Determination of the electromagnetic field produced by a magnetic oblique-rotator

Approximate solutions for the electromagnetic fields produced by a uniformly magnetized oblique rotator in vacuum are derived systematically in various regions by using general relativistic considerations. The results, which are expressed in compact vector notation, are compared with the component expression of Deutsch (1955) and Backus (1956). In our method, the potentials (, A) and fields (E, B) due to the rotating magnet are represented in terms of the vector potential for a rest magnet, with the proper correction for rotation. In doing this, the transformation laws for various quantities between the rest and rotating frames play important roles. The meaning of rotating magnetic field lines is also considered in connection with the apparent paradox in the gedanken-experiment of a unipolar inductor.     

Determination of the electromagnetic field produced by a magnetic oblique-rotator

The structure of the corotating region, which forms an inner portion of a stellar magnetosphere, is reconsidered in a quasi-neutral case by taking into account the inertial effects of electrons as well as that of ions up to the first order in their mass ratio (δ=m_?/m₊). It is emphasized first that the magnetosphere is not globally equipotential even in the frame rotating with a central star (i.e. ?#0, where ? is the ‘non-Backus’ potential) due at least to the inertial effects of plasma particles. However, it is shown that the condition ?=0 is asymptotically recovered in the corotating region owing to the presence of the drift current which can be taken into account only when δ is not entirely neglected. This fact suggests that the deviation of the plasma motion in the outer magnetosphere from the corotation can be attributed to the non-zero ?. A globally self-consistent solution is obtained under this condition (?=0). In contrast with the solutions in the ‘force-free’ and the ‘mass-less-electron’ approximations, this solution has a disk structure in the corotation zone in which the plasma and the current density are concentrated to a thin disk near the magnetic equator. Owing to this sheet current in the disk the lines of force of the stellar magnetic field are modified to form a very elongated shape (the magnetodisk) if the plasma β-value is fairly large. Such a disk structure seems to be a common feature in the high β inner magnetospheres of various types of stars.     

Determination of the electromagnetic field produced by a magnetic oblique-rotator

The inertial effect on the structure of the magnetosphere of a rotating star is investigated, in the corotation approximation for a surrounding quasi-neutral plasma. The equation of motion reduces to a usual static balance equation between the electromagnetic and the centrifugal forces, in the rotating frame. However the MHD condition, which can be regarded as a special form of the generalized Ohm's law, is modified by the inclusion of inertial effect, with a violation of the frozen-in condition in case of a general (i.e., not restricted to corotation) plasma motion. The inertial effect on the electromagnetic field is summarized in a partial scalar potential named the non-Backus potential, which is proportional to the centrifugal potential in the corotation approximation.An approximate solution of this corotation problem is given, in which another characteristic radiusr _M appears besides the light radiusr _L . This radius defines a distance beyond which the inertial effect becomes dominant over the electromagnetic one, and is useful in estimating the magnitude of the terminal velocity of a centrifugal wind. A few examples of the modification of dipole magnetic field due to the inertial effect are visualized. In an oblique-rotation case, it can be seen that such a warp of the neutral sheet (the surface ofB _r =0) is reproduced as observed in the Jovian magnetosphere.     

Determination of the electromagnetic field produced by a magnetic oblique-rotator

The electromagnetic field produced by a magnetic dipole moment, , which is rotating obliquely surrounded by a corotating plasma sphere, is investigated. This corotating-plasma approximation has the same order of accuracy as the force-free one but has somewhat different physical implications. In the former the effect of non-electromagnetic forces such as the inertial force are included, though in somewhat artificial manner, as a departure from the strict MHD condition and this fact seems to guarantee the existence of physical solutions.Analogous to the relativistic force-free equation, a set of two differential equations (the corotation equation) are derived for the scalar functions associated with the electric and magnetic fields. A self-consistent solution of these equations is given and it is shown that this solution has no singularity, in spite of apparent divergence in the formal solution, on the light cylinder. It is concluded from this solution that, even in the extreme case of the largest possible corotation-radius (i.e.b=r _L , wherer _L is the light radius), the existence of a corotating plasma does not alter the field structure drastically from the vacuum case. It is also suggested through this treatment that inclusion of the inertial term in generalized Ohm's law might be essential in considering the centrifugal-wind problem.     

The electromagnetic field of a slowly rotating magnetized neutron star in Saa's gravitation model with torsion

The equations of the electromagnetic field of a slowly rotating magnetized neutron star in Saa's gravitation model with torsion are derived and their exterior solution is investigated. The following conclusions are obtained: first, there is a specific solution for the electromagnetic field when A^ô = 0; second, there is no solution at all when A^ô ≠ 0. Therefore, we can judge whether or not torsion exists by observing the exterior electromagnetic field of the neutron star.     

Torsional oscillations of magnetized relativistic stars

Strong magnetic fields in relativistic stars can be a cause of crust fracturing, resulting in the excitation of global torsional oscillations. Such oscillations could become observable in gravitational waves or in high-energy radiation, thus becoming a tool for probing the equation of state of relativistic stars. As the eigenfrequency of torsional oscillation modes is affected by the presence of a strong magnetic field, we study torsional modes in magnetized relativistic stars. We derive the linearized perturbation equations that govern torsional oscillations coupled to the oscillations of a magnetic field, when variations in the metric are neglected (Cowling approximation). The oscillations are described by a single two-dimensional wave equation, which can be solved as a boundary-value problem to obtain eigenfrequencies. We find that, in the non-magnetized case, typical oscillation periods of the fundamental     torsional modes can be nearly a factor of 2 larger for relativistic stars than previously computed in the Newtonian limit. For magnetized stars, we show that the influence of the magnetic field is highly dependent on the assumed magnetic field configuration, and simple estimates obtained previously in the literature cannot be used for identifying normal modes observationally.     

Gravitational and electromagnetic emission by magnetized coalescing binary systems

We discuss the possibility to obtain an electromagnetic emission accompanying the gravitational waves emitted in the coalescence of a compact binary system. Motivated by the existence of black hole configurations with open magnetic field lines along the rotation axis, we consider a magnetic dipole in the system, the evolution of which leads to (i) electromagnetic radiation, and (ii) a contribution to the gravitational radiation, the luminosity of both being evaluated. Starting from the observations on magnetars, we impose upper limits for both the electromagnetic emission and the contribution of the magnetic dipole to the gravitational wave emission. Adopting this model for the evolution of neutron star binaries leading to short gamma ray bursts, we compare the correction originated by the electromagnetic field to the gravitational waves emission, finding that they are comparable for particular values of the magnetic field and of the orbital radius of the binary system. Finally we calculate the electromagnetic and gravitational wave energy outputs which result comparable for some values of magnetic field and radius.     

Polarization transfer of electromagnetic waves in a magnetized plasma

In this investigation, the polarization transfer equations in terms of the Stokes parameters are derived for electromagnetic waves propagating in an arbitrary direction in an inhomogeneous magnetized plasma. This system of transfer equations is then solved analytically in the case when the magnetized plasma is homogeneous. For simplicity in presentation, the source term in the equation of transfer has been omitted. Transitting to the special case of quasi-longitudinal propagation, the results obtained here are shown to be in agreement to that derived by Zheleznyakov earlier.     

Strong electromagnetic waves in a magnetized relativistic electron-positron plasma

It is shown that in a strongly magnetized relativistic electron-positron plasma, strongly localized large amplitude circularly polarized electromagnetic wave pulses exist. The localization is due to relativistic mass variation as well as ponderomotive force effects. Three types of pulses are found analytically: the sharply spiked pulse in a strongly magnetized cold plasma, the smooth pulse in a week magnetized warm plasma, and the moderately spiked pulse for a weakly magnetized cold plasma. The physical mechanisms giving rise to these pulses are distinct for each case. Possible implications of our investigation to pulsar radiation are discussed.     

The radiation of a hot magnetized plasma accreted by degenerate stars with a strong magnetic field

The absorption coefficients for extraordinary and ordinary electromagnetic modes are found for a tenuous hot magnetized plasma, taking into account the collisions between plasma particles and the scattering of photons. An approach is suggested which generalizes collisionless and cold-plasma approximations. The simple formulae obtained are valid both near, and at a distance from, the cyclotron harmonics. In particular, the ordinary mode is shown to have resonance at the cyclotron frequency. The number of noticeable reasonances of absorption coefficient at cyclotron harmonics is estimated for both modes.Using the coefficients obtained, the intensity, Stokes parameters and polarization of radiation of a homogeneous plasma slab are calculated for conditions which may be realized in the heated regions of accreted plasma in an AM Herculis-type system. The large difference between the absorption coefficient of extra-ordinary and ordinary modes near the cyclotron harmonics may result in the emission of the broad polarized continuum together with the narrow cyclotron lines. The polarization of these lines has a complicated spectral dependence.The results obtained are shown to be useful for explaining the main properties of AM Herculistype objects.     

Extinction properties of infinitely long graphite cylinders

We have calculated the extinction efficiencies of randomly oriented infinite graphite cylinders, including hollow cylinders, using the rigorous Kerker-Matijevi formulae. The peak in the mid-ultraviolet extinction varies in wavelength with particle radius and cavity size in a way that makes such particles of limited interest as models of interstellar grains.     

Self-organization of electromagnetic waves into vortices in a magnetized electron-positron plasma

This paper presents a new class of well localized dipolar vortex solutions to the newly derived set of coupled nonlinear equations governing the dynamics of low-frequency electromagnetic waves in a strongly magnetized electron-positron plasma.     

Suppression of resonant field line oscillations by a turbulent background

Stochastic fluctuations of the magnetospheric plasma and background magnetic field, especially intense during geomagnetically active periods, can provide an additional mechanism of damping of Alfvén field line oscillations. To quantify this hypothesis, we consider a driven Alfvén field line resonator with stochastic fluctuations of the Alfvén resonant frequency. This problem is first considered analytically for a low level of fluctuations, then a more general numerical approach is introduced. The results of analytical calculations and numerical modeling both indicate the deterioration of resonant properties of the resonator owing to stochastic background fluctuations.     

Nonlinear coupling between electromagnetic fields in a strongly magnetized electron-positron plasma

The nonlinear coupling between electromagnetic fields in a strongly magnetized electron-positron plasma is considered. We point out that compressional magnetic field perturbations are excited by the rotational part of the nonlinear current, and derive a new nonlinear system of equations that is basic for studies of modulational instabilities and coherent nonlinear structures in magnetized electron-positron plasmas.     

External electromagnetic fields of a slowly rotating magnetized star with gravitomagnetic charge

We study Maxwell equations in the external background spacetime of a slowly rotating magnetized NUT star and find analytical solutions for the exterior electric fields after separating the equations for electric field into angular and radial parts in the lowest order in angular momentum and NUT charge approximation. The star is considered isolated and in vacuum, with dipolar magnetic field aligned with the axis of rotation. The contribution to the external electric field of star from the NUT charge is considered in detail.     

Linear MHD stability of infinitely long coronal flux tubes

Twisted magnetic flux tubes are often used to model the filed in coronal loops, and much attention has been given to analysing their stability. Previous astrophysical studies have concentrated on establishing the existence of an instability or determining stability bounds, and little information seems available on the associated eigenvalues, which give crucial information on the energy released. This paper develops methods of determining eigenvalues for infinitely long flux tubes. The most striking feature of the results is that the eigenvalues are always small-of order 10^–2 (in dimensionless units) even for the fastest helical kink modes (m=1). The more localized higher-m modes have even smaller eigenvalues. A family of flux tubes with field line twist proportional tor is investigated, and it appears that the most energetic instabilities occur in the Gold-Hoyle tube with uniform twist (=0). Implications of these results are discussed.     

Cylindrical oscillations of force-free electromagnetic fields

This paper is devoted to Force-Free Electromagnetic Oscillations in a constant magnetic field. A correction is made in the derivation of the basic equation. The paper confirms the predicted spectrum of frequencies, namely _n = _o (n + 1)^1/2;n = 0, 1, 2, .... In addition it is suggested that hybrid frequency _n = ( _n ² + _H ² )^1/2 should be found in observational data.