Induced Raman scattering in pulsar magnetospheres

It is shown that induced Raman scattering of electromagnetic waves in the strongly magnetized electron–positron plasma of pulsar magnetospheres may be important for wave propagation and as an effective saturation mechanism for electromagnetic instabilities. The frequencies at which strong Raman scattering occurs in the outer parts of a magnetosphere fall into the observed radio band. The typical threshold intensities for the strong Raman scattering are of the order of the observed intensities, implying that pulsar magnetospheres may be optically thick to Raman scattering of electromagnetic waves.     

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.     

On accretion flow penetration of magnetospheres

We address the problem of plasma penetration of astrophysical magnetospheres, an important issue in a wide variety of contexts, ranging from accretion in cataclysmic variables to flows in protostellar systems. We point out that under well-defined conditions, penetration can occur without any turbulent mixing (driven, for example, by Rayleigh–Taylor or Kelvin–Helmholtz instabilities) caused by charge polarization effects, if the inflowing plasma is bounded in the direction transverse to both the flow velocity and the magnetic field. Depolarization effects limit the penetration depth, which nevertheless can, under specific circumstances, be comparable to the size of the magnetosphere. We discuss the effect of ambient medium on plasma propagation across the stellar magnetic field and determine the criteria for deep magnetosphere penetration. We show that, under conditions appropriate to magnetized white dwarfs in AM Her type cataclysmic variables, charge polarization effects can lead to deep penetration of the magnetosphere.     

Energetics of the aligned pulsar magnetosphere

An energy analysis is performed on two explicit models, due to Jackson, of a pulsar with aligned magnetic and rotational axes. The unknown parameters of these models are determined by calculating the minimum total energy states of the models. It is found that the minimum energy analysis favors states with extended, dynamically active magnetospheres with a high degrees of corotation. By calculating total power input to the magnetosphere via collisions in the stellar crust, and the total power radiated due to azimuthal drift motion, it is determined that the minimum energy states are the only states where a power balance can be achieved. Consideration of a local power balance condition and dissipative flows in the magnetosphere shows that neither model is completely self-consistent, but one is considerably better than the other. Properties of both models and implications for other models are discussed.     

Towards solution of the pulsar problem

The 24-year-old pulsar problem is reconsidered. New results are obtained by replacing the assumption of steady-state discharges near the polar caps by oscillatory discharges, and by creating the neutral-excess pair plasma via inverse-Compton collisions rather than via curvature radiation. As a result, the electrons and positrons which compose the pulsar wind have different bulk velocities and an oscillating space density, and (strong) coherent curvature radiation is implied (without invoking the excitation of instabilities, and contrary to existing proofs of its impossibility). The magnetospheres of young pulsars are likely to have considerable higher-order multipole components, in particular octupole. Radiation transfer through the pulsar magnetosphere results in fan beams whose polarization is dictated by the bottom of the radiation zone, hence, looks like curvature radiation from dipole-like polar caps.Wind generation depends mainly on the quantityB² which takes similar values for the ms pulsars; the latter compensate for (somewhat) weaker fields by wider polar caps and smaller curvature radii.     

Toroidal Magnetic Field Generation in the Magnetosphere of Crab Pulsar

Plasma mechanism for the generation of toroidal magnetic field in the magnetosphere of Crab pulsar is presented. The mechanism is based on the development of parametric type instability in the relativistic electron-positron plasma of the pulsar magnetosphere. As a result of plasma corotation with pulsar and its magnetic field, the effect of plasma radial braking takes place and the time dependence of plasma particle radial velocity is harmonic. This triggers the development of parametric type instability in the relativistic plasma of the pulsar magnetosphere. The energy for this process is drawn from the slowing down of pulsar rotation.     

Simulations of the axisymmetric magnetospheres of neutron stars

In this paper we present the results of time-dependent simulations of the dipolar axisymmetric magnetospheres of neutron stars carried out within the frameworks of both relativistic magnetohydrodynamics (MHD) and resistive force-free electrodynamics. The results of force-free simulations reveal the inability of our numerical method to accommodate the equatorial current sheets of pulsar magnetospheres, and raise a question mark about the robustness of this approach. On the other hand, the MHD approach allows us to make significant progress. We start with a non-rotating magnetically dominated dipolar magnetosphere and follow its evolution as the stellar rotation is switched on. We find that the time-dependent solution gradually approaches a steady state that is very close to the stationary solution of the pulsar equation found in 1999 by Contopoulos, Kazanas & Fendt. This result suggests that other stationary solutions that have the Y-point located well inside the light cylinder are unstable. The role of particle inertia and pressure on the structure and dynamics of MHD magnetospheres is studied in detail, as well as the potential implications of dissipative processes in the equatorial current sheet. We argue that pulsars may have differentially rotating magnetospheres which develop noticeable structural oscillations, and that this may help to explain the nature of the subpulse phenomena.     

Non-stationary pair plasma in a pulsar magnetosphere and the two-stream Instability

We test a new emission mechanism in pulsar magnetospheres, eventually responsible in part for the high level of observed radio radiation. This is carried out by comparing the efficiency of the two-stream instability of Langmuir waves in a pulsar emission region, where the stationary and non-stationary characters of pair plasma outflows produced in the gap region are characterized by two different time-scales. On the shorter time-scale, the Ruderman &38; Sutherland 'sparking' phenomenon leads to the creation of pair plasma clouds, in motion along magnetic field lines, that contain particles with a large spectrum of momenta. The overlapping of particles with different energies produced in successive clouds results in an efficient 'two stream'-like instability. This effect is a consequence of the non-stationary character of the pair plasma produced in the gap region, just above the magnetic poles of the neutron star. On a long time-scale, resulting pair plasma outflows in pulsar magnetospheres can be treated as stationary. In this case, the instability which results from interaction between existing primary beam particles and the pair plasma is negligible, whereas the instability owing to interaction between electrons and positrons of the pair plasma itself, and more precisely to their relative drift motion along curved magnetic field lines, is effective. We derive characteristic features of the triggered instability, using specific distribution functions to describe either particles in the assembly of clouds or relative drifting of electrons and positrons in these same plasma clouds. Although linear and local, our treatment suggests that non-stationary effects may compete with, or even dominate over, drifting effects in parts of pulsar emission regions.     

Neutron Stars in the Subsonic Propeller Stage

In the evolutionary tracks of magnetized compact stars the subsonic propeller state is intermediate between the supersonic propeller and accretor states. The rotation rate of a star in this stage decreases because of the interaction of its magnetosphere with the surrounding hot quasistatic shell. The radius of the magnetosphere is less than the corotation radius, and the boundary of the magnetosphere is stable with respect to inter-change instabilities. The mass flow rate from the inner radius of the shell to the surface of the compact object is limited by the rate at which plasma diffuses into the magnetic field of the star. Because of this, a subsonic propeller will show up as a low (or moderate) luminosity accretion pulsar with a soft x-ray spectrum.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 477–490 (August 2005).     

Formation of Hydrogen Emission Lines in the Magnetospheres of Young Stars

Astronomy Letters - The formation of hydrogen emission lines in the magnetospheres of young stars is considered. The magnetosphere is assumed to be formed by a dipolar magnetic field whose axis is...     

Pulsar radiation belts and transient radio emission

It is proposed that radiation belts similar to the ones in the planetary magnetosphere can exist for a pulsar with a relatively long period and a strong magnetic field. In the belts located in the closed field line region near the light cylinder relativistic pairs are trapped and maintained at a density substantially higher than the local Goldreich–Julian corotation density. The trapped plasma can be supplied and replenished by either direct injection of relativistic pairs from acceleration of externally supplied particles in a dormant outer gap or in situ ionization of the accreted neutral material in the trapping region. The radiation belts can be disrupted by waves that are excited in the region as the result of plasma instabilities or emitted from the surface due to starquakes or stellar oscillations. The disruption can cause an intermittent particle precipitation towards the star producing radio bursts. It is suggested that such bursts may be seen as rotating radio transients.     

Acceleration of charged particles in the magnetosphere of a collapsing star and their nonthermal electromagnetic radiation

We investigate a transformation of a magnetic field and plasma in nonhomogeneous magnetospheres of collapsing stars with a dipole initial magnetic field and certain initial energy distributions of particles in the magnetosphere as the power low, relativistic Maxwell and Boltzmann. The betatron mechanism of the charged particles acceleration in a collapsing star’s magnetosphere is considered. When a magnetized star is compressed in the stage of the gravitational collapse, the magnetic field increases strongly. This variable magnetic field generates a vortical electric field. Our calculations show that this electric field will accelerate charged particles up to relativistic velocities. Thus, collapsing stars may be sources of high energy cosmic rays in our galaxy as in others. The acceleration of particles during the collapse happens mostly in polar regions of the magnetosphere that leads to polar relativistic streams (jets) formation. When moving in a magnetic field, these particles will generate nonthermal electromagnetic radiation in a broad electromagnetic wavelength band from radioto gamma rays. Thus, in the stage of the gravitational collapse, relativistic jets are formed in stellar magnetospheres. These jets are powerful sources of the nonthermal electromagnetic radiation.     

A magnetohydrodynamic model of whistler duct structure in the magnetosphere

In this study, the physical structure for the propagation of whistler waves within a duct in the Earth's magnetosphere is investigated by means of magnetohydrodynamic (MHD) theory. Expressions for the current density and induced magnetic field are determined analytically and evaluated in terms of two models for the duct plasma density distribution. It is found that once the duct is formed, forces associated with the current structure will maintain it. MHD instabilities are examined briefly and found to be unlikely to threaten duct maintenance in regions where whistlers are typically observed. Examination of some effects of field-aligned currents suggest that this may be a viable mechanism for duct formation.     

The energy loss of a rotating magnetized neutron star

The analysis of observations of pulsar B1931+24 shows that the mechanism of the spin-down of a rotating magnetized neutron star is due to the plasma generation in its magnetosphere and, consequently, the radio emission generation. The unique observation of the switch on and switch off of this pulsar allows us to distinguish between the energy loss in the absence of radio emission (the magnetodipole radiation) and the current loss due to the rotation energy expenditure to the relativistic plasma generation and acceleration in the pulsar magnetosphere. The inclination angle χ, the angle between the rotation axis and the magnetic dipole axis, can be stationary for this pulsar,  χ=χ_st  . From observations and theory it follows that  χ_st= 59°  .     

The termination shock in a striped pulsar wind

The origin of radio emission from plerions is considered. Recent observations suggest that radio-emitting electrons are presently accelerated rather than having been injected at early stages of the plerion evolution. The observed flat spectra without a low-frequency cut-off imply an acceleration mechanism that raises the average particle energy by orders of magnitude but leaves most of the particles at an energy of less than approximately a few hundred MeV. It is suggested that annihilation of the alternating magnetic field at the pulsar wind termination shock provides the necessary mechanism. Toroidal stripes of opposite magnetic polarity are formed in the wind emanating from an obliquely rotating pulsar magnetosphere (the striped wind). At the termination shock, the flow compresses and the magnetic field annihilates by driven reconnection. Jump conditions are obtained for the shock in a striped wind. It is shown that the post-shock magnetohydrodynamic parameters of the flow are the same as if the energy of the alternating field had already been converted into plasma energy upstream of the shock. Therefore, the available estimates of the ratio of the Poynting flux to the matter energy flux, σ, should be attributed not to the total upstream Poynting flux but only to that associated with the average magnetic field. A simple model for the particle acceleration in the shocked striped wind is presented.     

On the analogy between the zebra patterns in radio emission from the sun and the crab pulsar

We investigate the close analogy between the solar radio emission with a quasi-harmonic spectrum structure and one of the microwave emission components of the Crab pulsar in the form of the so-called zebra pattern. The radio emission mechanism of this component can be provided by instability at double plasma resonance and can be realized in extraordinary (for a radio pulsar) conditions, namely in a nonrelativistic plasma with a relatively weak magnetic field. We point out possible models of the emission source in the form of a magnetic trap or a neutral current sheet with a transverse magnetic field localized in the corotating region of the pulsar magnetosphere far from the neutron star surface.     

On pair production in intense electromagnetic fields occurring in astrophysical situations

We show that the pair production rate in a strong magnetic field is substantially altered when an electric field is also included. We illustrate and emphasize this significant alteration by considering a few special cases. In the vicinity of the polar cap of a rotating magnetized neutron star it is currently though thatboth steady electric and magnetic fields must be present. The results presented here then indicate that some considerable degree of caution must be exercised in applying pair production rates calculated under the assumption of zero electric field to the problems of pulsar emission and the generation and maintence of pulsar magnetospheres. In general such rates are very different from the rate computed allowing for the existence of an electric field.     

Tearing instability in the pulsar magnetosphere

Equations governing the coupling of the scalar and vector potentials for a resistive electron-positron plasma in a strong magnetic field are derived. It is shown that in the presence of magnetic shear, a tearing instability may occur. The latter can lead to magnetic field line reconnection and the formation of magnetic islands which could affect the dynamics of the pulsar magnetosphere.     

Pulsar investigation using interstellar scintillation

We discuss the resolution of pulsar magnetospheres using interstellar scintillation. The two-dimensional spatial structure of pulsar emission zones can be obtained from analysis of diffractive scintillations at low frequencies. Based on refractive and diffractive scintillation of pulsars we can also reconstruct the distribution of turbulent plasma along the line of sight, and using this analysis a new approach to pulsar distance estimation can be made. This revised version was published online in July 2006 with corrections to the Cover Date.     

Numerical construction of magnetosphere with relativistic two-fluid plasma flows

We present a numerical model in which a cold pair plasma is ejected with relativistic speed through a polar cap region and flows almost radially outside the light cylinder. Stationary axisymmetric structures of electromagnetic fields and plasma flows are self-consistently calculated. In our model, motions of positively and negatively charged particles are assumed to be determined by electromagnetic forces and inertial terms, without pair creation and annihilation or radiation loss. The global electromagnetic fields are calculated by the Maxwell's equations for the plasma density and velocity, without using ideal magnetohydrodynamic condition. Numerical result demonstrates the acceleration and deceleration of plasma due to parallel component of the electric fields. Numerical model is successfully constructed for weak magnetic fields or highly relativistic fluid velocity, i.e. kinetic energy dominated outflow. It is found that appropriate choices of boundary conditions and plasma injection model at the polar cap should be explored in order to extend present method to more realistic pulsar magnetosphere, in which the Poynting flux is dominated.