共查询到20条相似文献，搜索用时 722 毫秒
1.
K. G. Ivanov 《Astrophysics and Space Science》1974,31(2):287-294
The theory of strong discontinuities in plasma with anisotropic pressure is applied for interpretation of the recent plasma and magnetic experiments on the boundary and in the tail of the magnetosphere. The properties of the discontinuities are described. It is supposed that on the boundary and in the tail of the magnetosphere anisotropic discontinuities occur with a nonzero normal component of the magnetic field. The general consequence of this assumption is the existence of the magnetospheric surface flow expanding from the subsolar point. The consequence does not contradict the data obtained on IMP 5. For the investigation of the low latitude part of the flow, the use of electrostatic analysers is desirable with the entrance oriented along the magnetospheric surface. The well-known qualitative scheme of the hydrodynamical flow with strong discontinuities in the tail is generalized by means of the three anisotropic discontinuities: the contact one coinciding with the magnetic neutral sheet, and the two discontinuities with the nonzero normal flow of the mass simulating the boundary of the plasma sheet. The result of the scheme of the components of the bulk velocity in the plasma sheet agrees with the recent observations on the Vela 4B. The scheme connects the well-known phenomena of the blocking and the thinning of the plasma sheet in the initial phase of the substorm. According to the general principles of the reconnection the dynamical dissipation is decreased by the blocking of the flow in the plasma sheet. The decrease leads to a drift of the plasma sheet boundary in the neutral sheet direction. The reverse picture is probably a result of a relaxation of the blocking. 相似文献
2.
首先是对太阳光球亮点近年来研究工作的总结。光球亮点是一种发生在太阳光球上宁静区域的的小尺度和短寿命增亮现象,平均直径在100～300knm之间,平均寿命约为几分钟。光球亮点的研究对于光球辐射和磁场性质的认识具有重要意义。过去的观测显示,绝大多数光球亮点的产生和演化与磁场,特别是光球上的小尺度磁场的演化密切相关,比如,两个同极性磁场的合并,或者反极性磁场的对消,或者一个同极性磁场的分裂,均可以促使光球亮点产生或消失。基于这样的观测结果,统计研究了2722个光球亮点(1600A)与光球上偶极磁元的关系,发现大约有1/3的光球亮点出现在偶极磁元中心附近。 相似文献
3.
本文在中子星磁层与吸积盘之间引入了一个速度、密度、压强和磁场都连续变化的有限厚度的剪切层,以代替Anzer理论中的切向间断面,用磁流体力学方法讨论了中子星磁层与吸积盘交界处等离子体可压缩情况下平面波扰动的K-H不稳定性。结果表明,K-H不稳定性依然存在,径向波矢扰动成为不稳定的主要模式。文中特别讨论了剪切层厚度取值对中子星自转的影响,表明适当调节剪切层厚度就可解释X射线脉冲星周期的变化。将此模型应用到脉冲X射线源Her X-1上,得到较好的结果。 相似文献
4.
To evalute the effect of the non-uniform surface on the radiation field, the upwelling radiation at the top of the atmosphere bounded by the checkerboard type of terrain is computed using the modified doubling method. The terrain is composed of the square Lambert surfaces with two different albedoes. The dimension of the each square is assumed to be 0.5–6 km. The radiance of the terrain is discussed with respect to the atmospheric effect. The observational site is located at altitude 30 km. The corresponding projected point on the ground is located at the center of a square. The solar and observational direction is located in the plane parallel to the checkerboard squares. The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecules, where the model aerosol is of the oceanic or the water soluble types.Numerical simulation exhibits the extraordinary effect near the edge of each squares. The radiance of the terrain depends upon the difference of albedoes and size of squares. It increases with the increase of the dimension of the square. It decreases with the optical thickness. At large optical thickness, the variation of radiation with zenith direction depends upon the aerosol characteristics. It shows little dependence on the solar zenith angle if less than 20°. 相似文献
5.
C. Alard 《Astrophysics and Space Science》1999,265(1-4):363-370
Microlensing is one of the most promising technique to probe the densityof dark matter in the Galactic Bulge. We review briefly
the history ofmicrolensing and comment on the discovery of high optical depthin the direction of the Bulge. This optical depth
is several timeslarger than the first theoretical predictions. We will show that someof the discrepancy can be resolved by
taking into account the effectof self amplification of stars into the Galactic Bar. We will alsoexplain that the optical depth
is contaminated with the contributionof faint unresolved stars. However, we emphasize that a categoryof sources, the bulge
giants are bright enough to escape the biasdue to unresolved sources. Finallywe show that even if self amplification in the
bar is taken into account, the optical depth to giants is hard to reproduce. We concludeby saying that in the near future
this excess in the bulge opticaldepth should be clarified and measured with good accuracy. In particulargood progress should
be made when the analyze of the last observationsof Bulge giants will be completed by the MACHO group. The futureimplementation
of the image subtraction technique in the data pipelinesshould also help to overcome the bias in the measurement of the opticaldepth
to turnoff stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Cezary Migaszewski Krzysztof Go&#;dziewski 《Monthly notices of the Royal Astronomical Society》2008,388(2):789-802
We present the secular theory of coplanar N -planet system, in the absence of mean motion resonances between the planets. This theory relies on the averaging of a perturbation to the two-body problem over the mean longitudes. We expand the perturbing Hamiltonian in Taylor series with respect to the ratios of semimajor axes which are considered as small parameters, without direct restrictions on the eccentricities. Next, we average out the resulting series term by term. This is possible thanks to a particular but in fact quite elementary choice of the integration variables. It makes it possible to avoid Fourier expansions of the perturbing Hamiltonian. We derive high-order expansions of the averaged secular Hamiltonian (here, up to the order of 24) with respect to the semimajor axes ratio. The resulting secular theory is a generalization of the octupole theory. The analytical results are compared with the results of numerical (i.e. practically exact) averaging. We estimate the convergence radius of the derived expansions, and we propose a further improvement of the algorithm. As a particular application of the method, we consider the secular dynamics of three-planet coplanar system. We focus on stationary solutions in the HD 37124 planetary system. 相似文献
7.
In the traditional axisymmetric models of the 11-year solar cycle, oscillations of the magnetic fields appear in the background of nonoscillating (over time scale considered) turbulent velocity fields and differential rotation. In this paper, an alternative approach is developed: The excitation of magnetic oscillations with the 22-year period is the consequence of hydrodynamic oscillations with the 11-year period. In the excitation of hydrodynamic oscillations, two processes taking place in high latitudes near the interface between the convective and radiative zones play a key role. One is forcing of the westerly zonal flow, the conditions for which are due to deformation of the interfacial surface. The other process is the excitation of a shear instability of zonal flow as a consequence of a strong radial gradient of angular velocity. The development of a shear instability at some stage brings about the disruption of the forcing of differential rotation. In the first (hydrodynamic) part of the paper, the dynamics of axisymmetric flows near the bottom of the convection zone is numerically simulated. Forcing of differential rotation having velocity shear in latitude and the existence of solutions in the form of torsional waves with the 11-year oscillation period are shown. In the second part the dynamics of the magnetic field is studied. The most pronounced peculiarities of the solutions are the existence of forced oscillations with the 22-year period and the drift of the toroidal magnetic field component from the mid latitudes to the equator. In high and low latitudes after cycle maximum, the toroidal component is of opposite sign in accordance with observations. In the third part, the transport of momentum from the bottom of the convection zone to the outer surface by virtue of diffusivity is considered. The existence of some sources of differential rotation in the convection zone is not implied. A qualitative correspondence of the differential rotation profile in the bulk of the convection zone and on its outer surface to experimental data is shown. The time correspondence between torsional and magnetic oscillations is also in accordance with observations. 相似文献
8.
对低纬子午环的仪器误差作了分类，包括制造误差、安装误差、仪器重力变形和热变形的影响，还分别列出了各种误差测量设备的制造误差、安装误差、读数零点偏差及其漂移的影响。文中强调了在该仪器误差修正中，不建立和使用任何误差模型，而是采用实时测定值，并且特别注意消除系统误差的影响，在这基础上，才能通过重复采数来压缩随机误差的影响。 相似文献
9.
A simple model is presented for the estimation of the surface potential of satellites with insulating coating. Any small area F of the surface is represented by a capacitor, which is charged up by the plasma and photoelectron currents. Using simple approximations for the currents, the surface potential is calculated for plasma parameters supposed to describe the solar wind between 0.3 a.u. and 1 a.u. distance from the Sun.The potential of the shaded side of the satellite is found to depend strongly on the capacitance of the surface. High capacitances or high spin frequency for the spacecraft can be used to limit the potential differences on the surface. In any case the angular surface potential distribution is asymmetric with respect to the solar wind velocity. 相似文献
10.
We study the motion of the free dual-spin gyrostat spacecraft that consists of the platform with a triaxial ellipsoid of inertia and the rotor with a small asymmetry with respect to the axis of rotation. The system with perturbations caused by a small asymmetry of the rotor and the time-varying moments of inertia of the rotor is considered. The dimensionless equations of the system are written in Serret–Andoyer canonical variables. The system’s phase space is described. It is shown that changes in the moments of inertia of the gyrostat leads to the deformation of the phase space. The internal torque control law is proposed that keeps the system at the center point in the phase space. The effectiveness of the control is shown through a numerical simulation. It’s shown that the uncontrolled gyrostat can lose its axis orientation. Proposed internal torque keeps the initial angle between the axis of the gyrostat and the total angular momentum vector. 相似文献
11.
A new type of optical fiber T.V. adaptor-link device is developed to bring a star image at the focus of a Schmidt telescope to the slit of a spectrograph located at the observing floor, as well as part of the field, to a T.V. monitor for acquisition and guiding purposes. The device is constructed by 60 000 thin glass fibers (18 micron in diameter and 1.2 m in length) used as the adaptor, and a single long (20 m) thicker (25 micron core diameter) fused silica fiber used as the link between the telescope and the spectrograph. The 60 000 thin fibers are arranged in a 4 by 5 mm rectangular array with the link built in the center of the field. With the adaptor-link the astronomer can easily acquire the star on the T.V. monitor, transfer the star image through the link to the slit of the spectrograph and guide the star during observation. 相似文献
12.
The temporal variation of the cosmic-ray flux at Earth directly influences the production rate of cosmogenic isotopes in the
Earth’s atmosphere. Here we put the emphasis on problems of the modulation of cosmic rays in the heliosphere. The physics
of the modulation region, e.g., the heliosphere, is described and also the transport theory of the cosmic rays through the
heliosphere is presented. The discussion includes more recent ideas of the modulation in the heliosheath. In the light of
these ideas the cosmic-ray fluxes during the Maunder minimum are discussed. It is also discussed, that the 22-year cycle observed
in the cosmogenic isotopes is a modulation effect of the cosmic rays, and hence directly connected with the physics of the
outer heliosphere. 相似文献
13.
The upwelling radiation at the top of the atmosphere is computed over a circular lake which is located in the uniform Lambert surface, using a modified version of the doubling-adding method. The radiance over the lake is discussed with respect to the atmospheric effect. The radius of the lake is assumed to be 0.5, 1, and 3 km. The observational site is located at altitude 30 km. The zenith of the observational site is located in the plane which is determined by the zenith of the center of the lake and incident solar direction. The zenith angle of the observational site to the center of the lake is fixed to 6.28°. The atmosphere is assumed to be homogeneous, which is composed of aerosol and molecule, where the model aerosol is of the oceanic or the water soluble types.Numerical simulation exhibits an extraordinary effect near the lake. The radiance of the lake against the surrounding depends upon the albedo of the surrounding surface. It increases with the increase of the size of the lake and decreases with the optical thickness. At large optical depth, the radiance depends upon the aerosol characteristics. It shows little dependence on the solar zenith angle if less than 60°. 相似文献
14.
15.
F. Søraas J.Å. Lundblad N.F. Maltseva V. Troitskaya V. Selivanov 《Planetary and Space Science》1980,28(4):387-405
A study of simultaneous groundbased observations of I.P.D.P. (intervals of pulsation of diminishing period) magnetic field fluctuation events and satellite observations of energetic protons have been performed. Some of our results are as follows. (1) The region of I.P.D.P. occurrence is always located equatorward of the isotropic proton precipitation. (2) The I.P.D.P. generation is not connected with the poleward leap of the aurora and the poleward expansion of the precipitating protons. (3) In the evening to afternoon sector enhanced pitch angle scattering is found near L = 4 during I.P.D.P. events, earlier shown to be associated with ion cyclotron resonance. (4) I.P.D.P. events seem to be associated with increased fluxes of (40–60) keV protons injected during substorms near the plasmapause in the equatorial plane.In order to explain the observations we invoke the following model: at substorm onset ring current protons are injected deep into the nightside magnetosphere covering a certain region in L and L.T., with the inner edge of the proton population following McIlwain's injection boundary. The protons drift azimuthally westward and generate ion cyclotron waves in a certain L interval at or inside the plasmapause. By taking into account the shape and position of the plasmapause and the injection boundary, the exterrt and position of the wave generating region can be determined. The frequency-time dispersion of the I.P.D.P. is largely attributed to the L-dependent drift velocity of protons in a narrow energy band. The model is able to explain the observations during several individual events. Also, the model predicts the general trends that have been found by statistical analysis of I.P.D.P. events and accounts for the constant frequency observed by satellites during I.P.D.P. events. 相似文献
16.
We consider the ejection of one stellar system from the centre of another stellar system, representing both by Plummer models. Using the impulsive appoximation, we derive analytically the overall and differential energy changes and also the mass escape from the systems. We compare the results with those obtained for colliding systems.We find that the disruptive effects are considerably less in the case of ejection. If the ejected system is compact, it escapes with negligible disruptive effects.In the case of ejections, stars are also accelerated in the direction of motion of the system. Using a dimensionless parameter λ defined as the ratio of the squares of the stellar velocity perturbations in the direction of motion of the system and perpendicular to it, we find a significant difference between ejecting systems and colliding systems. In fast head-on collisions of spherical stellar systems, the systems become elongated in the direction perpendicular to the direction of motion whereas in the case of ejecting systems, they also become elongated in the direction of motion. These effects are more pronounced in the outer regions of the smaller system and the innner regions of the bigger system. These effects are enhanced if the ejected system is compact. 相似文献
17.
H. Greiner-Mai 《Astronomische Nachrichten》1997,318(1):63-71
The geomagnetic field is maintained by amagnetohydrodynamic dynamo process within the liquid outer core. The distribution of the associated electric currents is modified if the outer core is bounded by electrically conducting material. Then, eddy currents and the related magnetic fields are generated within these regions. In particular, the relative rigid rotation of the inner core produces a secondary magnetic field, which is superimposed on the dynamo field. The angle between the dipole axis of the total field and the rotational axis of the inner core is an important quantity needed for the theory of polar motion of the Earth. This angle is investigated for a broad spectrum of angular velocities of the inner core. To simplify the mathematical procedure, we model the dynamo field using an axisymmetric field generated by a system of electric currents within the outer core. The conductivity of the mantle is neglected. We find that the position of the dipole axis depends on the angular velocity of the inner core as well as on the distribution of the current system within the outer core. Coincidence of both axes can be reached if the angular velocity is high enough and if the current system is concentrated within a thin sheet near the outer core-inner core boundary. 相似文献
18.
Kyoung W. Min 《Astrophysics and Space Science》1990,163(1):95-107
Magnetospheric substorm in the magnetotail region is studied numerically by means of a three-dimensional MHD code. The analytic solution for the quiet magnetotail is emloyed as an initial configuration. The localized solar wind is modeled to enter the simulation domain through the boundaries located in the magnetotail lobe region. As a result of the interaction between the solar wind and the magnetosphere, the magnetic field lines are stretched, and the plasma sheet becomes thinner and thinner. When the current-driven resistivity is generated, magnetic reconnection is triggered by this resistivity. The resulting plasma jetting is found to be super-magnetosonic. Although the plasmoid formation and its tailward motion is not quite clear as in the two-dimensional simulation, which is mainly because of the numerical model chosen for the present simulation, the rarification of the plasmas near thex-point is observed. Field-aligned currents are observed in the late expansive stage of the magnetospheric substorm. These field-aligned currents flow from the tail toward the ionosphere on the dawn side and from the ionosphere toward the tail on the dusk side, namely in the same sence of the region 1 current. As the field-aligned currents develop, it is found that the cross-tail current in the Earthside midnight section of the magneticx-point is reduced. 相似文献
19.
20.
木星探测轨道分析与设计 总被引：3，自引：0，他引：3
研究了与木星探测相关的轨道设计问题.重点关注木星探测轨道与火星、金星等类地行星探测轨道的不同及由此带来的轨道设计难点.首先分析了绕木星探测任务轨道的选择.建立近似模型讨论了向木星飞行需要借助多颗行星的多次引力辅助,对地木转移的多种行星引力辅助序列,使用粒子群算法搜索了2020年至2025年之间的燃料最省飞行方案并对比得到了向木星飞行较好的引力辅助方式为金星-地球-地球引力辅助.结合多任务探测,研究了航天器在飞向木星途中穿越主小行星带飞越探测小行星的轨道设计.最后,给出2023年发射完整的结合引力辅助与小行星多次飞越的木星探测轨道设计算例. 相似文献