimal. The equation governing the out-of-plane motion becomes one with time varying coefficients. The stability of this equation is then investigated using Lie Series.Presented as a paper AAS No. 70-313, at the AAS/AIAA Astrodynamics Specialists Conference 1971 at Fort Lauderdale Fla., U.S.A.     

Comments on torsion,massive electrodynamics and gravitation induced by scalar fields

Some discrepancies in Andrade (1992) and consequent corrections are mentioned.     

Basic concepts in the electrodynamics of the auroral ionosphere

Equations governing the large-scale electrodynamic processes in the auroral ionosphere are systematically discussed and the limits to drawing conclusions from incomplete sets of equations are evaluated. The vectors of electic current density,j, and electric field,E, are expressed as explicit functions of the densities, pressures and velocities of the constituents of the ionosphere.The equation div (·E)=0 is an identity satisfied by any solution of the full set of equations governing the problem and cannot be treated as a differential equation forE in which the components of the conductivity tensor are given parameters. The concept of the height-integrated conductivities and the conclusions based on it are inconsistent with the equations of momentum balance for the ionospheric constituents.The global structure of the auroral ionosphere is determined by the state of equilibrium between the pressure gradients, the inertial forces and thej×B-force associated with the auronal electrojets flowing along the auroral oval. The time-averaged, global, electric field is directed across the auroral oval. Its value is substantially affected by the motions of neutral particles. The velocity vector of the neutrals has a substantial component directed across the oval.     

Thermodynamic study of non-linear electrodynamics in loop quantum cosmology

In this work, we have discussed the Maxwell’s electrodynamics in non-linear forms in FRW universe. The energy density and pressure for non-linear electrodynamics have been written in the electro-magnetic universe. The Einstein’s field equations for flat FRW model in loop quantum cosmology have been considered if the universe is filled with the matter and electro-magnetic field. We separately assumed the magnetic universe and electric universe. The interaction between matter and magnetic field have been considered in one section and for some particular form of interaction term, we have found the solutions of magnetic field and the energy density of matter. We have also considered the interaction between the matter and electric field and another form of interaction term has been chosen to solve the field equations. The validity of generalized second law of thermodynamics has been investigated on apparent and event horizons using Gibb’s law and the first law of thermodynamics for magnetic and electric universe separately.