The role of multipolar magnetic fields in pulsar magnetospheres     

Estelle Asseo  David Khechinashvili 《Monthly notices of the Royal Astronomical Society》2002,334(4):743-759

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.  相似文献   

Synchrotron emission from anisotropic disordered magnetic fields     

R.A. Laing 《Monthly notices of the Royal Astronomical Society》2002,329(2):417-422

We derive expressions for the total and linearly polarized synchrotron emissivity of an element of plasma containing relativistic particles and disordered magnetic field that has been sheared or compressed along three independent directions. Our treatment follows that given by Matthews & Scheuer in the special case of a power-law electron energy spectrum. We show that the emissivity integrals depend on a single parameter, making it straightforward to generate one-dimensional look-up tables. We also demonstrate that our formulae give identical results to those in the literature in special cases.  相似文献   

1993年5月日面AR7500中暗条的动力学特征     

姜云春  栾蒂 《天文研究与技术》2001,(1):1-18

本文利用光球磁场、色球Hα单色像和Hβ速度场等观测资料，分析了1993年5月日面AR7500中3个暗条的演化和动力学行为，得出4个结论(1)3个暗条中两个是右旋暗条，一个是左旋暗条。(2)暗条附近两侧的色球纤维和光球横场几乎平行于暗条长轴，暗条端点处的黑子没有呈现明显的涡旋结构。(3)尖角处因为轴向场取向不同，一直没有发生暗条合并，即使其中一个右旋暗条消失后又重新形成也如此。(4)几天持续存在的左旋暗条，在两天的观测中未出现扰动激活，其中部为杂乱而不明显的运动图案。本文还讨论了可以用暗条的扭曲磁流绳模型来解释暗条的这些动力学行为，以及一些尚待进一步澄清的问题。  相似文献   

Spatial Distribution of Magnetic CP Stars     

D. O. Kudryavtsev  I. I. Romanyuk 《Astrophysics》2003,46(2):234-242

The spatial distribution of magnetic CP stars is studied using the catalog of CP stars compiled by the Special Astrophysical Observatory of the Russian Academy of Sciences. It is shown that the overwhelming majority of these objects are at distances closer than 500 pc from the sun. The axes of rotation and the magnetic axes of the field CP stars are distributed randomly, while a preferential orientation of the magnetic axes in certain directions is seen for stars that are members of open clusters. The closeness of the magnetic models for CP stars that are members of clusters may be evidence that there is a general factor which influences the origin of magnetic fields in stars. The magnetic field of the galaxy may be such a factor.  相似文献   

Temporal fluctuations in the differential rotation of cool active stars     

J.-F. Donati  A. Collier Cameron  P. Petit 《Monthly notices of the Royal Astronomical Society》2003,345(4):1187-1199

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Precession of isolated neutron stars – II. Magnetic fields and type II superconductivity     

Ira Wasserman 《Monthly notices of the Royal Astronomical Society》2003,341(3):1020-1040

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Probing sunspot magnetic fields with p-mode absorption and phase shift data   总被引：1，自引：0，他引：1  

P. S. Cally †  A. D. Crouch  D. C. Braun 《Monthly notices of the Royal Astronomical Society》2003,346(2):381-389

Long-standing observations of incoming and outgoing f- and p-modes in annuli around sunspots reveal that the spots partially absorb and substantially shift the phase of waves incident upon them. The commonly favoured absorption mechanism is partial conversion to slow magneto-acoustic waves that disappear into the solar interior channelled by the magnetic field of the sunspot. However, up until now, only f-mode absorption could be accounted for quantitatively by this means. Based on vertical magnetic field models, the absorption of p-modes was insufficient. In this paper, we use the new calculations of Crouch & Cally for inclined fields, and a simplified model of the interaction between spot interior and exterior. We find excellent agreement with phase shift data assuming field angles from the vertical in excess of 30° and Alfvén/acoustic equipartition depths of around 600–800 km. The absorption of f-modes produced by such models is considerably larger than is observed, but consistent with numerical simulations. On the other hand, p-mode absorption is generally consistent with observed values, up to some moderate frequency dependent on radial order. Thereafter, it is too large, assuming absorbing regions comparable in size to the inferred phase-shifting region. The excess absorption produced by the models is in stark contrast with previous calculations based on a vertical magnetic field, and is probably due to finite mode lifetimes and excess emission in acoustic glories. The excellent agreement of phase shift predictions with observational data allows some degree of probing of subsurface field strengths, and opens up the possibility of more accurate inversions using improved models. Most importantly, though, we have confirmed that slow mode conversion is a viable, and indeed the likely, cause of the observed absorption and phase shifts.  相似文献   

The generation of a large-scale Galactic magnetic field by electric currents of energetic particles     

A. Z. Dolginov  I. N. Toptygin 《Astronomy Letters》2003,29(6):374-382

We consider the generation of a magnetic field in the Galaxy by the electric currents excited by cosmic-ray particles in the disk and halo. We assume that the sources of relativistic particles are distributed continuously and uniformly in the Galactic disk, their total power is equal to the observed value, and the particles themselves undergo anisotropic diffusion in a homogeneous medium. We take into account the differential rotation of the Galactic disk but disregard the turbulence gyrotropy (the α effect). The strength of the generated magnetic field in our model is shown to strongly depend on the symmetry of the relativistic proton and thermal electron diffusion tensors, as well as on the relations between the tensor components. In particular, if the diffusion is isotropic, then no magnetic field is generated. For the independent tensor components estimated from observed parameters of the Galactic medium and with a simultaneous allowance made for the turbulent field dissipation processes, the mechanism under consideration can provide an observable magnetic-field strength of the order of several microgauss. This mechanism does not require any seed magnetic field, which leads us to suggest that relativistic particles can give an appreciable and, possibly, determining contribution to the formation of the global Galactic magnetic field. However, a final answer can be obtained only from a nonlinear self-consistent treatment, in which the symmetry and magnitude of the particle diffusion tensor components should be determined together with the calculation of the magnetic field.  相似文献   

A revised procedure for discharge measurement by means of the salt dilution method     

Dag Hongve 《水文研究》1987,1(3):267-270

Discharge measurement by means of injection of a NaCl-solution and integration of the electrical conductivity as a function of time is a traditional and well-documented method for use in turbulent streams. The ease of the chemical measurements permits results to be calculated in the field. This work has shown that dry fine-grained salt may substitute the salt solution. Correction factors for water temperature and background conductivity are derived from experiments. A simple procedure for calculation of the discharge is described.  相似文献   

BOOK REVIEWS     

《The Professional geographer》1988,40(1):116-131

Books reviewed in this article: Boundary Control and Legal Priniciples . Curtis M. Brown , Walter G. Robillard and Donald A. Wilson . Lost Initiatives: Canada's Forest Industries, Forest Policy, and Forest Conservation . R. Peter Gillis and Thomas R. Roach . The Navajo Atlas: Resources, People, and History of the Dine Bikeyah . James M. Goodman . The City and the Sign . M. Gottdiener AND Alexandros Ph . Lagopoulos , EDS. The Comfortable House: North American Suburban Architecture 1890–1930 . Alan Gowans . Arabic-Islamic Cities. Building and Planning Principles . Besim Salim Hakim . Contemporary Climatology . Ann Henderson -Sellers and Peter J. Robinson . A Trace of Desert Waters . Samuel G. Houghton . Geography, Resources and Environment. Vol. 1: Selected Writings of Gilbert F. White. Vol. 2: Themes from the Work of Gilbert F. White . Robert W. Kates AND Ian Burton , ED. Spatial Dimensions of Unemployment and Underemployment (A Case Study of Rural Punjab) . Gopal Krishan . Maps of the Holy Land: Cartobibliography of Printed Maps, 1475–1900 . Compiled by Eran Laor , assisted by Shoshana Klein . The Geography of Third World Cities . Stella Lowder . A Perspective on U.S. Farm Problems and Agricultural Policy . Lance Mc Kinzie , Timothy G. Baker AND Wallace E. Tyner . Soil Erosion and Its Control. R.P.C . Morgan , ED Geopolitics . Patrick O'Sullivan . Landscape, Meanings and Values . Edmund C. Penning -Rowsell AND David Lowenthal , EDS.  相似文献