The drift model of “magnetars”

It is shown that the drift waves near the light cylinder can cause the modulation of emission with periods of order several seconds. These periods explain the intervals between successive pulses observed in AXPs, SGRs and radio pulsars with long periods. The model under consideration gives the possibility to calculate real rotation periods P of host neutron stars. It is shown that P≤1 s for the investigated objects. The magnetic fields at the surface of the neutron star are of order 10¹¹–10¹³ G and equal to the fields usual for the known radio pulsars.      

To the problem of solar coronal heating

We consider the adequacy of various solar coronal heating models. We show that the correlation between the intensity of the coronal Fe XIV 530.5 nm green line and the calculated magnetic field strength in the solar corona can be a useful tool for this purpose. We have established this correlation for coronal structures and magnetic fields of large spatial and temporal scales. The correlation found exhibits a strong dependence on both solar cycle phase and heliolatitude. The efficiency of a particular coronal heating mechanism is probably determined by the relative area occupied by low and high loops (including open structures). The direct current models based on slow field dissipation (DC) and the wave models based on Alfvén and magnetosonic wave dissipation (AC) are more efficient in the equatorial and polar zones, respectively.     

Ni I 6768 Å line profile variations during a solar flare and their effect on the SOHO/MDI magnetic field measurements

Observational data on the Ni I 6768 Å line profile variations during the impulsive and post-impulsive phases of the July 18, 2002 while light flare (WLF) in the kernel of WLF emission and in other flare kernels are presented. The line profiles at the sites of intense photospheric motions in active regions are also studied. The effect of the observed Ni I 6768 Å line profile variations on the SOHO/MDI magnetic field measurements is estimated. The following conclusions have been reached. (1) The thermodynamic structure of the photo-spheric layers changes significantly during the flare. As a result, the Ni I line profile changes, particularly at the site of WLF emission. At this time, the line depth decreases significantly, but the line does not show any emission reversal. Subsequently, a relatively slow return to the conditions of an undisturbed photosphere is observed. (2) The technique of SOHO/MDI magnetic field measurements is insensitive to such line variations. Therefore, the detected variations during the flare did not result in any noticeable errors in the MDI longitudinal magnetic field measurements. (3) The line profile is broadened, shifted as a whole, and asymmetric at the sites of active regions where intense photospheric motions appear. In the MDI measurements, such changes in the profile lead to an underestimation of the magnetic field by approximately 10% if the line-of-sight velocity of the photo-spheric ejection is about 1.6 km s^?1.     

An Alternative Explanation for a Screw-like Meteoric Train Photographed by Double-Station Observations

Two-station observation of meteors, especially a meteor trains, provides an effective approach to the measurement of the physical parameters. We have collected four special groups of photographs of meteoric trains taken at two stations during Leonids 2001. One representative group has been measured and analyzed in detail. An analysis has been reported in our first paper. In this paper, an alternative explanation for the screw-like meteoric train is suggested based on some physical calculations. The results reveal that this train has a screw-like structure and, apparently, spoke beams. The mother meteor of this train may be negatively charged and moves forward along a left-hand screw trajectory under the effect of the geomagnetic field. The spoke beams might be the visual effect of the long time exposure of many particles released from the disintegrated meteoroid.     

The role of multipolar magnetic fields in pulsar magnetospheres

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, 'neutral points' and 'neutral lines' exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these 'favourable' subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.     

The influence of Hall drift on the magnetic fields of neutron stars

We consider the evolution of magnetic fields under the influence of Hall drift and Ohmic decay. The governing equation is solved numerically, in a spherical shell with   r _i / r _o = 0.75  . Starting with simple free-decay modes as initial conditions, we then consider the subsequent evolution. The Hall effect induces so-called helicoidal oscillations, in which energy is redistributed among the different modes. We find that the amplitude of these oscillations can be quite substantial, with some of the higher harmonics becoming comparable with the original field. Nevertheless, this transfer of energy to the higher harmonics is not sufficient to accelerate significantly the decay of the original field, at least not at the   R _B = O (100)  parameter values accessible to us, where this Hall parameter   R _B   measures the ratio of the Ohmic time-scale to the Hall time-scale. We do find clear evidence though of increasingly fine structures developing for increasingly large   R _B   , suggesting that perhaps this Hall-induced cascade to ever-shorter length-scales is eventually sufficiently vigorous to enhance the decay of the original field. Finally, the implications for the evolution of neutron star magnetic fields are discussed.     

Magnetic flux separation in photospheric convection

Three-dimensional non-linear magnetoconvection in a strongly stratified compressible layer exhibits different patterns as the strength of the imposed magnetic field is reduced. There is a transition from a magnetically dominated regime, with small-scale convection in slender hexagonal cells, to a convectively dominated regime, with clusters of broad rising plumes that confine the magnetic flux to narrow lanes where fields are locally intense. Both patterns can coexist for intermediate field strengths, giving rise to flux separation: clumps of vigorously convecting plumes, from which magnetic flux has been excluded, are segregated from regions with strong fields and small-scale convection. A systematic numerical investigation of these different states shows that flux separation can occur over a significant parameter range and that there is also hysteresis. The results are related to the fine structure of magnetic fields in sunspots and in the quiet Sun.     

Helicoidal magneto-electron waves in interstellar molecular clouds

We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons – weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds.