脉冲星磁场的演化

综述了前人对于单个脉冲星磁场的起源和演化的研究结果及其最新进展。脉冲星磁场的起源有多种模型，其所对应的初始磁场有两种位形：磁场束缚在核内和磁场束缚在壳层中。脉冲星的磁场如何演化，没有一致的结论。有各种观测证据可能直接或间接表明磁场的演化行为，如根据特征年龄和运动学年龄的差异可以推断出脉冲星磁场按指数规律衰减，而根据特征年龄与超新星遗迹年龄的差异或几颗年轻脉冲星的制动指数可以认为年轻脉冲星的磁场可能是增强的。脉冲星的样本合成研究（数值模拟）是研究脉冲星磁场演化的重要方法。模拟结果表明，假定脉冲星磁场按指数衰减，特征衰减时标必须为10^7yr或更长。而壳层中磁场的欧姆耗散模型数值计算显示脉冲星磁场演化行为因冷却模型和状态方程的取法不同而异，但最终无明显的衰减。由自转变慢诱导的脉冲星核内部磁场向壳层中扩散模型的计算表明脉冲星磁场的衰减只发生在10^7-10^8yr这段时间内，磁场衰减1－2个量级。     

银道面上下的奇对称环向磁场

在分析河外射电源和脉冲星的法拉弟旋度（RotationMeasure，RM）时，发现银道面上下有规则的奇对称环向磁场分量，这是任何银盘磁场模型所无法解释的．这种奇对称的环向磁场说明在银冕中运行着AO模式的Dynamo。     

Solar magnetic fields and convection

The traditional model of solar magnetic fields is based on convection which dominates generally weak, diffuse fields and so tends to create increasingly tangled fields. Surplus fields must be eliminated by merging of opposite polarities; for example a solar dynamo of period≈10 yr requires fields to be reduced to a scale of<100 km or diffusivity to be increased by a factor of≈10⁷ over molecular diffusivity. It is now shown that the true requirements of any diffuse-field theory are far more stringent, and that surplus fields must be eliminated within a single eddy period of 1 day (10 min) for the supergranules (granules). The reason is that during that period fresh fields are created with flux and energy comparable with those of the old fields. The numerical models of Weiss and Moss are used to confirm this result which is fatal to all diffuse-field theories. The basic error in these theories is found in the assumption that because heat and other passive properties of a fluid diffuse much faster in the presence of turbulence, passive magnetic fields should do likewise. The error is that the heat content of an eddy is not increased by the motion while the magnetic flux and energy are increased rapidly. It is shown that the observed concentrations of surface fields into strengths of?100 G cannot be accounted for by observed surface motions. Nor are they accounted for by the numerical models of turbulence of Weiss or Moss whatever values of the magnetic Reynolds number are assumed. A detailed comparison is made between both small-scale and large-scale surface magnetic features and the predictions of the diffuse-field theory. The differences appear irreconcilable and the features only explicable in terms of the twisted flux-rope model.     

Deep colour–magnitude diagrams of LMC field stars imaged with HST

We present deep photometry ( V ≲26) in V and I bands obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope for 7 fields ∼5° away from the Large Magellanic Cloud centre. The fields contain, typically, 2000 stars each. Isochrones were fitted to the colour–magnitude diagrams in order to identify different star populations in these fields. An old population ( τ >10 Gyr) has been found in all fields. Some events of enhanced star formation, with ages between 2 and 4 Gyr, were identified in the fields localized in the north to north-west regions. Luminosity functions of low-mass stars were also obtained for all fields. Kolmogorov Smirnov test results suggest differences smaller than 30 per cent in the mixture of stellar populations contributing to the fields. Finally, density profiles were derived for old and intermediate-age stars. The former shows a slightly steeper decline than the latter.     

Note concerning gravitation and electromagnetism

The Maxwell equations for gravitational fields previously assumed by Sciama are derived from elementary considerations. The Lagrangian for a gravitating mass in a non-inertial coordinate system yields equations of motion leading to force definitions for a gravitational field intensity and a gravitational induction field. The non-inertial velocity of the coordinate system plays the role of a vector potential contributing to the generalized momenta of bodies moving in the system. A Lagrangian density constructed from the force-defined fields then lead to the source definitions of gravitational fields. It is found that positive field energy densities require repulsive gravitational forces, whereas attractive forces imply the violation of the conservation of energy. This paradox is resolved by representing gravitational quantities as pure-imaginary entities. Thus characterized, the equations which define gravitational fields become identical to Maxwell's equations but are pure-imaginary. This suggests a combined representation for gravitational and electromagnetic fields which, in covariant form, indicates both the well known equivalence of mass and energy and a possible equivalence of charge and energy. From orthogonality considerations, it is conjectured that this latter energy is gravitational, and that, whereas gravitational fields interact with electromagnetic energy, electromagnetic fields interact with gravitational energy. Parts of this work were completed at Air Force Cambridge Research Laboratories, Bedford, Mass., U.S.A.     

Investigation of Weakly Magnetic CP Stars. I

The magnetic fields of a sample of chemically peculiar stars, in which they had not been detected earlier, are reinvestigated on the basis of our own measurements and literature data. Despite the considerably higher measurement accuracy, again no magnetic fields were detected in any of them. An upper limit on the field of HD 10221 of 40 G was obtained. Despite the weakness of the fields, the parameters characterizing an anomaly in chemical composition correspond to those for strong fields. The same parameters in stars with strong fields generally correspond in value, although they are lower than expected in the case of HD 47152.     

The calculation of force-free fields from discrete flux distributions

This paper presents particularly simple mathematical formulas for the calculation of force-free fields of constant α from the distribution of discrete sources on a flat surface. The advantage of these formulas lies in their physical simplicity and the fact that they can be easily used in practice to calculate the fields. The disadvantage is that they are limited to fields of ‘sufficiently small α’. These formulas may be useful in the study of chromospheric magnetic fields by the comparison of high-resolution Hα photographs and photospheric magnetograms.     

Differential rotation of solar magnetic fields

The connection of the differential rotation of solar magnetic fields with the field sign and strength is studied. The synoptic maps of magnetic fields over the last three solar cycles taken at the Kitt Peak Observatory served as input data for the study. The algorithm of magnetic field filtering over 14 chosen strengt intervals and successive 5-degree latitude zones was applied to these data. The Fourier transform of the time series obtained was then used. Analysis of the power spectra led to the conclusion that there are two types of magnetic fields. These differ in strength (0–50 and 50–700 G) and rotation characteristics. The rotation differentiality for strong magnetic field is almost twice as large as that for weak magnetic fields.     

A new numerical scheme for structures of rotating magnetic stars

We have developed a new numerical scheme for obtaining structures of rapidly rotating stars with strong magnetic fields. In our scheme, both poloidal and toroidal magnetic fields can be treated for stars with compressibility and infinite conductivity. By introducing the vector potential and its integral representation, we can treat the boundary condition for the magnetic fields across the surface properly. We show structures and distributions of magnetic fields as well as the distributions of the currents of rotating magnetic polytropic stars with polytropic index   N = 1.5  . The shapes of magnetic stars are oblate as long as the magnetic vector potential decreases as 1/ r when   r →∞  . For extremely strong magnetic fields, equilibrium configurations can be of toroidal shapes.     

Relation of the green coronal line intensity to sunspot areas and magnetic fields of different scales

The intensity of Fe XIV 530.3-nm green coronal line is compared quantitatively with the strength of magnetic fields of small and large scales and also with total sunspot areas for 1977–2001. A degree of similarity of appropriate synoptic maps is evaluated using correlation analysis. The green line intensity maps are constructed from data of its daily monitoring. Strengths of magnetic fields are calculated in a potential approximation using the photosphere observations of Wilcox Solar Observatory for a distance of 1.1 The calculations are performed separately for fields of large and small spatial scales. The total area of sunspots is obtained using data from the Greenwich Catalogue and its continuation by USAF/NOAA. The correlation has been calculated for the aggregate of areas (with a size of 20° in latitude and 30° in longitude) coinciding spatially on all maps. It is found that the most correlation between the green line intensity and coronal fields of small scales is observed in a zone of 0°–20°. The correlation with total sunspot areas (i.e., with local fields at the photosphere level) is substantially less here. In the higher-latitude zone 20°–40°, correlation of the green-line intensity with spot areas and small-scale coronal fields decreases. The large-scale fields have little influence on the green-line emission in the spot-formation zone. These results are the evidence of a complex nature of the effect of different-scale fields, arising as a result of dynamo activity in the subsurface (leptocline) and deep-lying (tachocline) layers of the convective zone, on the processes of the Sun’s corona heating and green coronal line emission.     

Computation of inner coronal magnetic fields from longitudinal field components on a spherical photosphere

A method is developed which for a certain day permits the approximate calculation of closed small and large scale magnetic field lines. From the photospheric longitudinal components of the magnetic field measured at this day normal components are derived taking into account the curvature of the solar surface. The magnetic fields are assumed to be potential or force-free fields.The method is applied to observations of September 5 and September 7, 1973. The projected magnetic field lines are compared with the loop structures which are visible in XUV pictures taken on these days. In the cases where no good agreement could be obtained for potential fields, force-free fields are calculated and fitted to the observed structures.     

On the screening of the interstellar electric and magnetic fields by the solar wind

Fast-streaming solar-wind plasma with high conductivity screens the heliosphere from the penetration of the interstellar electric and magnetic fields. The simplest model with the constant solar wind conductivity and radial velocity is considered and the boundary electrodynamic problem is solved for static external fields. The results show that screening of the external fields takes place in the heliosphere according to the exponential law.     

Comparison of Polar and Equatorial Magnetic Fields Near Sunspot Minimum

We investigate the polar magnetic fields near sunspot minimum using high-resolution videomagnetograph data from Big Bear Solar Observatory. To avoid the problem of center-to-limb variation of the projected longitudinal field, we compare polar with equatorial field strengths for the same limb distance. Polar fields are stronger than the quiet equatorial field, but no greater than equatorial limb data containing unipolar regions. The difference is entirely in the stronger field elements. The polar background fields are of mixed polarity but show a net weak field opposite in sign to that of the stronger polar elements. We believe this to be the first evidence of widespread background field. No dependence of the measured signal on the B-angle was found, so the high-latitude fields do not change strength near the pole. Further, there was no significant change in the polar fields in the 15-month period studied. We tried to derive a high-latitude rotation rate; our data show motion of high-latitude magnetic elements, but the diurnal trajectory is not much bigger than random motions and field changes, so the result is inconclusive. We suggest that the polar fields represent the accumulation of sunspot remnants, the elements of which last for years in the absence of other fields.     

Reversals of the Sun’s Polar Magnetic Fields in Relation to Activity Complexes and Coronal Holes

A spatiotemporal analysis of long-term measurements of the Sun’s magnetic field was carried out to study changes in its zonal structure and reversals of the polar fields in Cycles 21?–?24. A causal relationship between activity complexes, their remnant magnetic fields, and high-latitude magnetic fields has been demonstrated in the current cycle. The appearance of unipolar magnetic regions near the poles is largely determined by the decay of long-lived activity complexes. The nonuniform distribution of sunspot activity and its north–south asymmetry result in the asymmetry of remnant fields that are transported poleward due to meridional circulation. The asymmetry of high-latitude magnetic fields leads to an asynchrony of polar-field reversals in both hemispheres. The interaction of high-latitude unipolar magnetic regions with the polar fields affects the embedded coronal holes. The evolution of large-scale magnetic fields was also studied in a time–latitude aspect. It is shown that regular reversals of the Sun’s polar fields resulted from cyclic changes in high-latitude magnetic fields. A triple polarity reversal of the polar fields in Cycle 21 and short-term polarity alternations at the poles were interpreted taking into account the interaction of the remnant fields with the Sun’s polar fields.     

关于LAMOST天体测量标准天区选择的建议

简述已有几个天体测量标准天区的大小、星数以及恒星位置和自行的精度；给出LAMOST天体测量标准天区选择的原则；介绍研究疏散星团和古德带附近星团观测的意义；列出13个LAMOST天体测量标准天区J2000．0历元的赤经和赤纬、银经和银纬，如果疏散星团位于这个标准天区，也给出它们的日心距、红化值、金属丰度和年龄。     

无力磁场的稳定性

无力场被广泛用来模拟太阳活动区的强磁场,本文从Bernstein能量原理出发,导出了无力场能量原理的普遍形式,并给出了若干稳定性的充分条件,它们可方便而有效地对无力场进行稳定性判断。     

Magnetic field evolution in OBA stars

The applications of the spectral analysis methods discovered by Kirchhoff for the investigation of stellar magnetic fields are considered. The statistical properties of the mean magnetic fields for OBA stars have been investigated by analyzing data from two catalogs of magnetic fields. It is shown that the mean effective magnetic field ℬ of a star can be used as a statistically significant characteristic of its magnetic field. The magnetic field distribution functions F(ℬ) have been constructed for B-type and chemically peculiar (CP) stars, which exhibit a power-law dependence on ℬ. A sharp decrease in F(ℬ) in the range of weak magnetic fields has been found. The statistical properties of the magnetic fluxes for main-sequence stars, white dwarfs, and neutron stars are analyzed.     

非线性无力场模型的线性稳定性

本文从能量原理出发，对无力场稳定性进行了研究，给出了一般情形下无力场稳定性的充要判据。它可以把线性无力场稳定性判据作为特例包括在内。还对Ｋｒｕｇｅｒ给出的一个充分判据作了进一步的探讨，对无力场稳定性的物理图象也作了讨论。     

The α effect with imposed and dynamo-generated magnetic fields

Estimates for the non-linear α effect in helical turbulence with an applied magnetic field are presented using two different approaches: the imposed-field method where the electromotive force owing to the applied field is used, and the test-field method where separate evolution equations are solved for a set of different test fields. Both approaches agree for stronger fields, but there are apparent discrepancies for weaker fields that can be explained by the influence of dynamo-generated magnetic fields on the scale of the domain that are referred to as meso-scale magnetic fields. Examples are discussed where these meso-scale fields can lead to both drastically overestimated and underestimated values of α compared with the kinematic case. It is demonstrated that the kinematic value can be recovered by resetting the fluctuating magnetic field to zero in regular time intervals. It is concluded that this is the preferred technique both for the imposed-field and the test-field methods.